Bardsey's Willdlife 2020

BREEDING BIRDS

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DUNBAR, PRETTYMAN AND STANSFIELD

1 INTRODUCTION

Since 1953, Roberts (1985), Jones (1988), Leitch (1992), Loxton and Jones (1995) and Eagle (2012) have
produced summaries of the breeding birds on Bardsey, in addition there are annual summaries of the
previous year’s nesting season in the systematic list in each of the Observatory’s reports. Since 1998,
however, increased effort has meant that more data has been collected systematically and a more
comprehensive report has been produced each year with the aim of providing a quick reference of the
breeding data from year to year, comparing the data for 2020 with that collected in the previous nine
years and showing the number of pairs nesting, productivity figures (where available and relevant) and
notes on successes or failures compared with previous years.

Since 2009, the four-visit Breeding Bird Survey (BBS) method has also been used for breeding landbirds,
the data from this survey help with the assessment of land management impacts on non-seabird species
and inform conservation management reviews.

The seabird data that is collected, as well as being used by Natural Resources Wales (NRW) to assess
Bardsey’s seabird populations and health, are fed into the national Seabird Monitoring Programme
(SMP). The SMP was established in 1986 to monitor annually the 26 species of seabird that regularly breed
in Britain and Ireland. It aims to ensure that sample data on breeding numbers and breeding success of
seabirds are collected both regionally and nationally, to enable their conservation status to be assessed.
The SMP is led and co-ordinated by JNCC in partnership with others including NRW - in November 2009
all partners signed a Statement of Intent that established the way forward for future data sharing and
collaborative working.

2 METHODS

The Observatory is funded through the NRW management agreement to monitor the breeding landbird
and seabird sites on Bardsey as part of the NNR.

As in previous years, the monitoring and mapping of seabirds followed the methods set out by Walsh et
al. (1995) in the Seabird Monitoring Handbook. Land-based counts were undertaken, and a boat trip on
15 June allowed sets of photographs to be taken of the larger cliff-nesting seabird colonies; the number
of birds were then counted on a monitor.

As has been done since 1998, the territories (usually singing males) of the landbirds were mapped, as
were any nests and/or young that were located. The maps were compiled by BBFO staff from data
collected during their daily census’ of the island, and specific excursions were made to map species
such as Oystercatcher, Rock Pipit, Meadow Pipit, Swallow, Wren and Linnet.

Additionally, four Breeding Bird Census visits covering the whole island (each visit spread over several
mornings) were made between 18-21 April, 11-14 May, 1-4 June and 22-25 June. The data generated
from these will be helpful when comparing Bardsey with other sites. A full explanation of the methods
employed for each species is given in the BBFO Report for 2009 (Brown and Stansfield, 2010).

Original maps can be found at the Observatory.

3 RESULTS

The following systematic list summarises the nesting season for each species during 2020. The number of
nesting pairs and productivity figures from 2020 are compared with data from 2011 to 2019. The
number of nesting pairs and productivity estimates from 2011 to 2019 are shown in table format where
sufficient data are available.

144 REP. BARDSEY BIRD FLD OBS.

BREEDING BIRDS

BREEDING CATEGORIES:

STATUS DEFINITION
Scarce 1-10 pairs breeding annually
Uncommon 11-50 pairs breeding annually
Fairly common 51-250 pairs breeding annually
Common 251-1000 pairs breeding annually
Abundant 1001+ pairs breeding annually

SYSTEMATIC LIST OF BREEDING SPECIES

C A N A D A G O O S E Branta canadensis Scarce breeder on Ynysoedd Gwylan

This year two pairs of Canada Geese were present on Bardsey in spring but did not show any signs of
nesting.

S H E L D U C K Tadorna tadorna Scarce

Only two pairs were present this year. Both pairs only produced one brood of chicks, and both broods
appeared on 10 May. One brood at the North End only had two chicks remaining when it was
discovered, but no adults were present; these chicks were quickly predated by gulls. The other brood, of
six chicks, was in Solfach, these too were rapidly picked off by gulls; only one chick remained the
following morning and had vanished by the afternoon.

№ pairs with young 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
№ of fledglings 6 5 5 3 2 2 4 2 6 2
0 0 13 2 0 0 0 0 0 0

Shelduck © Steven Stansfield

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M A L L A R D Anas platyrhynchos Scarce, population stable

A total of nine broods was located, six fewer than in 2019. Broods were found from April through to July.
Four on Pwll Cain, one on Pwll ‘G’, two on Nant Pond, one on Tŷ Pellaf Pond and one on the Lowlands
pond. There was a minimum of 73 ducklings throughout the season, giving an average brood size of 8.11.
Duckling survival rates were very low as usual, often predated by Ravens, Carrion Crows and gulls. Most
chicks were seen to be predated before reaching any considerable size, though some flightless, but
sizable young were noted and probably did fledge.

C U C K O O Cuculus canorus Occasionally breeds as a brood parasite

A singing male and a female were present on the island from 10 May until 17th and were seen around
Cristin, Tŷ Pellaf, Nant and the Withies. On 4 July, a recently fledged juvenile appeared in Cristin Garden
where it was being fed by its Meadow Pipit foster parents. Another juvenile was seen at Tŷ Pellaf on 6th,
though its parents were unknown; the two fledglings were subsequently seen together in the Lowlands
for around a week afterwards.

Juvenile Cuckoo © Steven Stansfield

W O O D P I G E O N Columba palumbus Scarce, population stable

A total of ten territories was located this year, three fewer than last year and 9.91% below the ten-year
mean (11.10±s.d.2.08), yet still 54.80% greater than the long-term average from 1953-2020 (6.46 ±s.d.4.68).
In total, at least seven juveniles were counted throughout the late summer.

20

Number of pairs 15

10

5

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953 Woodpigeon Year
Mean 6.46 ±s.d.4.68

146 REP. BARDSEY BIRD FLD OBS.

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2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
№ of pairs 9 9 9 10 11 13 15 12 13 10

M O O R H E N Gallinula chloropus Scarce/uncommon, population fluctuating

Six pairs were recorded this year, a decrease of one compared to the previous year and 29.41% lower
than the ten-year mean (8.50±s.d.3.47). A total of ten broods containing 19 young was recorded, giving
an average brood size of 1.90. The productivity this year was 3.16; this is 92.21% higher than the ten-year
mean (1.64±s.e.0.26).

25

Number of pairs 20

15

10

5

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953 Moorhen Year
Mean 7.53 ±s.d.4.32

2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
Productivity 1.63 1.25 2.00 3.00 0.75 1.42 1.08 1.15 1.00 3.16

O Y S T E R C A T C H E R Haematopus ostralegus Fairly common, population stable

In total 77 pairs nested this year; the majority nested on the rocky coastline, but some 15 pairs nested in
the fields and amongst the gorse. This total is 9.94% lower than the ten-year mean (85.50±s.d.8.63), but
18.99% higher than the long-term mean from 1953-2020 (64.71 ±s.d.19.87). Of the 22 nests that were
found, 55 eggs were counted, giving an average clutch size of 2.50. This is 1.07% below the ten-year
mean (2.53±s.d.0.19).

Oystercatcher chick © Steven Stansfield

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DUNBAR, PRETTYMAN AND STANSFIELD

Number of pairs 120
100
1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
80 Oystercatcher Year
60 Mean 64.71 ±s.d.19.87
40
20

0
1953

№ of pairs 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
Mean clutch 85 85 87 101 98 82 84 84 72 77
2.71 2.75 2.69 2.63 2.36 2.14 2.46 2.65 2.38 2.50

K I T T I W A K E Rissa tridactyla Fairly common, population declining

This year saw 143 AONs, 22 (18.18%) more than last year and 23.70% greater the ten-year mean
(115.60±s.d.35.70) and the largest number since 2011. The number of AONs this year was 2.18% lower
than the long-term 1953-2019 average (146.19 ±s.d.103.29). From the 143 nests, 119 large juveniles were
counted and believed to have fledged giving a productivity of 0.83, very similar to last year’s 0.84 and
40.0% higher than the ten-year mean (0.59±s.e.0.09).

Nesting Kittiwakes © Steven Stansfield

148 REP. BARDSEY BIRD FLD OBS.

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400

Number of pairs 300

200

100

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953 Kittiwake Year
Mean 146.19 ±s.d.103.29

2011 2012 2013 2014 2015 2016 2017 2018 2019 2020

№ of pairs 188 76 62 104 125 122 125 90 121 143

№ of young 16* 23* 27* 20* 52* 27* 9* 36* 101 119

Productivity 0.52 0.52 0.71 0.21 0.84 0.41 0.11 0.92 0.84 0.83

*Only partial census carried out; more birds may have fledged. Productivity calculated at study plots from a smaller number of

AON.

G R E A T B L A C K - B A C K E D G U L L Larus marinus Scarce

Bardsey had two pairs again this year, 55.56% below the ten-year mean (4.50±s.d.1.90) and 8.33% below
the long-term 1953-2019 average (2.18 ±s.d.1.82). Two juveniles were seen on the East Side in June, giving
a minimum productivity of 1.00, 24.53% above the ten-year mean (0.80±s.e.0.15).

10

Number of pairs 8

6

4

2

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953 Year

Great Black Backed Gull 2.18 ±s.d.1.82

№ of pairs 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
Productivity 4 6 8 5 6 4 5 3 2 2

1.00 0.50 0.63 0.20 1.83 1.00 0.20 0.67 1.00 1.00

Unfortunately, due to inclement weather conditions and events caused by the SARS-CoV- 2 pandemic,
monitoring trips to the Gwylans were not possible this year, but data up to and including 2019 are
presented here for completeness.

№ of pairs 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
Productivity 44 - - 79 92 72 35 50 50 -
0.93 - - 0.75 0.64 0.79 0.68 0.54 0.82 -

H E R R I N G G U L L Larus argentatus Common, population recovering

A total of 423 AONs was counted, 78 (22.61%) greater than the 2019 total of 345, 11.08% above the ten-
year mean (380.80±s.d.35.60) and 28.14% greater than the long-term 1953-2019 average (330.11
±s.d.143.95) and the highest total since 2006. A sample productivity was calculated from 109 nests in the

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DUNBAR, PRETTYMAN AND STANSFIELD

North End colony, where 59 young fledged giving a productivity figure of 0.54, 16.3% less than the ten-
year mean (0.65±s.e.0.04).

Number of pairs 700
600
500 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
400 Herring Gull Year
300 Mean 330.11 ±s.d.143.95
200
100

0
1953

2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
№ of pairs 398 395 323 399 400 331 377 417 345 423
Productivity 0.62 0.46 0.64 0.69 0.54 0.84 0.64 0.77 0.71 0.54*

* Productivity was calculated from study plots from 109 Nests.

Unfortunately, due to inclement weather conditions and the SARS-CoV- 2 pandemic, monitoring trips to
the Gwylans were not possible this year, but data up to and including 2019 are presented here for
completeness.

№ of pairs 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
Productivity 135 - - 61 52 113 145 165 89 -
0.45 - - 1.15 0.54 0.88 0.84 0.76 -
-

L E S S E R B L A C K - B A C K E D G U L L Larus fuscus Fairly common, population stable

This year saw 169 AON, five (3.05%) greater than the 2019 total, but 12.98% lower than the 2011-2020
mean (194.20±s.d.45.90) and 10.46% lower than the 1953-2020 mean (188.68±s.d.191.96). Productivity
from a colony of 36 nests at the North End saw 18 juveniles fledge, giving a productivity figure of 0.5
chicks per pair, 32.74% higher than the ten-year mean (0.38±s.e.0.06).

№ of pairs 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
Productivity 209 188 167 315 212 177 164 177 164 169
0.13 0.37 0.44 0.59 0.04 0.38 0.56 0.38 0.5
-

Number of pairs 700
600
500 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
400 Year
300
200 Lesser Black-backed Gull 188.68 ±s.d.191.96
100

0
1953

G U I L L E M O T Uria aalge Common, becoming abundant, Bardsey population increasing

A total of 1232 AOLs were recorded this year, 181 (12.8%) fewer than the count in 2019, 1.03% lower
than the ten-year-mean (1234.40±s.d.174.59) but 169.67% greater than the 1953-2020 mean (456.86

150 REP. BARDSEY BIRD FLD OBS.

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±s.d.429.44). A total of 22 chicks fledged from a sample plot of 50 AOLs gives a productivity of 0.44,
22.81% lower than 2019, but 32.13% higher than the ten-year mean (0.33±s.e.0.04).

2000

Number of pairs 1500

1000

500

0 1983 1988 1993 1998 2003 2008 2013 2018
1953 1958 1963 1968 1973 1978 Year
Mean 456.86 ±s.d.429.44
Guillemot aol

Guillemots © Steven Stansfield

2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
№ of AOL 981 1234 1071 1141… 1240 1346 1574 1112 1413 1232
№ of young 48* 18* 22* 18* 50* 70* 94* 63* 22*
Productivity 0.30 0.22 0.28 51* 0.38 0.26 0.40 0.22 0.57 0.44
0.26
* Productivity calculated at study plots of 50 AOL.

Unfortunately, due to inclement weather conditions and the SARS-CoV- 2 pandemic, monitoring trips to
the Gwylans were not possible this year, but data up to and including 2019 are presented here for
completeness.

№ of AOL 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
№ of young 77 - - 32 44 51 49 67 81 -
Productivity - - - 1 - - 23 - - -
- - - 0.03 - - - - - -

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R A Z O R B I L L Alca torda Common to abundant, population recovering

Based on flush counts, with a peak of c1400 birds from the whole of the East Side, there were 938 pairs
this year, with the x0.67 (1 adult = 0.67 pair) conversion factor applied (Walsh et al., 1995). The population
was 26.94% lower than the 2019 population. A minimum of 111 young and eggs was counted. This is a
67.83% decrease compared to last year and 61.03% lower than the ten-year mean (284.80±s.d.78.30)
and a 58.66% decrease from the 1953-2020 mean (268.51±s.d.115.11).

2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
Eggs/young 400 278 287 300 317 321 246 277 311 111

Unfortunately, due to inclement weather conditions and events caused by the SARS-CoV- 2 pandemic,
monitoring trips to the Gwylans were not possible this year, but data up to and including 2019 are
presented here for completeness.

2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
Eggs/young 17 - - 18 - 24 42 16 9 -

P U F F I N Fratercula arctica Uncommon, increasing on Bardsey, stable on Gwylan

This year 164 AOBs were counted on the East Side. There were 113 on the slopes above Ogof Morlas and
51 between Bae Felen and Seal Cave. This total is 21 (14.69%) greater than 2019 and 103.73% above the
ten-year mean (80.50±s.d.53.33) and a dramatic 1053.68% increase on the 1953-2020 mean
(14.22±s.d.35.25), continuing the upward trend for this species on Bardsey.

200

Number of pairs 150

100

50

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953 Puffin

Year

Mean 14.22 ±s.d.35.25

2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
AOB 16 16 29 55 75 75 100 132 143 164

Unfortunately, due to inclement weather conditions and events caused by the SARS-CoV- 2 pandemic,
monitoring trips to the Gwylans were not possible this year, but data up to and including 2019 are
presented here for completeness.

2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
AOB 759* - 839* 871 - 682 663 819 619* -

* Only partial census completed for safety reasons

S T O R M P E T R E L Hydrobates pelagicus Fairly common

Adults were heard calling in the usual spots around Briw Gerrig, Seal Cave and Bae Felen in June and
July. One bird was seen on a nest in an artificial nest box and the chick was ringed in August.

F U L M A R Fulmarus glacialis Uncommon, population fluctuating

The East Side cliffs produced 16 AOS, two more than the previous two years, 11.11% lower than the ten-
year mean (18.00±s.d.5.68) and 15.46% below the 1953-2020 mean (19.77±s.d.15.46). Productivity is not
often calculated for this species on Bardsey, but this year in mid- to late August, two trips seven days

152 REP. BARDSEY BIRD FLD OBS.

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apart were undertaken to count Fulmar chicks and produced a total of four well grown, near fledging
young, giving a productivity figure of 0.25.

Number of pairs 60
50
40 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
30 Fulmar Year
20 Mean 19.77 ±s.d.15.35
10

0
1953

№ of AOS 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
Productivity 22 21 11 17 31 15 19 14 14 16
0.40 - - - - - - - - 0.25

M A N X S H E A R W A T E R Puffinus puffinus Abundant, population stable

This season 114 burrows containing an adult bird incubating an egg were monitored. In total 27 failed at
the egg stage, three failed at the chick stage and the remaining 94 pairs successfully reared young,
giving a productivity figure of 0.74 chicks per breeding pair; a slight decrease of 3.90% on last year’s
productivity (0.77), and 8.82% higher than the ten-year mean (0.68 ±s.e.0.02). This year’s productivity is
exactly the same as the mean since 1998 (0.74 ±s.e.0.02) when Bardsey Bird Observatory began
monitoring productivity. The highest productivity was 0.88 in 2003 and the lowest was 0.60 in 2018.

A full report on the breeding season for Manx Shearwaters can be found on page 164.

Productivity 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
Sample size 0.73 0.72 0.61 0.70 0.64 0.64 0.65 0.60 0.77 0.74
122 148 132 101 119 125 127 125 122 114

S H A G Phalacrocorax aristotelis Uncommon, population increasing again

Shag nests were counted from both land and sea during several visits to the East Side of Bardsey. A total
of 39 nests was counted, six (13.33%) fewer than in 2019. The total is 25.81% above the 1953-2020 mean
(31.00±s.d.11.07), but equal to the ten-year-mean (39.00±s.d.4.97). A total of 41 young fledged, giving a
productivity of 1.10, which is 38.38% below the 2011-2020 mean (1.79±s.e.0.14).

80

Number of pairs 60

40

20

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953 Shag

Year

Mean 31.00 ±s.d.11.07

№ of pairs 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
Productivity 39 34 31 34 39 39 45 45 45 39
2.08 2.59 2.00 1.65 1.28 1.71 1.82 2.09 1.62 1.10

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DUNBAR, PRETTYMAN AND STANSFIELD

Unfortunately, due to inclement weather conditions and events caused by the SARS-CoV- 2 pandemic,
monitoring trips to the Gwylans were not possible this year, but data up to and including 2019 are
presented here for completeness.

№ of pairs 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
Productivity 49 - 24 46 48 41 44 66 52 -
2.12 - - 1.98 1.59 1.66 1.15 1.23 2.04 -

C O R M O R A N T Phalacrocorax carbo Uncommonon Ynysoedd Gwylan

Unfortunately, due to inclement weather conditions and events caused by the SARS-CoV- 2 pandemic,
monitoring trips to the Gwylans were not possible this year, but data up to and including 2019 are
presented here for completeness. A number of nesting pairs were visible from Bardsey using a high-
powered telescope.

Year 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
Y G Fawr pairs 34 - 31 0 0 0 0 0 0 -
Y G Bach pairs 12 - 5 9 9 8 12 14 20 -
46 - 36 9 9 8 12 14 20 -
Total pairs 2.15 - - -
Productivity 1.56 1.44 2.13 1.25 0.64 1.25

P E R E G R I N E Falco peregrinus Scarce (Schedule 1), population stable

One nest was located on 10 May, with a female sitting in one of the regular nest sites. Due to the location
of the nest the contents were not recorded. On 1 July two juveniles were seen flying over the Mountain
near to the nest site; this gives a productivity of at least 2.00.

№ of pairs 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
Productivity 2 2 2 2 2 2 1 1 1 1
0 0
2.50 2.00 2.00 2.00 1.00 1.00 2.00 2.00

L I T T L E O W L Athene noctua Scarce, population stable

Five males held territories in April, when birds were heard calling throughout the season on a daily basis,

however, this dropped to three by the start of June. This is three more territories than in 2019, 14.68%

greater than the long-term 1953-2020 mean (4.36±s.d.1.72) and 28.21% greater than the ten-year mean

(3.90±s.d.1.73). Only two juveniles were known to have fledged; one was along the fence line below

Carreg Bach on 19 July and the other at Nant on 28th, giving a productivity of 0.40, but it was suspected

that others did fledge and due to their secretive nature and dense vegetation, they went undetected.

10

Number of pairs 8

6

4

2

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953 Little Owl Year
Mean 4.36 ±s.d.1.72

2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
№ of pairs 3 3 2 3 5 7 6 3 2 5

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BREEDING BIRDS

M A G P I E Pica pica Scarce, population fluctuates, but fairly stable

Eight pairs nested this year, predominantly in gorse patches, both on the Mountainside and at the North
End, two fewer than last year. However, there were nests in the Lowlands and Wetlands, on the South
End and at Tŷ Pellaf Withy. A total of 23 juveniles was counted, giving a productivity of 2.88 young per
pair, 66.9% above the ten-year mean (1.73±s.e.0.26). This is the fourth consecutive year that the
population has fallen and is now 27.27% lower than the ten-year mean (11.00±s.d.2.45), but it is still 8.7%
greater than the long-term average 1953-2020 (7.35 ±s.d.3.31).

20

Number of pairs 15

10

5

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953 Magpie Year
Mean 7.35 ±s.d.3.31

№ of pairs 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
Productivity 8 10 11 9 14 15 14 12 10 8
1.80 1.55 0.93 0.87 1.57 2.83 2.80
1.13 0.90 2.88

C H O U G H Pyrrhocorax pyrrhocorax Scarce (Schedule 1), population stable

Seven pairs nested this year, the same number as last year, being 14.63% lower than the ten-year mean
(8.20±s.d.0.79), but 53.51% greater than the long-term average (4.56 ±s.d.2.25). The total of 15 juveniles
fledged gives a productivity figure of 2.14, which is 8.96% greater than the 2011-2019 mean
(1.96±s.e.0.15), but 19.3% lower than the long-term 1953-2020 average (2.65 ±s.e.0.13).

№ of pairs 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
№ of young 8 9 9 8 9 8 9 8 7 7
Productivity 14 15 13 24 18 14 22 15 11 15

1.75 1.67 1.44 3.00 2.00 1.75 2.44 1.88 1.57 2.14

C A R R I O N C R O W Corvus corone Scarce, population stable

Seven pairs nested this year, the same as last year, but the lowest since 2014, as the population swelled
to 13 pairs between 2015 and 2017. The population is now just 2.43% greater than the long-term average
1953-2020 (6.83 ±s.d.2.43) and 26.35% lower than the ten-year mean (9.50 ±s.d.2.64). At least 15 young
fledged giving a productivity of 2.14.

Number of pairs 14
12
10 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
Carrion Crow Year
8 Mean 6.83 ±s.d.2.43
6
4
2
0

1953

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2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
№ of pairs 7 9 10 7 13 13 13 9 7 7

R A V E N Corvus corax Scarce

One pair nested at Ogof Barcut this year with a brood of three, fledging on 1 May. A pair was seen
occasionally at the North End during April and May, but no breeding attempt was confirmed.

5

Number of pairs 4

3

2

1

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953 Raven Year
Mean 1.15 ±s.d.0.79

Raven © Steven Stansfield

G R E A T T I T Parus major Scarce

Three pairs bred this year; one each at Nant, Cristin and at Carreg Fawr. This is the seventh time this
species has nested and the highest ever total for the island, with the previous highest being two pairs
which bred in 1985. Great Tits last bred in 2014. A minimum of 30 young fledged during the summer, with
some pairs producing multiple broods.

4

Number of pairs 3

2

1

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953 Great Tit Year
Mean 0.15 ±s.d.0.51

156 REP. BARDSEY BIRD FLD OBS.

BREEDING BIRDS

S W A L L O W Hirundo rustica Uncommon, population decreasing

Eleven pairs nested this year, one more than last year, but still 29.03% lower than the ten-year mean
(15.50±s.d.4.43). However, this is 30.47% greater than the long-term 1953-2020 mean (8.43±s.d.4.82). There
were two Pairs at Nant, Cristin, Tŷ Pellaf and Carreg, and one pair each at Plas Bach, in the Boathouse,
and in the Lighthouse Compound. Most pairs managed to fledge two broods, and a minimum of 57
young was recorded, 32 from the first brood and a further 25 from the second and third broods, to give
a productivity figure of 5.18 young per pair; this is 1.91% below the ten-year mean (5.28±s.e.0.46).

25

Number of pairs 20

15

10

5

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953 Swallow Year
Mean 8.43 ±s.d.4.82

№ of pairs 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
№ of young 21 19 18 17 21 14 15 9 10 11
Productivity 106 100 62 105 77 79 54 71 69 57
5.05 5.26 3.44 6.18 3.67 5.64 3.60 6.90 5.18
7.89

H O U S E M A R T I N Delichon urbicum Scarce occasional breeder

This is the 17th time and eighth consecutive year that this species has nested on Bardsey. Four pairs
nested at the Lighthouse this year, and one pair built a nest at Plas and fledged three juveniles; the first
successful nesting away from the Lighthouse on the island. Of the four nests at the Lighthouse, only one
fledged young as the other three were destroyed in early July. The total of five pairs this year is 286.90%
greater than the long-term 1953-2020 average (1.29±s.d.3.15) and 25% above the ten-year mean
(4.00±s.d.2.54). A minimum of five young fledged, giving a productivity of 1.00. This is a low productivity,
due to the destruction of the nests in the Lighthouse Compound by what were thought to be Carrion
Crows.

20

Number of pairs 15

10

5

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953 House Martin Year
Mean 1.29 ±s.d.3.15

C H I F F C H A F F Phylloscopus collybita Scarce

One pair nested this year; singing birds were heard on a daily basis throughout May, but reduced to one
singing bird at Nant, and breeding was confirmed when an adult was seen feeding two fledged young
in August at Nant. The single pair this year was 44.4% lower than the ten-year mean (1.80±s.d.1.23) but
still 13.6% greater than the long-term average (0.88±s.d.1.23).

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5

Number of pairs 4

3

2

1

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953 Chiffchaff Year
Mean 0.88 ±s.d.1.23

2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
№ of pairs 1 4 2 3 1 0 2 1 3 1

W I L L O W W A R B L E R Phylloscopus trochilus Common migrant, occasional breeder

Only the 14th time this species has nested on Bardsey. Four singing males held territory in 2020; at Cristin,
Nant, in the Wetlands and in Cristin Withy. This total of four pairs is the joint highest number of breeding
pairs recorded on the island (with 2011) and is 263.64% above the ten-year mean (1.10±s.d.1.66). A
minimum of eight juveniles fledged, giving a productivity of 2.0.

5

Number of pairs 4

3

2

1

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953 Willow Warbler Year
Mean 0.38 ±s.d.0.88

2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
№ of pairs 4 0 0 2 0 0 0 0 1 4

S E D G E W A R B L E R Acrocephalus schoenobaenus Scarce

Six pairs bred this year, a decrease of one from last year, and 15.49% lower than the ten-year mean
(7.10±s.d.2.33), but still 239.94% greater than the long-term 1953-2020 mean (1.52±s.d.2.66). A minimum
of fifteen juveniles was recorded from the six territories.

Number of pairs 12
10
1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
8 Sedge Warbler Year
6 Mean 1.52 ±s.d.2.66
4
2
0

1953

Juvenile Sedge Warbler © Steven Stansfield

158 REP. BARDSEY BIRD FLD OBS.

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DUNBAR, PRETTYMAN AND STANSFIELD

2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
№ of pairs 8 8 9 3 5 10 10 5 7 6

B L A C K C A P Sylvia atricapilla Scare

Four pairs bred or attempted to breed this year, three successfully; the highest ever number of pairs to
have bred on Bardsey in the eight years they have nested, and 300% above the ten-year mean
(1.00±s.d.1.05). These pairs were found in Cristin Withy and Nant Withy, with the third pair breeding
successfully in the garden at Cristin. There were also signs of breeding at Tŷ Nesaf, although no food-
carrying or fledged young were subsequently seen. From the three successful pairs, a minimum of 13
young fledged, the first of which appeared in the garden at Cristin on 2 July.

5

Number of pairs 4

3

2

1

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953 Blackcap Year
Mean 0.20 ±s.d.0.64

2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
№ of pairs 0 1 0 0 2 1 1 0 2 4

W H I T E T H R O A T Sylvia communis Scarce

Six pairs again held territories this year and of those six, at least five fledged young. The successful pairs
nested on the Mountain behind Cristin, at Nant, Carreg Bach, and in the Lowlands. The earliest signs of
successful breeding came on 11 June with a brood of four newly-fledged young in Tŷ Capel garden. A
minimum of 16 juveniles fledged, giving a productivity of 2.67 young per pair.

20

Number of pairs 15

10

5

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953 Whitethroat Year
Mean 3.18 ±s.d.3.58

2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
№ of pairs 5 7 6 7 1 2 2 1 6 6

W R E N Troglodytes troglodytes Fairly common, population fluctuates

There were 142 territorial singing males recorded during the Breeding Bird Survey (BBS) dawn counts in
2020; this total is 34 (31.48%) greater than last year’s total of 108 territories and only 1.59% below the ten-
year mean (144.30±s.d.48.20), but 163.88% greater than the long-term average since 1953 (53.81

160 REP. BARDSEY BIRD FLD OBS.

BREEDING BIRDS

±s.d.50.88). It is encouraging to see that the population has increased from last year, and hopefully will
continue to do so, after suffering quite severely due to very cold weather in early 2018.

Number of pairs 300
250
200 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
150 Wren
100

50
0
1953

Year

Mean 53.81 ±s.d.50.88

2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
№ of territories 81 121 101 159 186 163 248 134 108 142

Wren © Steven Stansfield

B L A C K B I R D Turdus merula Scarce/uncommon, population increasing

This year a total of 19 pairs was recorded, 32.87% above the ten-year mean (14.30±s.d.3.52), five more

pairs than last year, back in line with 2018’s total and 44.11% greater than the long-term average (13.18

±s.d.12.94). A minimum of 54 young fledged, giving a productivity figure of 2.84, 23.86% above the ten-

year mean (2.29±s.e.0.26). The majority of birds nested in the island’s gardens, although there were also

many nesting birds in the wetlands and the withies. The first juveniles appeared in Nant Withy on 21 April.

50

Number of pairs 40

30

20

10

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953 Blackbird Year
Mean 13.18 ±s.d.12.86

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№ of pairs 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
Productivity 7 12 16 12 14 14 16 19 14 19
2.83 1.56 1.83 1.71 2.14 1.05 2.05 3.21 2.84
3.71

S O N G T H R U S H Turdus philomelos Scarce

This year, three pairs bred on the island, with one pair at Nant, one at Cristin and one in Cristin Withy. This
is the highest ever total on the island, and a good sign of a population increase after they returned as a
breeding species in 2018 after their 57-year absence. A minimum of nine juveniles fledged between the
three pairs, with the first juveniles appearing at Cristin on 27 May.

4

Number of pairs 3

2

1

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953 Song Thrush Year
Mean 0.17 ±s.d.0.55

R O B I N Erithacus rubecula Uncommon

There were two territories held this year, with one pair fledging four young up at Nant

S T O N E C H A T Saxicola torquatus Scarce, population recovering / stable

Stonechats had an exceptional year, with 15 breeding pairs, equal to the previous highest count in 1960.
This was five pairs greater than last year. This was 92.31% higher than the ten-year mean (7.80±s.d.3.55)
and 180.3% higher than the long-term 1953-2020 average (5.35 ±s.d.4.11). A minimum of 59 juveniles
fledged, giving a productivity figure of 3.93, 31.52% lower than the ten-year mean (5.74±s.e.0.53).

20

Number of pairs 15

10

5

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953 Stonechat Year
Mean 5.35 ±s.d.4.11

№ of pairs 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
№ of young 3 7 5 7 10 10 7 4 10 15
Productivity 29 44 28 43 49 52 26 27 52 59
4.90 5.2 5.20 3.93
9.67 6.29 5.60 6.14 3.71 6.75

W H E A T E A R Oenanthe oenanthe Scarce/uncommon, population recovering

This year a total of 32 pairs was recorded, nine more than last year, but still six fewer than the 2017 record
total of 38 pairs. This year’s total is 29.55% above the ten-year mean (24.70±s.d.10.09) and 114.21% greater

162 REP. BARDSEY BIRD FLD OBS.

BREEDING BIRDS

than the long-term average from 1953-2020 (14.94 ±s.d.9.75). A minimum of 45 young fledged this year,
giving a productivity figure of 1.41 young per pair; this is 26.35% below the ten-year mean (1.91±s.e.0.23).
The first breeding behaviour was recorded on 25 April and the first juvenile bird was seen on 1 June.

40

Number of pairs 30

20

10

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953 Wheatear Year
Mean 14.94 ±s.d.9.75

2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
23 32
№ of pairs 11 11 17 34 35 28 38 18 36 45
1.57 1.41
№ of young 27* 28 55 65 54 31 51 46

Productivity 3.86* 2.55 3.24 1.91 1.54 1.11 1.34 2.56

*Number of young/productivity was from a sample of monitored nests and not the entire population.

D U N N O C K Prunella modularis Uncommon, population fluctuating

This year 15 males held territory, five more than last year, but 34.21% below the ten-year mean
(22.80±s.d.14.67). As displayed in the graph below, the ten-year mean is high due to a large spike in the
population from 2010 to 2015. The total this year is 7.03% above the long-term mean from 1953-2020
(14.02±s.d.8.22). A minimum of 31 juveniles were recorded this year, the majority coming from the
Mountainside, including at least three broods at Cristin.

Number of pairs 60
50
40 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
30 Dunnock Year
20 Mean 14.02 ±s.d.8.22
10

0
1953

2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
№ of males 34 49 45 20 20 11 15 9 10 15

P I E D W A G T A I L Motacilla alba Scarce, population stable

Seven pairs nested this year, one more pair than last year, but low compared to the 14 pairs that were
present in 2015. This total is 24.73% below the ten-year mean (9.30±2.45), but 56.90% greater than the
long-term 1953-2020 average (4.46 ±s.d.3.44). A minimum of 25 juveniles were recorded, giving a
productivity figure of 3.57, which is 0.50% below the ten-year mean (3.59±s.e.0.21).

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15

Number of pairs 10

5

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953 Pied Wagtail Year
Mean 4.46 ±s.d.3.44

№ of pairs 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
№ of young 11 10 10 10 14 10 9 6 6 7
Productivity 42 31 48 46 40 29 30 17 21 25
3.82 3.10 4.80 4.60 2.86 2.90
3.33 3.40 3.50 3.57

M E A D O W P I P I T Anthus pratensis Fairly common, population decreasing

The population bounced back quite dramatically this year, with singing males present throughout the
lowlands and wetlands being very noticeable. There were 102 singing males recorded during four BBS
visits, this total being 121.74% greater than last year, 21.43% greater than the ten-year mean
(84.00±s.d.23.00), and 90.01% greater than the long-term 1953-2020 average (53.68 ±s.d.27.15). The first
fledged birds were recorded on 24 May in the Wetlands, being fed by their parents, with 30 July being
the last date when any new young birds were seen. Over the past few seasons, farming practices have
changed dramatically, in that there are no longer any hay meadows and long sward during the summer
months. When the removal of the hay making regime was announced in 2016 it was predicted this would
have a negative effect on the Meadow Pipit population, but at the same time having a positive effect
on other species. The population did decline heavily, but began to bounce back in 2019 to near-2017
numbers, however it is too early to be able to say for certain.

Number of pairs 140
120
100 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
Meadow Pipit Year
80 Mean 53.68 ±s.d.27.15
60
40
20

0
1953

2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
№ of pairs 59 66 72 97 118 93 106 81 46 102

R O C K P I P I T Anthus petrosus Uncommon, population increasing

This year’s total of 44 pairs is two greater than last year, 1.57% below the ten-year mean (44.70±s.d.7.60),
but 31.46% greater than the long-term 1953-2020 average (33.42 ±s.d.10.29). There were 45 juveniles
recorded on the coastline from 40 of the pairs that were monitored, giving a productivity figure of 1.13
young per pair.

164 REP. BARDSEY BIRD FLD OBS.

BREEDING BIRDS

Number of pairs 70
60
50 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
40 Rock Pipit Year
30 Mean 33.42 ±s.d.10.29
20
10

0
1953

2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
№ of pairs 31 43 47 50 59 40 51 40 42 44

C H A F F I N C H Fringilla coelebs Scarce, population declining

Five pairs bred this year, two at Nant, one at Cristin, one at Tŷ Pellaf and one in Cristin Withy. This is two
more pairs than last year, some 28.57% below the ten-year mean (7.00±s.d.4.16), but still 194.64% greater
than the long-term 1953-2020 average (1.70 ±s.d.3.18). This species’ population has previously been
bolstered by supplementary seed being a food source in the winter as shown by the increase from 2005
– 2015 when they were being fed by over-wintering residents on Bardsey. A total of 17 juveniles was
recorded between 16 May and 6 July.

Number of pairs 14
12
10 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
Chaffinch Year
8 Mean 1.70 ±s.d.3.18
6
4
2
0

1953

2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
№ of pairs 9 11 11 12 11 4 2 2 3 5

L I N N E T Linaria cannabina Uncommon, population declining

This year 22 pairs were recorded breeding, one fewer than last year and 21.8% lower than the ten-year
mean (28.20±s.d.7.66) and 7.59% lower than the long-term 1953-2020 average (23.88 ±s.d.15.88). Nesting
took place across the island including the Wetlands, Lowlands, South End, North End and the
Mountainside. The Mountain and grassy fields on the West Coast were less productive areas. This year 39
juveniles were recorded, giving a productivity figure of 1.77 young per pair, 49.14% below the ten-year
mean (2.81±s.e.0.46).

№ of pairs 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
№ of young 19 27 31 25 41 30 41 23 23 22
Productivity 106 58 148 68 39 38 114 62 80 39
5.58 2.15 4.77 2.72 0.95 1.27 2.71 2.70 3.48 1.77

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G O L D F I N C H Carduelis carduelis Scarce

This is the 14th year that Goldfinches have nested on Bardsey, all since 2002. Four pairs nested this year,
with one pair at Nant, one in Cristin garden and one each in Cristin and Tŷ Pellaf withies. Four pairs is the
joint highest total, along with those in 2013-2015, and is 81.82% above the ten-year mean (2.20±s.d.1.62).
All four pairs were successful, with the pair at Cristin rearing two broods; in total, a minimum of 13 juveniles
fledged from across the island.

5

Number of pairs 4

3

2

1

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953 Goldfinch Year
Mean 0.45 ±s.d.1.07

№ of pairs 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
1 1 4 4 4 1 2 1 0 4

4 SUMMARY

Razorbill productivity plummeted this year due to the strong south-westerly winds that swept through the
Irish Sea in late May. The total number of eggs and young was 111, a 67.83% decrease compared to last
year and 61.03% lower than the ten-year mean. Guillemots did better, with the breeding population
experiencing only a slight decline. 1232 AOL were counted this year, 12.8% less than in 2019. This means
the upward trend that has been experienced since the late 1990s is still clear to see. There were 22 chicks
in a sample plot of 50 AOL gave a productivity of 0.44, 22.81% lower than 2019. The Puffin colony
continues to grow on Bardsey, the count of AOBs increased to 164.

There were 143 pairs of Kittiwakes, 18.2% more than last year’s 121 pairs. The productivity figure of 0.83,
very similar to last year (0.86) and 40.0% higher than the ten-year mean (0.59±s.e.0.09). There were 39
Shag nests on Bardsey, a decrease of 13.33% from 2019, and the 41 chicks fledged gave a productivity
of 1.10. In total, 423 Herring Gull nests were counted, 22.6% more than in 2019. 2020, represents an
increase of 11.1% compared to the ten-year mean 380.80±s.d.35.60. Productivity studies from sample of
109 nests saw 59 juveniles fledged, giving a productivity of 0.54, 16.3% less than the ten-year mean
(0.65±s.e.0.04). Lesser Black-backed Gulls saw 169 AON counted, 3.05% more than last year, but 12.98%
below the ten-year mean (194.20±s.d.45.90). Productivity, calculated from a colony of 36 nests where 18
juveniles fledged gave a figure of 0.50, 32.74% higher than the ten-year mean (0.38±s.e.0.06). Two pairs
of Great Black-backed Gulls nested, 55.56% below the ten-year mean (4.50±s.d.1.90). The productivity of
1.00 was 24.53% above the ten-year mean (0.80±s.e.0.15).

There were 16 pairs of Fulmars which nested, 11.60% lower than the ten-year mean (18.10±s.d.5.70). Only
four young fledged, giving a productivity figure of 0.25. Manx Shearwater productivity figure was 0.74
chicks per breeding pair; a slight decrease of 3.90% on last year’s productivity, and 8.82% higher than
the ten-year mean (0.68 ±s.e.0.02). This is this is exactly the same as the mean since 1998 (0.74 ±s.e.0.02).

On the land bird front, a pair of Cuckoos were present on the island for a week in mid-May and breeding
was confirmed when two juveniles appeared in early July. The parents of the youngster at Cristin were

166 REP. BARDSEY BIRD FLD OBS.

BREEDING BIRDS

Meadow Pipits; however, the parents of the other juvenile from Tŷ Pellaf were unknown. Oystercatchers
increased by five pairs on last year, giving a total of 77 pairs around the island, still 9.94% below the ten-
year mean (85.50±s.d.8.63). Little Owls held five territories and breeding was confirmed for a second
consecutive year when a juvenile was seen on 19 July.

On the whole, passerines enjoyed a wealth of success stories on the island. Great Tits bred for the first
time since 2014 and had their best ever year on the island with three pairs breeding, beating the previous
highest total of two, which was back in 1985. Only one pair of Chiffchaffs bred this year, but Willow
Warblers enjoyed an excellent year with four pairs breeding across the island, the joint highest number
for Bardsey. Sedge Warblers were slightly down on last year’s numbers. Whitethroat numbers remained
constant, while Blackcaps had their best year ever with three pairs breeding. Wrens increased
significantly on last year’s numbers and are now almost back in line with the ten-year average. Having
only returned to Bardsey as a breeding species two years ago, Song Thrushes had their best ever year
with three pairs breeding, following their 57-year absence as a breeding species. Robins held two
territories this year, with one pair fledging four young up at Nant; Stonechats had an increase of five pairs
on last year and fifteen pairs bred, 92.31% above the ten-year mean (7.80±s.d.3.55). Wheatear numbers
also increased this year; nine more pairs than last year gave a total of 32 pairs, 29.55% above the ten-
year mean (24.70±s.d.10.09). Dunnocks also increased in numbers, with an extra five territories totalling
15 males holding territories this year. Pied Wagtails were up by one pair, making a total of seven, still low
compared to the 14 pairs present back in 2015. Rock Pipits increased by two pairs on last year, totalling
44 pairs and keeping the population in line with the ten-year mean (44.70±s.d.7.60). Meadow Pipits
experienced a dramatic bounce-back in numbers, with 102 pairs apparently breeding, compared to
last year’s total of 46, a 121.74% increase. Goldfinches had their joint-highest number of breeding pairs
this year, with four pairs fledging young across the island.

5 REFERENCES

Eagle, G. 2012. Bardsey’s breeding birds, 1992-2011. Rep. Bardsey Bird Fld. Obs. 55: 138-143.
Jones, P.H. 1988. The Natural History of Bardsey. National Museum of Wales, Cardiff.
Loxton, R.G. and Jones, P.H. 1995. The breeding birds of Bardsey, Skomer, Skokholm and the Calf of Man.

Part 1: Introduction and species accounts. Rep. Bardsey Bird Fld. Obs. 38: 84-159.
Mavor, R.A. et al. 2008. Seabird numbers and breeding success in Britain and Ireland, 2006.

Peterborough, JNCC UK Nature Conservation, No 31.
Walsh, P.M. et al. 1995. Seabird monitoring handbook for Britain and Ireland. JNCC/RSPB /ITE/Seabird

Group, Peterborough

Stonechat © Steven Stansfield

64: 142-167 167

DUNBAR, PRETTYMAN AND STANSFIELD

MANX SHEARWATER

Productivity, ringing and chick growth rates
George Dunbar

Manx Shearwater chick REP. BARDSEY BIRD FLD OBS.

© Steven Stansfield

168

64: 142-167 169

DUNBAR

1 INTRODUCTION

The Manx Shearwater is a truly pelagic bird, only returning to land to breed on a small number of select
offshore North Atlantic islands. It is currently identified as a conservation priority in the EC Birds Directive -
migratory species. It is Amber listed in Birds of Conservation Concern 3 (2009 update) (further information
on Conservation Designations for UK Taxa) and is Amber listed in Birds of Conservation Concern in Ireland
2008-2013 (2013 update).

Bardsey is considered to be the fourth largest Manx Shearwater colony in the world and is the third largest
of the three major colonies in Wales, following Skomer and Skokholm. As a significant proportion of the
world’s breeding population of Manx Shearwaters is found on Bardsey it is well-studied on the island. The
breeding population is known, its wintering quarters are known through ringing recoveries, and the
average productivity (number of young fledged per pair) since 1998 is known from a sample of up to
150 burrows monitored annually. Because of the global importance of the colony on the island, the Manx
Shearwater is one of the avian ‘features’ of the island as an SSSI and NNR.

Following a complete census undertaken on the island by Bardsey Bird Observatory between 2014 and
2016, a total of 20,675 Apparently Occupied Burrows (AOBs) were counted (Stansfield and Carter 2017),
showing an increase of almost 5000 AOBs (+30.37%) from the previous census in 2008 to 2010. Data from
year one of the current 2019-2022 census shows a further increase of 19.6%, which means that if the
increase is the same across the whole island, the population would now stand at somewhere in the
region of 25,000 pairs.

Data from this project is used by the Observatory and Natural Resources Wales (NRW) to assess the health
of the population. The data also forms part of the Seabird Monitoring Programme (SMP); established in
1986, this is an ongoing annual programme monitoring 26 species of seabird that regularly breed in Britain
and Ireland. It aims to ensure that sample data on breeding numbers and breeding success of seabirds
are collected, both regionally and nationally, to enable their conservation status to be assessed. The
SMP is led and co-ordinated by the Joint Nature Conservation Committee (JNCC) in partnership with
others including NRW. In November 2009 all partners signed a Statement of Intent that established the
way forward for future data-sharing and collaborative working.

2 AIMS

The aims of this study are to continue the productivity monitoring programme and expand the monitoring
area, to increase the proportion of the ringed population, particularly juvenile birds, and to re-trap as
many individuals as possible. The increased ringing of juveniles will provide details of more birds of known
age, therefore the age of first breeding will be known, as well as juvenile survival and dispersal. The
increased number of re-traps should give a clearer indication of the mean life expectancy, highlight any
losses in particular age classes and provide a robust indication of the population changes.

3 METHODS
3.1 PRODUCTIVITY MONITORING

A sample of Manx Shearwater burrows was selected over the previous few summers to be studied as
part of the long-running productivity monitoring survey. The burrows were visited several times throughout
the nesting season, initially during the mean egg-laying period when the Manx Shearwaters had just
begun to incubate. The next visits were made whilst the adults were rearing their young, and the final
visits were made during the fledging period. Shallow and easily accessible burrows were selected for the
survey so that the contents could be accurately monitored. Accessible burrows containing Manx

170 REP. BARDSEY BIRD FLD OBS.

SHEARWATER PRODUCTIVITY, RINGING AND CHICK GROWTH

Shearwaters and eggs were located at a number of clustered sites across the island covering a broad
range of habitats and areas.

Each burrow was mapped and marked with an individually numbered wooden stake and a grid
reference taken to aid relocation, both later in the season and the following year. When the chicks were
large enough, they were ringed, and biometrics were taken (wing length and weight). A final visit was
made to calculate how many chicks had successfully fledged.

3.2. RINGING

In the UK, birds are ringed by specially trained ringers under licence from the British Trust for Ornithology
(BTO) who, on behalf of the UK’s government, run the British and Irish Ringing Scheme and issue the rings
for the birds.

3.2.1 RINGING ADULT BIRDS

Adult Manx Shearwaters generally come ashore
after dark during the new moon period, therefore,
they are trapped at night-time for ringing as they
return to their burrows; head torches are used to
dazzle the birds so that they cannot see the ringer
approaching. They are then picked up by the ringer
and, using specialist pliers, a uniquely numbered
metal ring is fitted to the bird’s leg. Once the ring
has been fitted, the bird’s age, sex (if possible),
biometric data, date, time and location are all
noted. These data are entered onto Bardsey Bird
Observatory’s own database, and also onto the
BTO’s database and submitted as part of the
ringer’s licensing requirements.

3.2.2 RINGING YOUNG BIRDS

When the chicks are approximately one month old,

they are taken from the burrow during the daytime

and ringed using the same process as with the

adults. Some of the shorter burrows have chambers Fig. 1 George ringing a Manx Shearwater
which are around 30cm or so beneath the surface © Steven Stansfield
allowing easy access by the ringer. However,

burrows of more than arm’s length may be

accessed using a piece of bent fencing wire with a rubber coating on the end to prevent harm to the

bird. The chick is then removed by hooking the bird around its leg, and gently easing it towards the

surface. Many of the birds that nest on the island use chambers which are several metres underground

and are therefore, inaccessible. During late August and early September some of the more developed

young are captured at night when they come out of their burrows to exercise their wings. These birds are

distinguished from adults by several plumage characteristics (mainly the presence or absence of down

on the body). Birds with <20% body cover of down are aged as juveniles that are able to fledge (Euring

age code 3, i.e. hatched during the current calendar year). Birds with a body cover of down of >20%

are aged as pullus (Euring age code 1, i.e. a bird still in its nest). The birds aged as 3 are technically still

pulli but a figure of 20% down cover is used to differentiate between birds that are capable of flight and

ones that are not.

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3.2.3 RE-TRAPPING RINGED BIRDS

When a ringer captures a bird, a preliminary check is made to identify whether the bird is already ringed.
If an existing ring is found, the number is written down and double checked, the time, location, age and
sex are noted, and the bird is then released. These data are then entered onto the BTO’s database as
re-traps. The database is sufficiently powerful to be able to match up all the previous captures of any
individual bird, including its original ringing data, and can provide a complete capture history for each
bird. The physical state of the ring at the time of capture is also examined and, depending on the level
of wear, it may then be replaced; after 15 to 20 years rings can become thin and the numbers can
become illegible, so worn rings are carefully removed with circlip pliers. The numbers of both rings are
noted and later matched up on the database; the original ringing data is then applied to the new ring.
This method of re-ringing is necessary with long-lived birds such as shearwaters, especially since the ring
will be submerged in saltwater for the majority of its life.

Fig. 2 Adult Manx Shearwater exiting burrow © George Dunbar

3.3 GROWTH RATE OF CHICKS

In addition to monitoring the productivity, 36 of the productivity burrows were chosen for a survey looking
at the growth rate of chicks from hatching through to fledging. The selected nests were visited each
week, the young carefully removed from their burrows, and measurements of wing-length and weight
were taken.

172 REP. BARDSEY BIRD FLD OBS.

SHEARWATER PRODUCTIVITY, RINGING AND CHICK GROWTH

4 RESULTS
4.1 PRODUCTIVITY

This season 114 burrows containing an adult bird incubating an egg were monitored. These were marked
with a red-topped, individually numbered wooden post. Shearwaters from these burrows were followed
through to their conclusion. In total 27 failed at the egg stage, three failed at the chick stage and the
remaining 94 pairs successfully reared young, giving a productivity figure of 0.74 chicks per breeding pair;
a slight decrease of 3.90% on last year’s productivity (0.77), and 8.82% higher than the ten-year mean
(0.68 ±s.e.0.02). This is 0.59% lower than the mean since 1998 (0.74 ±s.e.0.02) when Bardsey Bird
Observatory began monitoring productivity. The highest productivity was 0.88 in 2003 and the lowest was
0.60 in 2018.

0.85

Productivity 0.75

0.65

0.55
1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020

Year Mean 0.74 ±s.e.0.02
Productivity

Fig.3 Manx Shearwater productivity 1998-2020

Table 1 Productivity 2011 – 2020

Year 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
Productivity 0.73 0.72 0.61 0.70 0.64 0.64 0.65 0.60 0.77 0.74
Sample size 122 148 132 101 119 125 127 125 122 114

Table 2 Area specific nest and productivity details

Area Incubating Eggs Fledged Eggs hatched Productivity
adults hatched/young (per pair) (chicks per
6
South End 12 6 16 0.50 pair)
Cristin 19 17 14 0.89 0.50
18 14 3 0.78 0.84
Pen Cristin 4 3 4 0.75 0.78
West Coast 5 3 0 0.60 0.75
Mountain Top 0 0 0 0.80
Mountain North 0 0 -!
Mnt North East - --
-

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North Stream 15 12 10 0.80 0.67
North West Fields 13 13 13 1.00 1.00
0 0 0- -
Barcut 9 5 4 0.56 0.44
Nant Valley East 5 4 4 0.80 0.80
Nant Valley West 13 10 10 0.77 0.77
1 0 0 0.00 0.00
Ty Pellaf 0 0 0- -
Treath Fynnon 114 87 84 0.76 0.74
Mountain South

Total

4.2 RINGING NEW BIRDS

During the new moon periods from mid-March through to September, while the shearwaters were ashore
after dark, a total of 2054 full-grown birds were handled by observatory staff. Of these, 1260 new birds
were fitted with a ring, the remaining birds being recorded as re-traps (see below). In addition to these,
214 young birds were ringed in the nest, giving a grand total of 1474 new birds ringed compared to 1005
last year.

4.3 RE-TRAPPING RINGED BIRDS

In addition to the 1474 new birds ringed, 794 subsequent encounters of birds already wearing rings were
made.

4.4 CHICK GROWTH RATES

Each week during the summer months, a visit was made to 31 of the most easily accessible productivity
shearwater nests to weigh and measure the chicks to monitor their growth rate. The first chicks were
measured on 1 July, one week later than last year; the final measurement was taken on 19 September,
although the first had fledged by 21 August, a week earlier than 2019. Several nests failed during the
season. The following results are based on the remaining 27 chicks from the original sample size of 31.
The maximum wing-chord to the nearest 1mm was taken, though this was not an easy measurement to
take when the chicks had first hatched, as the tip of the wing was fluffy. The birds were weighed using a
1 kg electronic balance. From the data collected in 2020 we chose to use measurement from ten chicks
for analysis as the remaining 21 were well over one week old when first encountered and these data
would have skewed the initial measurement means etc.

4.5 WING-LENGTHS

Most chicks had a wing-length of between 20 and 30mm, but some were as large as 29mm or as small
as 20mm when first measured, with the mean first measurement being 25.20mm±s.d.3.08. The chicks
continued to grow and fledged with an average wing-length of 232.70mm±s.d.10.54, 3.26%, shorter than
an average adult bird from Bardsey at 240.27mm±s.d.4.75 (n2359).

174 REP. BARDSEY BIRD FLD OBS.

Wing LengthSHEARWATER PRODUCTIVITY, RINGING AND CHICK GROWTH

250 30/06
02/07
200 04/07
06/07
150 08/07
10/07
100 12/07
14/07
50 16/07
18/07
0 20/07
22/07
Date24/07
26/07
Fig. 4 Growth rate of chicks – wing-length by date 28/07
30/07
4.6 WEIGHTS 01/08
03/08
All chicks had an initial weight of under 150g, with the mean initial weight being 75.10±s.d.26.51. The05/08
heaviest weight recorded of all the chicks measured was 781.4g (17.0% heavier than last year’s heaviest07/08
bird at 668g); this was a chick that was first checked on 1 July and, on that date, weighed 200g. It09/08
reached its peak weight on 7 August, slightly ahead of when the majority were at their heaviest; the final11/08
weight recorded before it fledged was 549.1g, though it will have shed more of this weight before leaving13/08
the burrow between 28 August and 5 September. Most of the chicks reached their peak weights in early 15/08
to mid-August; the mean peak weight was 625.08±s.d.78.75, 17.79% heavier when compared to last 17/08
year’s mean peak weight of 530.67g±s.d.119.18. The minimum peak weight of any of the chicks was 19/08
500.1g (compared to 461g last year) from a chick whose fledging weight was eventually 356.1g, 21/08
amounting to a loss in weight of at least 144g, 28.79% of its peak mass. All chicks this year were generally 23/08
of a slightly heavier weight than last year. 25/08
27/08
800 29/08
700 31/08
600 02/09
500 04/09
400 06/09
300 08/09
200 10/09
100 12/09
14/09
0 16/09
18/09
Date 20/09

WeightFig. 5 Growth rate of chicks – weight by date

30/06
02/07
04/07
06/07
08/07
10/07
12/07
14/07
16/07
18/07
20/07
22/07
24/07
26/07
28/07
30/07
01/08
03/08
05/08
07/08
09/08
11/08
13/08
15/08
17/08
19/08
21/08
23/08
25/08
27/08
29/08
31/08
02/09
04/09
06/09
08/09
10/09
12/09
14/09
16/09
18/09

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Fig. 6 Manx Shearwater from nest TP3 - photos taken weekly from mid-July to mid-
September © George Dunbar

176 REP. BARDSEY BIRD FLD OBS.

WeightSHEARWATER PRODUCTIVITY, RINGING AND CHICK GROWTH

Weight4.7 FLEDGING

Once the shearwaters reached a size where their wing-lengths were about 180-190mm (c75% of their
fledging wing-length) in mid-August, they began to slim down and lose weight prior to fledging. The
mean fledging wing-length was 232.70mm±s.d.10.54 (231.55mm±s.d.6.87 in 2019), with the smallest
fledgling having a wing-length of 216mm and the largest 247mm. The mean fledging weight was 469.40g
±s.d.50.45, 0.82% lighter than 2019’s 475.37g ±s.d.41.67, the lightest being 356.1g and the heaviest being
549.1g, 11g lighter than the heaviest of 2019, which was 560g. The average weight loss, from peak weight
to fledging weight, was 155.68±s.d.49.71, 99.30% more than 2019’s mean weight loss of 78.11g±s.d.45.11,
with the largest weight loss being 238.9 (188g in 2018) from 742g to 503.1g (32.19% of the chick’s mass).

800
700
600
500
400
300
200
100

0
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260

Wing Length

Fig. 7 Growth rate of chicks – weight against wing-length

560
540
520
500
480
460
440
420
400
380
360

216 218 220 222 224 226 228 230 232 234 236 238 240 242 244 246 248 250

Wing

Fig. 8 Fledging size of chicks – weight against wing-length

Adult Manx Shearwaters on Bardsey have an average wing-length of 240.27mm ±s.d.4.75 (n2359), with
a range of 223mm to 256mm. The mean fledging wing-length of the chicks in 2019 was 232.42mm, 3.27%
shorter than the average for adult birds.

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5 GROWTH RATES
5.1 SAMPLE SIZE

Since 2015, an additional study of the productivity burrows has been carried out each summer to monitor
the growth rate of Manx Shearwater chicks. The nests were visited once a week from when the chicks
first hatched until they had fledged. On these visits the maximum wing-chord and weight was measured.
In addition, the chick was ringed once it had reached a suitable age. These data were compiled and
only birds that fledged were included in the results. The sample size varied from year to year depending
on the productivity of the burrows chosen; 28 were monitored in 2015, 26 in 2016, 21 in 2017, 16 in 2018,
20 in 2019 and 31 in 2020.

5.2 WING-LENGTH

When the first measurements were taken, chicks generally had a wing-length between 20 and 29mm,
with an average of 25.20mm±s.d.3.08; thereafter wing-length grew significantly for the next five weeks,
with a mean growth of 23.52mm±s.e.0.47 per week. Growth per week was variable through the season,
with wing-lengths reaching a maximum mean growth of 32.93mm per week three weeks in; however,
some grew as much as 44mm in a single week. In the final weeks wing growth generally began to fall
before chicks reached their fledging size, reaching their lowest growth per week in the final two weeks.
Chicks fledged with a mean wing-length of 232.70mm ±s.d.10.54 and with a wing-chord ranging from
215mm to 247mm.

50

45

40

Growth (mm) 35

30

25

20

15

10

5

0
1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10

Week

Mean Max Min

Fig. 9 Average weekly growth of wing-length in 2020 (mm/week)

Table 3 Growth of wing-length (mm/week)

Week 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9
Mean 16.25 30.39 32.93 30.21 24.57 24.14 22.64 19.00
5.30 9.41 6.04 5.85 4.62 5.45 6.72 7.26
s.d.±
Max 25 44 42 39 33 33 38 33
Min 9 16 22 20 15 16 13 9

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5.3 WEIGHTS

On the first visit, the chicks had a mean weight of 75.10g±s.d.26.51, but ranged from as low as 33.1g up
to 107.6g (this was a large chick that was not found in the first two weeks, making it a large chick on its
first visit). In the initial weeks, both adults returned to shore most nights to feed their chick, which rapidly
gained weight, with an average increase of 127.67g in the first two weeks. Over the first four weeks the
chicks increased on average by approximately 108.08g per week. However, by the fifth week adults
returned to the nest less frequently and chicks grew by a mean weight of just 64.06g±s.d.89.28 per week.
Most chicks reached their peak weight seven weeks after hatching at a maximum mean weight of
625.08±s.d.78.75. One bird reached a peak weight of 750.2g whilst another significantly smaller bird
peaked at just 500.1g. Weight gain continued to fall throughout the last weeks of chick rearing until the
last two or three weeks, when birds began to slowly lose weight, having been abandoned by their
parents. One bird lost as much as 139.10g in a single week before fledging, which constituted 28.30% of
its fledging weight of 417g. Another bird lost just 99.2g and fledged at a weight of 466g, down from its
peak weight of 565.2g.

150
100

Growth (mm) 50

0

-50

-100
1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9
Week

Fig. 10 Average weekly weight difference (±g/week)

Table 4 Weight difference (±g/week)

Week 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9
Mean 127.67 131.84 80.50 92.30 64.06 17.08 -2.06 -63.69
20.44 37.95 35.94 38.08 89.28 95.41 85.31 57.67
s.d.± 163.20 175.90 149.90 136.80 191.40 200.10 134.40 23.10
Max 96.60 76.90 39.40 29.20 -90.10 -70.30 -124.60 -139.10
Min

5.4 FLEDGING

Having reached their peak weight, the shearwaters began to lose weight in the final three weeks before
fledging. Using the more closely monitored growth monitoring burrows, we can work out that the median
fledging time, with 14 of the 30 birds fledging, was after nine weeks; however, two fledged after eight
weeks, and two remained until 11 weeks, with the remaining ten fledging after ten weeks. The mean
fledging wing length was 232.70mm±s.d.10.54, but one individual fledged with a wing as short as 215mm,
12.60% below that of an average adult Manx Shearwater on Bardsey; another individual fledged with a
wing of 247mm.

The mean fledging weight was 469.40g±s.d.50.45, with the smallest fledgling leaving the nest with a
weight of just 356.1g and the largest with a weight of 530g. In the final weeks, before fledging, chicks lost

63: 168-181 179

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a substantial amount of weight as they converted their fat reserves into muscle and feathers, the mean
weight-loss being 155.68g±sd.49.71. One bird lost 234g, 201.20% more than the average of 77.69g, having
reached its peak weight of 684.2g in the fifth week. It dropped to 450.2g in week nine, having lost 34.20%
of its peak weight, before fledging shortly thereafter.

6 DISCUSSION

Productivity decreased slightly on last year, after 2018 experienced the worst productivity since
monitoring began in 1998. The overall productivity of Manx Shearwaters on Bardsey this year was only
slightly lower than the long-term mean, with a 0.59% decrease this year. Over the years the productivity
has ranged from 0.88 in 2003 to 0.60 in 2018. Whilst the productivity has fluctuated dramatically
throughout the years, the trend clearly shows a general decline. One of the reasons for the decline could
be the increasing population on Bardsey, with the 2014-2016 census producing 20,675 pairs, up from
15,859 only six years earlier and the current census appears to indicate a further 58.62% increase, with a
current estimate of over 35,000 pairs. This dramatic rise in population will result in more new and
inexperienced pairs breeding on the island, which could in turn lead to a lower productivity. The more
favourable weather experienced this year is likely to have played a part in the increase, but it will be the
monitoring in the coming years that will determine whether or not the population is growing.

Much like 2019, there was no clear pattern in terms of productivity in relation to habitat or location;
productivity simply varied across the island. Nant Valley East does show a low figure, but the sample size
was small. The weather this year was mostly very favourable, probably helping the productivity remain
in line with the average. The failures mostly occurred at the egg stage; once the chicks hatched, all but
three reached the fledgling stage. As in 2019, some down was found at the entrance of several of the
failed burrows, indicating that predation played a part in the lack of success for some pairs.

7 SUMMARY

In total 114 burrows containing birds incubating eggs were found and subsequently marked. Productivity
was calculated from these burrows and a total of 84 chicks fledged. This gave a productivity of 0.74
chicks per pair, an increase of 23.33% on 2018 (0.60) and 8.82% higher than the ten-year mean
(0.68±s.e.0.02); this is 4.67% lower than the mean since 1998 (0.74±s.e.0.02) when monitoring productivity
began.

Ringing and re-trapping birds was again successful, with 2268 birds handled, and of these 1260 new full-
grown birds were ringed. A further 794 birds were recaptured, including 24 that were re-ringed. 53
individuals over 20 years old were caught, six of which were over 30. One individual captured in 2020
was originally ringed in 1981, it was handled a few times between then and 1990, but had not been
handled since then. The bird was ringed as a juvenile, so its actual age is 39 years old. In addition to the
1260 full grown birds, a further 214 young birds were ringed in their nests, giving a grand total of 1474 birds
fitted with new rings during the year.

A sample of young was weighed and measured weekly during the nesting season, from hatching to
fledging. The mean fledging wing-length was 232.70mm±s.d.10.54, the smallest being 215mm and the
largest 247mm. The mean fledging weight was 469.40g±s.d.50.45, 0.42% lighter than 2019’s 476.40g, the
lightest being 356.1g and the heaviest being 530g. The mean weight-loss was 155.68g±sd.49.71, with a
maximum loss of 234g.

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SHEARWATER PRODUCTIVITY, RINGING AND CHICK GROWTH

8 ACKNOWLEDGEMENTS

Thanks are due to Natural Resources Wales for providing funding for the productivity monitoring to be
completed. Thanks also to Alex Starace, Sam Prettyman, Mark Carter and Josie Hewitt and others who
helped with monitoring the burrows and measuring and ringing of the chicks.

9 REFERENCES

Else, R.J. 2009. Census of the breeding population and productivity monitoring of Manx Shearwaters
Puffinus puffinus on Bardsey in 2008. Rep. Bardsey Bird Fld. Obs. 52: 92-96.

Else, R.J. 2011. Manx Shearwater population census and productivity monitoring. Rep. Bardsey Bird Fld.
Obs. 54: 111-113.

Leaper, G. 2002. Census of the breeding population of Manx Shearwaters Puffinus puffinus on Bardsey
2001. Rep. Bardsey Bird Fld. Obs. 45: 93-99.

Perfect, E. 2018. Manx Shearwater Studies: productivity, ringing and chick growth. Rep. Bardsey Bird Fld.
Obs. 61:190-201.

Stansfield, S.D. 1998 - 2016. Reports to Natural Resources Wales. Unpublished.
Stansfield, S.D. & Carter, M. 2017. Manx Shearwater Studies: Manx Shearwater 2014-2016 population

census. Rep. Bardsey Bird Fld Obs. 60:152-158.
Walsh, P.M. et al. 1995. Seabird Monitoring Handbook for Britain and Ireland. JNCC/RSPB/ITE/Seabird

Group, Peterborough.

.

Fig. 11 Manx Shearwater chick removed from burrow for measuring © Steven Stansfield

63: 168-181 181

HOOPER

MANX SHEARWATER
Population census 2019-2022 – year 2
Sam Prettyman

Manx Shearwaters REP. BARDSEY BIRD FLD OBS.

© Steven Stansfield
182

MANX SHEARWATER CENSUS

63: 168-181 183

PRETTYMAN

1 INTRODUCTION

The Manx Shearwater Puffinus puffinus is a pelagic seabird in the family Procellariidae, more commonly
known as the tube-noses (Brooke, M., 2013). It is the most northerly Atlantic breeding shearwater species,
and the UK holds more than 90% of the world’s population (Perrins et al. 2012). Bardsey is the fourth largest
Manx Shearwater colony in the world and the third largest colony in Wales, with only Skomer and
Skokholm holding larger populations (Perrins et al., 2012). The national and international importance of
Bardsey’s Manx Shearwater population is the reason for its listing as one of the avian ‘features’ of the
Bardsey Site of Special Scientific Interest (SSSI) and National Nature Reserve (NNR).

Owing to the island’s importance for Manx Shearwaters, it is crucial that the population is monitored on
a regular basis. This is done by way of a population census which is carried out every six years. Over the
last two decades the Manx Shearwater population on Bardsey has undergone a significant increase,
from less than 7,000 apparently occupied burrows (AOBs) in 1996 to over 21,000 in 2016 (Stansfield &
Carter, 2017). The last census took place over the 2014-2016 breeding seasons (Stansfield & Carter, 2017),
however, the recent population increase means that the current census will take four seasons to
complete, from 2019-2022.

2 AIMS

To comply with Natural Resources Wales (NRW) requirements and the regulations for monitoring features
of the SSSI and NNR, the Manx Shearwater population on Bardsey Island is censused on a regular basis
using a consistent method. Doing so enables comparable, long-term monitoring of the population and
detection of any population changes.

3 METHODS

In previous censuses, the island has been divided Fig. 1. 2020 Manx Shearwater census
into three sections, with one section surveyed per area (unshaded).
year. However, the expected population increase
for the current census means it is now necessary to
further sub-divide the East Side section - 75% of the
island’s population - in two. The south-western
section (grey shaded area of Fig. 1) was surveyed in
2019, the west side of the Mountain and North End
was surveyed in 2020 (unshaded), with the two
halves of the East Side due to be surveyed in 2021
and 2022 (shaded pale blue). The 2020 survey of the
North End and west side of the Mountain is an ‘L’
shaped area, grid lines SH120-SH123 east were used
to divide the Mountain, and grid line SH220 north
gives the area for the North End.

The methods used in the current census follow those
used in previous years which are outlined in the
Seabird Monitoring Handbook (Walsh et al. 1995).
However, following the low response rates obtained
on Bardsey in both the 2008 and 2014 surveys, R.
Brown and C. M. Perrins (pers. comm.) advised that
using dual-sex playback would improve response
rates.

Results from Perkins et al. (2017) show that dual-sex
calls give higher response rates with lower variance
than male-only calls, since females frequently reply
to female calls but rarely to male calls. In simulated
3–5-day calibration trials by Perkins et al. (2017),
response rates and correction factors were up to 50% more precise with dual-sex calls. This suggests that
females are more likely to respond to female rather than male calls, especially if they already have a
partner and are incubating an egg. As shown by Perkins et al. (2017) and Perrins et al. (2012), male

184 REP. BARDSEY BIRD FLD OBS.

MANX SHEARWATER CENSUS

response rates remain the same since they respond to the calls of other males in defence of the burrow
and their partner. The findings from these studies show that dual-sex calls generate increased response
rates from birds in burrows, so this minor change to the method was adopted for the current census.

R. Brown and C. M. Perrins (pers. comm.) also advised that the low response rates in 2014 and 2018 could
have been compounded by the poor sound quality of the playback equipment used. To combat this
issue, an EasyAcc Mini Bluetooth Speaker – the same as is used on Skokholm (R. Brown pers. comm.) -
was purchased, and the MP3 dual-sex playback calls used on Bardsey from 2019 onwards are those used
by Perkins et al. (2017).

The 2020 census was conducted between 16 May and 20 June. This five-week period was chosen to
coincide with the peak incubation period as per Perkins et al. (2017). All Manx Shearwater burrows within
the census area were surveyed, and three separate counts were made simultaneously. All surveys were
conducted during the day, as fewer non-breeding or prospecting birds will be present in the burrows
(Walsh et al., 1995).

3.1 COUNT 1: THE TOTAL NUMBER OF BURROWS

Every burrow encountered was counted. Each burrow was checked to make sure the burrow was
complete (at least 30cm deep), and not just the start of a burrow, or a complex of burrows with multiple
entrances - see Fig. 2.

3.2 COUNT 2: APPARENTLY-OCCUPIED BURROWS (AOB)

Rabbits are no longer found on Bardsey and there are relatively few Puffins breeding on the island (c.150
pairs; Dykes & Stansfield, 2019), so any suitably sized burrow is most likely that of a Manx Shearwater. All
burrows are examined to determine whether or not they are ‘apparently occupied’; the majority of
burrows show clear signs of use (smooth, compacted soil entrance, parallel grooves made by the birds'
feet, fresh digging, droppings). However, not all burrows are this obvious, so this method can be rather
subjective. This is particularly true for burrows whose entrances lie between stones in walls or are almost
completely hidden by grass on the tops of walls and mounds; these burrows show little or no external
signs of use.

Fig. 2 Apparently Occupied Burrows on Pen Cristin © Josie Hewitt

63: 182-191 185

PRETTYMAN

For burrows with little or no external signs of use, call playback helped to determine whether they were
occupied or not – any burrow where playback elicited a response, and which had a long tunnel of
suitable diameter was counted as an AOB. Burrows with dense vegetation growing in the entrance and
tunnel, or old eroded burrows were not counted. To complicate matters, some burrows have multiple
entrances, while in other cases entrances with signs of occupation can sometimes lead only to shallow
excavations just a few inches long or a large entrance might lead to two or more nesting chambers.

Burrow entrances were thoroughly checked for these features and if entrances were linked, only one
burrow was counted. Similarly, a response from two or more entrances was only counted once if it was
clearly from the same bird. In certain habitats - boulders and areas of dense European Gorse Ulex
europaeus - it was not possible to carry out a complete count of AOBs. Some burrow entrances are
distinguishable under boulders (soil worn smooth, droppings etc.) and under gorse bushes (tunnels in
vegetation under bushes, droppings), and these were counted, but many more may have been hidden.
In these areas a slight alteration to the survey methods was made c.f. 3.3 below.

3.3 COUNT 3: RESPONSE TO THE VOICE PLAYBACK

For the current census, a dual-sex call (male and female birds calling simultaneously) was used for the
voice playback survey. The recording was played into every burrow at a natural volume for 25 seconds
and was followed by a 25 second period of listening for any responses. If a bird called back during the
playback or listening time, then a response was noted. If a response was noted from two or more
neighbouring entrances but was clearly from the same bird, then it was counted only once.

Although some of the habitat in the 2020 census was clear stone walls, banks and grazed fields, most
areas contained dense vegetation including European Gorse, tall grass and bracken, which made it
difficult to count burrows and AOBs accurately. Some burrow entrances are still distinguishable in these
habitats, e.g. under boulders (soil worn smooth, droppings etc.) and under gorse bushes (tunnels in
vegetation under bushes, droppings). Those that could be distinguished were counted, but some are
likely to have been missed in areas of very dense vegetation. Due to the constraints posed by dense
vegetation, an adapted methodology was used in these areas; the dual-sex recording was played at
maximum volume into and around the tunnels or any areas showing signs of activity (droppings or
scrapings) and also at large areas of gorse for a period of 30 seconds, with a 60 second listening period.
Any responses were counted as an AOB.

3.4 SURVEY EFFORT

Previously, the island has been divided into three sections and surveyed over a three-year period.
However, due to the population increase over recent decades, the island has now been split into four
areas and will be surveyed over a four-year period (2019-2022). The 2020 census area was sub-divided
by grid lines; burrows were counted using click-tally counters and marked on maps in the field. These
were then transcribed onto a complete map of the whole census area at the end of the census period.

The 2020 census was predominantly completed by a single member of Observatory staff with occasional
assistance. Due to COVID-19 restrictions it was not possible to have volunteers assisting with the census
in 2020. Census fieldwork was always carried out during the day in favourable weather conditions and,
whilst the weather was conducive for most of the census period, strong winds and rain did prevent
fieldwork on a handful of days. In total almost 100 hours was spent undertaking the field work in 2020.

4 RESULTS

A total of 8532 burrows was counted within the 2020 census area, of which 7969 (93.40%) were
considered to be apparently occupied. Responses to voice playback were recorded from 2496 (31.32%)
of apparently-occupied burrows (AOBs).

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Table 1: Results of the Bardsey 2020 Manx Shearwater population census.

Area total Number of Apparently % of burrows Playback % of responses
burrows occupied occupied responses from AOBs
(from AOBs)
8532 burrows 93.40 31.32
7969 2496

Table 2: Comparison of the 2020, 2016 and 2010 Manx Shearwater surveys on Bardsey.

Whole area 2020 2016 2010 % change
total 8532 5379 4729
7969 4937 4071 2016 – 2020 +58.62%
Number of 2496 933 1387 2010 – 2020 +80.42%
burrows 2016 – 2020 +61.41%
AOBs 2010 – 2020 +95.75%
2016 – 2020 +167.42%
Playback 2010 – 2020 +79.97%
responses

5.1 DISCUSSION

5.1.1 POPULATION INCREASE

The results of the 2020 census showed a marked increase in the overall number of burrows, with the
combined number (AOBs and unoccupied) rising from 4729 in 2010 to 5379 in 2016 (+13.75%) and then
to 8532 in 2020 (Table 1). This is an increase of 58.62% from the previous census in 2016 and +80.42% over
the last decade (Table 2).

Fig. 3 Freshly excavated burrow © Steven Stansfield

The number of AOBs also showed a similar percentage increase to that of the overall number of burrows,
rising from 4071 in 2010 to 4937 in 2016 (21.27%), followed by a further 61.41% increase to 7969 AOBs in
2020. The number of AOBs has almost doubled between the 2010 and 2020 surveys, increasing by a
staggering 3898 (+95.75%).

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If we combine the 2019 and 2020 results, 12,082 burrows have so far been counted for the current census,
46.11% above the total for the same areas in the last census. Likewise, the total number of AOBs
increased by 48.22%, from 7453 in the same area in the 2014-2016 census to 11,047 in the current census.

Assuming that the East Side populations have remained unchanged since the last census, Bardsey now
has a minimum of 26,083 total burrows and 24,269 AOBs. However, if we presume that the rate of increase
on the East Side of the Mountain is the same as the rest of the island, and apply a 46% increase in total
burrows and a 48% increase in AOBs, the total number of burrows is projected to be 38,081 and 35,918
AOBs.

These results show that the total number of burrows and those that are apparently-occupied are
increasing at a similar rate, perhaps suggesting that prospecting and excavating birds are returning to
take up residency and breed. It also suggests that the carrying capacity for breeding Manx Shearwaters
on Bardsey has not yet been reached.

5.1.2 OCCUPANCY RATES

Burrow occupancy rate is broadly similar across the three survey periods (2010, 2016 and 2020); however,
there has been a slight increase in the occupancy rate of the 2020 census areas. In the 2020 survey,
93.40% of burrows were apparently occupied, compared to 91.78% in 2016 and 86.09% in 2010. This
increase indicates that more breeding birds are occupying burrows, in contrast to the trend seen in the
South End census area (Hooper et al., 2019).

5.1.3 RESPONSE RATES

The number of birds that responded to voice playback in both 2019 and 2020 was much greater than in
previous years. In the 2019 survey of the South End of the island, birds responded in 46.23% of AOBs,
compared to 33.20% in 2008 and 13.99% in 2014.

Across the 2020 survey area, 2496 (31.32%) of the 7969 AOBs contained responsive birds; a large increase
compared to the 2016 response rate of 18.90%, but similar to the response rate recorded in 2010 where
birds responded from 34.07% of the AOBs. Due to time constraints, the gender of each bird responding
was not noted, so the true effect of female responses in this study is unknown.

There is a substantial difference in the response rate of birds in the 2020 survey areas compared to those
surveyed in the southern part of the island in 2019. The reasons for this are unclear, however it is possible
that the earlier breeding season in 2020 meant that chicks had begun hatching by the end of the survey
and so fewer adult birds were still occupying their burrows during the playback surveys. Alternatively, the
contrast in response rates could be linked to the different patterns of occupancy observed between the
2019 and 2020 survey areas.

5.1.4 CORRECTION FACTOR
5.1.4.1 METHOD

In 2020, a sample of 39 burrows known to be occupied were used to calculate a correction factor which
could then be applied to the voice playback results in order to estimate the number of occupied burrows
in an area, taking into account that only males tend to respond.

Five visits were made to each of the 39 occupied burrows between 3 and 16 June and the standard
voice playback as described in 3.3 were used and the responses noted. A physical check of the nest
using either a hand or endoscope was also carried out to see if the bird was incubating an egg.

The correction factor is calculated by dividing the number of AOBs by the number of burrows from which
a bird responded to the playback. A correction factor is calculated for each of the five visits and the
average of these is then applied as the correction factor for the survey area.

Unfortunately, all of the burrows used in this small study were occupied by incubating adults, with none
occupied by non-breeding birds. This is likely to influence the results, so a greater sample of burrows will
be used in 2021.

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5.1.4.2 RESULTS

The response rates from AOBs varied from 35.9% to 61.54%, with an average of 43.82%. The correction
factor across each of the five burrow visits varied from 1.63 to 2.79, with an average of 2.39. This average
correction factor figure can now be applied to burrow counts from the 2019 and 2020 survey areas in
order to provide a more robust estimation of the number of occupied burrows, compared to just relying
on the number of AOBs (Perkins et al., 2017).

Table 3: 2019 and 2020 Manx Shearwater surveys on Bardsey. including x2.39 correction
factor applied .

Number of AOBs % of Playback % of Responses x
burrows burrows responses responses correction
3070 occupied from AOBs factor (2.39)
2019 3550 7969 (from
2020 8532 11,039 86.7 AOBs) 46.23 3401
2019+2020 12,082 93.40 1432 31.32 7041
90.05 2496 38.78 10,442
3,928

5.1.5 IMPROVED AUDIO QUALITY AND DUAL-SEX CALLS

It is well documented that male Manx Shearwaters have a response rate of 93-98% to calls of other males
during the breeding season and that females respond less than 1% of the time to the call of a male
(Perkins et al., 2017). As mentioned previously, dual-sex calls were used in the 2019 and 2020 voice
playback surveys on Bardsey as a direct result of a study by Perkins et al. (2017) which found that females
respond at higher rates to calls of another female compared to male-only calls. The response rate of
AOBs decreased significantly from 46.23% in 2019, to 31.32% in 2020, despite the same equipment being
used. It’s thought that this could be due to an earlier breeding season in 2020, meaning that towards the
end of the survey chicks had already hatched and parents were no longer in burrows.

The same voice-playback methods were employed in the 2008-2010 and 2014-2016 surveys whereby
male-only calls were used. The only difference was the type of portable speaker used: two small
Dictaphones with built in speakers were used in 2008-2010, while two iLuv speakers and iPods were used
for the 2014-2016 surveys. A significant deterioration in the sound quality of the iLuv speakers over the
census period is believed to be the main reason for such poor response rates from birds in 2014-2016.

During 2015 and 2016 a test was carried out using the iLuv speakers and iPods to try and elicit responses
from incubating birds in 30 study burrows. Over a 14-day period, each burrow was surveyed four times
following the same voice playback methodology as used in the 2008 and 2014 censuses (25 seconds of
playback using male-only calls, followed by a 25 second listening period). Given that the process
occurred over the 14-day period, both males and females should have been incubating eggs in burrows
at different times and so a response rate of ~50% was to be expected. Consequently, it was surprising to
find that the response rates varied from c15-25% (pers obs./Stansfield and Leatherbee - unpublished
data). It was concluded that the combination of poor-quality audio playback equipment and the use
of male-only calls was the reason for this low response rate.

As previously stated, the 2020 results saw a marked increase in responses compared with 2016,
presumably due to the methodology (dual-sex playback) along with the equipment used. The new
EasyAcc Mini Bluetooth Speaker and digital MP3 dual-sex recording of Manx Shearwaters produced a
much better sound quality at a greater volume; the improvement in sound quality alone could be
expected to improve the likelihood of a bird calling back to it. Studies conducted on Skomer (Perrins et
al. 2012) suggest that the call back rates obtained in 1998 and 2011, using very different audio tape
qualities, produced similar response rates; however, we do not know how poor the 1998 equipment was
compared to the Bardsey equipment of 2008 and 2014. Booker et al. (2019) suggested that volume could
also be a contributing factor for poor response rates as they had issues with this among their surveyors
on Lundy.

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5.1.6 OTHER OBSERVATIONS AND CONSIDERATIONS

The results of the 2019 and 2020 surveys suggest that Bardsey’s Manx Shearwater population is still rapidly
expanding and that the island’s carrying capacity for nesting Manx Shearwaters has not yet been
reached. It is important to consider that this is only the second year of what will be a four-year survey, so
these results do not necessarily represent the population dynamics of nesting Manx Shearwaters across
the remaining parts of the island. Despite this, the number of new excavations observed across the
remaining survey areas does suggest that similar increases will be evident there too - see Fig. 3.

The productivity rate for Manx Shearwaters was 77% in 2020, meaning that as many as 27,656 chicks
could have hatched. If we add this to the 71,836 breeding adults, then there may have been as many
as 99,492 Manx Shearwaters on the island during the summer months (excluding non-breeders).

6 SUMMARY

The results from the 2020 surveys suggest that the population of Manx Shearwaters on Bardsey has rapidly
increased in this section of the island. A total of 8532 burrows (occupied and unoccupied) were counted
across the 2020 study area. Of these, 7969 (93.40%) were deemed to be apparently occupied, and voice
playback elicited responses from 2496 (31.32%) of these. Furthermore, the total number of burrows and
burrow occupancy rate are increasing similarly (approx. +60% since 2016), while the number of AOBs has
almost doubled between the 2010 and 2020 surveys, from 4071 to 7969 (+95.75%).

Combining the 2019 and 2020 census results, the overall number of burrows counted on the western side
of the island is 12,082, 46.11% above the total for the same areas in the previous census. Likewise, the
total number of AOBs has increased by 48.22%, from 7453 in the 2014-2016 census to 11,047 in the current
census.

If we were to assume that the East Side populations remain unchanged since the last census, Bardsey
now has a minimum of 26,083 total burrows and 24,269 AOBs. However, if we assume that the rate of
increase on the East Side of the Mountain is the same as the rest of the island and apply a 46% increase
in total burrows and a 48% increase in AOBs, the total number of burrows is projected to be 38,081 and
35,918 AOBs.

The island’s population is now estimated between 25,000 and 36,000 AOBs.

7 ACKNOWLEDGEMENTS

Thanks are due to George Dunbar for assistance with the census and for undertaking the correction
factor fieldwork and Emma Stansfield and Josie Hewitt for commenting on an early draft of this paper.

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CHOUGH

A summary of the 2020 breeding season
Samuel Prettyman

Chough REP. BARDSEY BIRD FLD OBS.

© Richard Layton

192