BREEDING BIRDS
C U C K O O Cuculus canorus Occasionally breeds as a brood parasite
Adults were regularly recorded at Nant and Cristin during May and June; however, no fledglings were
recorded on the island this year.
W O O D P I G E O N Columba palumbus Scarce, population stable
In total of nine territories were located this year, one fewer than last year and 18.92% below the ten-year
mean (11.10±s.d.2.08), yet still 38.46% greater than the long-term average from 1953-2021 (6.50 ±s.d.4.64).
At least eight 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 6.50 ±s.d.4.64
2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
№ of pairs 9 9 10 11 13 15 12 13 10 9
M O O R H E N Gallinula chloropus Scarce/uncommon, population fluctuating
Eight pairs were recorded this year, two more than the previous year, but 5.88% lower than the ten-year
mean (8.50±s.d.3.47) though 6.14% higher than the long-term 1953-2021 mean (7.54±s.d.4.25). A total of
eight broods containing 21 young was recorded, the productivity being 2.62, 50.32% higher than the ten-
year mean (1.74±s.e.0.28).
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 7.54 ±s.d.4.25
№ of pairs 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
№ of young 4 4 7 12 12 12 13 7 6 8
Productivity
5 8 21 9 17 13 15 7 19 21
1.25 2.00 3.00 0.75 1.42 1.08 1.15 1.00 3.16 2.62
O Y S T E R C A T C H E R Haematopus ostralegus Fairly common, population stable
In total 54 pairs nested this year: the majority were on the rocky coastline, with 13 pairs in the fields and
amongst the gorse. This total is 34.47% lower than the ten-year mean (82.40±s.d.13.19), and 16.35% lower
than the long-term average from 1953-2021 (64.55 ±s.d.19.76). Of the 30 nests that were found, 82 eggs
were counted, giving an average clutch size of 2.73. This is 7.95% above the ten-year mean
65: 138-167 143
COSSEY & STANSFIELD
(2.53±s.d.0.20). A minimum of 14 fledged young was counted in July, giving a productivity of just 0.26
chicks per pair.
Number of pairs 120
100
1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
80 Oystercatcher Year
60 Mean 64.55 ±s.d.19.76
40
20
0
1953
№ of pairs 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
Mean clutch 85 87 101 98 82 84 84 72 77 54
2.75 2.69 2.63 2.36 2.14 2.46 2.65 2.38 2.50 2.73
Productivity 1.15 0.78 0.38 0.07 0.46 0.31 0.39 0.40 - 0.26
R I N G E D P L O V E R Charadrius hiaticula Scarce
A pair was recorded breeding on the Narrows in June. There were four eggs with three young hatching
and two finally fledging giving a productivity of 2.00. This is the first time that this species has bred since
2014.
K I T T I W A K E Rissa tridactyla Fairly common, population declining
This year saw 181 AONs, 38 (26.57%) more than last year and 57.53% greater the ten-year mean
(114.90±s.d.34.16), only three less than the decade’s highest total in 2011. The number of AONs this year
was 23.35% above the long-term 1953-2021 average (146.73 ±s.d.102.56). From the 181 nests, 268 large
juveniles were counted and believed to have fledged, giving a productivity of 1.48, 115.51% higher than
the ten-year mean (0.69±s.e.0.09).
Kittiwakes © Steven Stansfield REP. BARDSEY BIRD FLD OBS.
144
BREEDING BIRDS
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 146.73 ±s.d.102.56
2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
№ of pairs 76 62 104 125 122 125 90 121 143 181
№ of young 23* 27* 20* 52* 27* 9* 36* 101 119 268
Productivity 0.52 0.71 0.21 0.84 0.41 0.11 0.92 0.84 0.83 1.48
*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, 53.49% below the ten-year mean (4.30±s.d.2.06) and 8.22% below
the long-term 1953-2021 average (2.18 ±s.d.1.81). 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 Mean 2.18 ±s.d.1.81
№ of pairs 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
Productivity 6 8 5 6 4 5 3 2 2 2
0.50 0.63 0.20 1.83 1.00 0.20 0.67 1.00 1.00 1.00
There were 51 AONs on Ynysoedd Gwylan in total: 42 AONs with 83 adults and 28 juveniles on Ynys
Gwylan Fawr and a further nine visible AONs on Ynys Gwylan Bach (these were viewed from Ynys Gwylan
Fawr and from the boat). This was one higher than the 2019 total but 16.78% lower than the ten-year
mean (61.29±s.d.20.10). The productivity figure calculated from numbers on Ynys Gwylan Fawr, of
0.67±s.e.0.03 was 4.5% below the ten-year average (0.70±s.e.0.04).
№ of pairs 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
Productivity - - 79 92 72 35 50 50 - 51
- - 0.75 0.64 0.79 0.68 0.54 0.82 - 0.67
H E R R I N G G U L L Larus argentatus Common, population recovering
A total of 337 pairs was counted, 86 (20.33%) fewer than the 2020 total and 10.06% below the ten-year
mean (374.70±s.d.37.50). This figure was 2.06% greater than the long-term 1953-2021 average
65: 138-167 145
COSSEY & STANSFIELD
(330.21±s.d.142.86). A sample of 248 AONs at the northern end of the island where 119 young fledged,
gave a productivity figure of 0.48, 23.93% lower than the ten-year mean (0.63±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 330.21 ±s.d.142.86
200
100
0
1953
2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
№ of pairs 395 323 399 400 331 377 417 345 423 337
Productivity 0.46 0.64 0.69 0.54 0.84 0.64 0.77 0.71 0.54* 0.48*
* Productivity calculated from study plots.
On 13 June, 57 AONs were counted on Ynys Gwylan Fawr, 41.5% below the ten-year mean
(97.43±s.d.45.03). A minimum of 54 young was counted, giving a productivity figure of 0.95, 11.33% above
the 2012 to 2021 mean (0.85±s.e.0.06).
№ of pairs 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
Productivity - - 61 52 113 145 165 89 - 57
- - 1.15 0.54 0.88 0.84 0.76 - 0.95
-
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 102 AON, 67 (39.64%) lower than the 2020 total and 44.41% lower than the ten-year mean
(183.50±s.d.53.85). This figure is 45.57% lower than the long-term 1953-2021 mean (187.39±s.d.190.79). This
is the lowest number recorded since 1985 when just 100 pairs were noted and 550 fewer than the peak
of 652 in 2003. A colony of 69 nests at the North End saw 37 juveniles fledge, giving a productivity figure
of 0.54 chicks per pair, 8.00% greater than the previous year and 27.89% higher than the ten-year mean
(0.42±s.e.0.05).
Lesser Black-backed Gulls © Steven Stansfield REP. BARDSEY BIRD FLD OBS.
146
BREEDING BIRDS
2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
315 212 177 164 177 164 169 102
№ of pairs 188 167 0.44 0.59 0.04 0.38 0.56 0.38 0.50* 0.54*
Productivity 0.37 -
* Productivity calculated from study plots
Number of pairs 700
600
500 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
400 Year Mean 187.39 ±s.d.190.79
300
200 Lesser Black-backed Gull
100
0
1953
G U I L L E M O T Uria aalge Common, becoming abundant, Bardsey population increasing
A total of 1432 AOLs were recorded this year, 200 (16.23%) more than in 2020, 11.92% greater than the
ten-year-mean (1279.50±s.d.159.45) and 203.48% greater than the 1953-2021 mean (471.86 ±s.d.442.91).
27 chicks fledged from a sample plot of 50 AOLs giving a productivity of 0.54, 22.73% greater than 2020
and 51.26% higher than the ten-year mean (0.36±s.e.0.04).
2000
Number of pairs 1500
1000
500
0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953 Guillemot AaoOlL Year
Mean 471.86 ±s.d.442.91
2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
1141
1240 1346 1574 1112 1413 1232 1432
№ of AOL 1234 1071 18* 50* 70* 94* 63* 22* 27*
51* 0.38 0.26 0.40 0.22 0.57 0.44 0.54
№ of young 18* 22* 0.26
Productivity 0.22 0.28
* Productivity calculated from study plots.
There was a minimum of 82 AOLs counted on Ynys Gwylan Bach, with a further 16 chicks and five eggs
counted on Ynys Gwylan Fawr. The combined total of 103 AOLs is 68.85% above the ten-year average
(61.00±s.d.24.41).
№ of AOL 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
№ of young - - 32 44 51 49 67 81 - 103
Productivity - - 1 - - 23 - - - 16
- - 0.03 - - 0.47 - - - 0.16
R A Z O R B I L L Alca torda Common to abundant, population recovering
A count of 1220 birds from the whole of the East Side was made on 15 June. With the x0.67 (1 adult = 0.67
pair) conversion factor applied (Walsh et al., 1995) there were 817 pairs (625 pairs or 33.88% fewer than
2020). The population was 35.33% lower than the ten-year mean 1263.89±s.d.235.25 and 12.65% lower
65: 138-167 147
COSSEY & STANSFIELD
than the long-term 1953-2021 average (1396.63±s.d.747.40). A minimum of 195 young and eggs was
counted. This is a 75.68% increase compared to last year, though 26.88% lower than the ten-year mean
(266.70±s.d.66.77).
Pairs 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
Eggs/young 1058 1146 1479 1587 1542 1167 1321 1284 1236 817
278 287 300 317 321 246 277 311 111 195
On Ynys Gwylan Fawr 12 AOLs were counted with none seen on Ynys Gwylan Bach. Only one chick was
recorded, 94.55% lower than the 2012 to 2021 average (18.33±s.d.14.04).
2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
- 18 - 24 42 16 9 - 1
Eggs/young -
P U F F I N Fratercula arctica Uncommon, increasing on Bardsey, stable on Gwylan
A total of 404 adults (202 Pairs) was counted on the East Side. This total is 38 pairs (23.17%) greater than
2020 and 103.83% above the ten-year mean (99.10±s.d.60.31) continuing the upward trend for this
species on Bardsey since it first began to colonise in 2000.
250
Number of pairs 200
150
100
50
0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953 Puffin
Year
Mean 17.06 ±s.d.41.93
Estimated no pairs 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
16 29 55 75 75 100 132 143 164 202
A monitoring trip to Ynys Gwylan Fawr took place on 13 June. 349 AOBs were counted with a flush count
of 578 individuals. This is 48.43% lower than the ten-year average (676.80±s.d.203.47).
2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
- 682 663 819 619* - 349
AOB - 839* 871
* 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 places around Briw Gerrig, Seal Cave and Bae Felen in June and
July. An adult and a chick were seen in the nest box in August.
F U L M A R Fulmarus glacialis Uncommon, population fluctuating
The East Side cliffs produced 21 AOS (Apparently-Occupied Sites), five more than the 2020 total, 17.32%
above the ten-year mean (17.90±s.d.5.61) and 6.13% higher than the long-term average from 1953-2021
(19.79±s.d.15.23). Counts made on 27 July and 10 August produced a total of 12 near-fledging young,
giving a productivity figure of 0.57 chicks per pair.
148 REP. BARDSEY BIRD FLD OBS.
BREEDING BIRDS
Puffins © Steven Stansfield 149
65: 138-167
COSSEY & STANSFIELD
Number of pairs 60
50
40 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
30 Fulmar Year
20 Mean 19.79 ±s.d.15.23
10
0
1953
№ of AOS 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
Productivity 21 11 17 31 15 19 14 14 16 21
- - - - - - - - 0.25 0.57
M A N X S H E A R W A T E R Puffinus puffinus Abundant, population stable
In total 124 burrows containing birds incubating eggs were monitored to completion. Productivity was
calculated and a total of 80 chicks fledged. This gave a productivity of 0.65 chicks per pair, a decrease
of 12.16% on last year’s productivity (0.74), and 3.27% lower than the ten-year mean (0.67 ±s.e.0.02). This
is 10. 69% lower than the mean since 1998 (0.73 ±s.e.0.02) when Bardsey Bird Observatory began
monitoring productivity.
Manx Shearwater © Steven Stansfield
Productivity 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
Sample size 0.72 0.61 0.70 0.64 0.64 0.65 0.60 0.77 0.74 0.65
148 132 101 119 125 127 125 122 114 119
A full report on the breeding season for Manx Shearwaters can be found on page 168.
150 REP. BARDSEY BIRD FLD OBS.
BREEDING BIRDS
C O R M O R A N T Phalacrocorax carbo Uncommon on Ynysoedd Gwylan
Seven AONs were counted on Ynys Gwylan Fawr, the first time they have bred on the larger of the two
islands since 2013. A minimum of three AONs on Ynys Gwylan Bach were visible from Ynys Gwylan Fawr.
From the seven AONs on Ynys Gwylan Fawr there were 16 young, giving a productivity of 2.29. This is
51.8% higher than the ten-year mean (1.51±s.e.0.18).
Year 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
Y G Fawr pairs - 31 0 0 0 0 0 0 - 7
Y G Bach pairs - 5 9 9 8 12 14 20 - 3
- 36 9 9 8 12 14 20 - 10
Total pairs - - -
Productivity 1.56 1.44 2.13 1.25 0.64 1.25 2.29
S H A G Gulosus aristotelis Uncommon, population increasing again
Shag nests were counted from both land and sea during several visits to the East Side, 59 nests were
counted, 20 (51.28%) more than in 2020 and the third highest count since 1953. The total is 43.9% higher
than the ten-year-mean (41.00±s.d.8.04) and 87.79% above the long-term 1953-2021 average
(31.42±s.d.11.51). A total of 133 young fledged, giving a productivity of 2.25, which is 24.24% above the
2012-2021 mean (1.81±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 31.42 ±s.d.11.51
№ of pairs 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
Productivity 34 31 34 39 39 45 45 45 39 59
2.59 2.00 1.65 1.28 1.71 1.82 2.09 1.62 1.10 2.25
A total of 39 AONs were found on Ynys Gwylan Fawr with a further six seen across on Ynys Gwylan Bach.
The combined total of 45 AONs is 1.64% lower than the ten-year mean (45.75±s.d.11.67). A total of 71
young was counted on Ynys Gwylan Fawr giving a productivity of 1.82, 11.07% higher than the 2012-2021
mean (1.64±s.e.0.11).
№ of pairs 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
Productivity - 24 46 48 41 44 66 52 - 45
- - 1.98 1.59 1.66 1.15 1.23 2.04 - 1.82
L I T T L E O W L Athene noctua Scarce, population declining
Three territories were recorded with a pair seen on Pen Cristin and calling males at Carreg Bach and
Nant Valley. This is two fewer territories than in 2020, and 18.92% lower than the ten-year mean
(3.70±s.d.1.57). No juveniles were recorded but it is suspected that a small number did fledge.
65: 138-167 151
COSSEY & STANSFIELD
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 4.37 ±s.d.1.67
2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
№ of pairs 3 2 3 5 7 6 3 2 5 3
*NB an error in the number of breeding pairs in 2012 has been corrected in the data this year. Previous reports may refer to one
pair rather than three.
P E R E G R I N E Falco peregrinus Scarce (Schedule 1), population stable
One nest was located in May, with a pair regularly sitting by one of the usual nest sites. Due to the location
of the nest the contents were not recorded. An adult and two juveniles were seen flying around the East
Side near to the nest site on 29 June; this gives a productivity of 2.00.
№ of pairs 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
Productivity 2 2 2 2 2 1 1 1 1 1
0 0
2.00 2.00 2.00 1.00 1.00 2.00 2.00 2.00
Juvenile Peregrine and Herring Gull © Steven Stansfield
M A G P I E Pica pica Scarce, population fluctuates, but fairly stable
Eight pairs nested again this year, predominantly in gorse. A total of 14 juveniles was counted, giving a
productivity figure of 1.75, 2.13% below the ten-year mean (1.79±s.e.0.25). This is the fifth consecutive
152 REP. BARDSEY BIRD FLD OBS.
BREEDING BIRDS
year that the population has fallen and is 27.27% lower than the ten-year mean (11.00±s.d.2.45), but it is
still 8.72% greater than the long-term average 1953-2021 (7.36 ±s.d.3.28).
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 7.36 ±s.d.3.28
№ of pairs 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
Productivity 10 11 9 14 15 14 12 10 8 8
1.80 1.55 0.93 0.87 1.57 2.83 2.80
0.90 2.88 1.75
C H O U G H Pyrrhocorax pyrrhocorax Scarce (Schedule 1), population stable
During 2021, six pairs attempted to breed (25% below the ten-year mean 8.00 ±s.d.1.05) and of these,
four successfully fledged young. The other two pairs failed during the nest building and early chick rearing
stages.
Number of pairsThe resulting 2.00 chicks per pair is 0.53% greater the ten-year mean (1.99±s.e.0.15). The number of young
fledged (12) is 24.53% below the ten-year mean (15.90±s.d.4.23). These figures are outside the
acceptable limits set out for the management of the NNR, SPA and SAC. However, looking long-term,
the population is 30.94% greater than the long term (1953-2021) mean (4.58±s.d.2.24), with the
productivity being 24.24% lower than the long-term mean (2.64 ±s.e.0.13).
10
8
6
4
2
0
1953
1955
1957
1959
1961
1963
1965
1967
1969
1971
1973
1975
1977
1979
1981
1983
1985
1987
1989
1991
1993
1995
1997
1999
2001
2003
2005
2007
2009
2011
2013
2015
2017
2019
2021
Year Mean 4.60 ±s.d.2.25
Number of pairs
№ of pairs 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
№ of young 9 9 8 9 8 9 8 7 7 7
Productivity 15 13 24 18 14 22 15 11 15 11
1.67 1.44 3.00 2.00 1.75 2.44 1.88 1.57 2.14 1.57
C A R R I O N C R O W Corvus corone Scarce, population stable
Eight pairs nested this year, one more than last year and 16.78% higher than the long-term average
(6.85±s.d.2.42) but 16.67% lower than the ten-year mean (9.60±s.d.2.55). A minimum of 11 young fledged
giving a productivity figure of 1.38.
65: 138-167 153
COSSEY & STANSFIELD
Number of pairs 14
12
10 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
Carrion Crow Year
8 Mean 6.85 ±s.d.2.42
6
4
2
0
1953
2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
№ of pairs 9 10 7 13 13 13 9 7 7 8
R A V E N Corvus corax Scarce
Two pairs were seen around the East Side in April and May, however, only one pair was successful with
a brood of four fledging on 10 May. This is the 11th consecutive year that Ravens have nested on 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 Raven
Year
Mean 1.16 ±s.d.0.79
Raven © Steven Stansfield REP. BARDSEY BIRD FLD OBS.
154
BREEDING BIRDS
G R E A T T I T Parus major Scarce
Four pairs bred this year: two at Cristin and one each at Nant and Carreg Withy. This is the eighth time
this species has nested and the highest-ever total for the island. A minimum of seven young fledged
during the summer, with some pairs attempting multiple broods.
5
Number of pairs 4
3
2
1
0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953 Great Tit Year
Mean 0.21 ±s.d.0.69
S W A L L O W Hirundo rustica Uncommon, population decreasing
Ten pairs nested this year, one less than last year, and 30.56% lower than the ten-year mean
(14.40±s.d.4.27). However, this is 18.28% greater than the long-term 1953-2020 mean (8.45±s.d.4.78). There
were three pairs at Tŷ Pellaf and one pair each at Nant, Lloft Carreg, Plas Bach, Cristin and in the
Lighthouse compound. Most pairs managed to fledge two broods, and a minimum of 52 young was
recorded, giving a productivity figure of 5.20 young per pair, 1.81% below the ten-year mean
(5.30±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 8.45 ±s.d.4.78
№ of pairs 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
№ of young 19 18 17 21 14 15 9 10 11 10
Productivity 100 62 105 77 79 54 71 69 57 52
5.26 3.44 6.18 3.67 5.64 3.60 6.90 5.18 5.20
7.89
H O U S E M A R T I N Delichon urbicum Scarce occasional breeder
This is the ninth consecutive year that this species has nested on Bardsey. Two pairs nested at the
Lighthouse and one pair occupied one of the nest boxes at Cristin, the first use of these boxes since they
were installed in 2009. After their nest structurally failed in early July the second pair at the Lighthouse re-
built, but the chicks did not fledge. The total of three pairs this year is 127.59% greater than the long-term
1953-2021 average (1.32±s.d.3.13) but 28.57% below the ten-year mean (4.20±s.d.2.35). A minimum of
seven young fledged, giving a productivity figure of 2.33.
65: 138-167 155
COSSEY & STANSFIELD
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 1.32 ±s.d.3.13
W I L L O W W A R B L E R Phylloscopus trochilus Common migrant, occasional breeder
Just one male held territory at Nant for most of the spring. This is three fewer than 2020 but 25.00% above
the ten-year mean (0.80±s.d.1.32). No juveniles were recorded, and it is considered that the breeding
attempt failed.
Number of pairs 5
4
3 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
2
1
0
1953
Year
Willow Warbler Mean 0.39 ±s.d.0.87
2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
0 2 0 0 0 0 1 4 1
№ of pairs 0
C H I F F C H A F F Phylloscopus collybita Scarce
One pair nested this year at Nant, with five fledged young on 12 July. This was 44.4% lower than the ten-
year mean (1.80±s.d.1.23) but still 13.79% greater than the long-term average (0.88±s.d.1.22).
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.1.22
2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
2 3 1 0 2 1 3 1 1
№ of pairs 4
156 REP. BARDSEY BIRD FLD OBS.
BREEDING BIRDS
S E D G E W A R B L E R Acrocephalus schoenobaenus Scarce
Nine pairs attempted to breed. This is 26.75% higher than the ten-year mean (7.10±s.d.2.28). Four juveniles
was recorded, but migrants passing through in early autumn masked the true figures of young produced.
Number of pairs 12
10
1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
8 Sedge Warbler Year
6 Mean 1.62 ±s.d.2.75
4
2
0
1953
2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
№ of pairs 8 9 3 5 10 10 5 7 6 9
B L A C K C A P Sylvia atricapilla Scarce
Three pairs attempted to breed this year, one fewer than last year but 114.29% above the ten-year mean
(1.40±s.d.1.35). Pairs were found in Cristin Withy, Nant and Cristin. A minimum of three young fledged,
two at Cristin and one in the Withies.
Number of pairs 5
4
3 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
2 Blackcap Year
1 Mean 0.24 ±s.d.0.72
0
1953
2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
№ of pairs 1 0 0 2 1 1 0 2 4 3
Blackcap © Steven Stansfield 157
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W H I T E T H R O A T Curucca communis Scarce
Four pairs held territories, two fewer than last year and 4.76% lower than the ten-year mean
(4.20±s.d.2.49), but 25.12% higher than the long-term 1953-2021 average (3.20±s.d.3.55). There were pairs
at Tŷ Capel Withy, Cristin, Carreg Bach and Nant. The earliest signs of successful breeding came on 9
July with a juvenile in Tŷ Capel Withy. A minimum of four juveniles fledged, giving a productivity of 1.00.
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 3.20 ±s.d.3.55
2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
№ of pairs 7 6 7 1 2 2 1 6 6 4
W R E N Troglodytes troglodytes Fairly common, population fluctuates
There were 131 singing males recorded during the Breeding Bird Survey (BBS) dawn counts in 2021; this
total is 13 (9.03%) lower than last year’s total and 12.37% below the ten-year mean (149.50±s.d.43.21), but
138.05% greater than the long-term average since 1953 (55.03 ±s.d.51.44).
Wren © Steven Stansfield REP. BARDSEY BIRD FLD OBS.
158
BREEDING BIRDS
Number of pairs 250
200
150 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
100 Wren
50
0
1953
Year
Mean 55.03 ±s.d.51.44
2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
№ of territories 121 101 159 186 163 248 134 108 142 131
S O N G T H R U S H Turdus philomelos Scarce
Five males held territory, with one pair each at Nant, Carreg Withy, Tŷ Pellaf, Cristin and in the Wetlands.
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 following a 57-year absence. At least two juvenile birds were seen or
trapped.
Number of pairs 6
5
4 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
3 Song Thrush Year
2 Mean 13.17 ±s.d.12.76
1
0
1953
B L A C K B I R D Turdus merula Scarce/uncommon, population increasing
This year 12 pairs were recorded, seven fewer than 2020, 18.37% below the ten-year mean
(14.70±s.d.2.63) and 8.86% lower than the long-term 1953-2021average (13.17 ±s.d.12.76). A minimum of
15 young fledged, giving a productivity figure of 1.25, 38.94% below the ten-year mean (2.05±s.e.0.23).
The majority of birds nested in the island’s gardens, although there were also several nesting birds in the
wetlands and the withies.
Number of pairs 50
40
30 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
20 Blackbird
10 Year
Mean 13.17 ±s.d.12.76
0
1953
№ of pairs 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
Productivity 12 16 12 14 14 16 19 14 19 12
2.83 1.56 1.83 1.71 2.14 1.05 2.05 3.21 2.84 1.25
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R O B I N Erithacus rubecula Uncommon
There were four pairs this year, two more than in 2020 but still 28.57% below the ten-year average
(5.60±s.d.4.33). Pairs were at Cristin, Tŷ Pellaf, Nant and Tŷ Bach. A minimum of five young fledged.
15
Number of pairs 10
5
0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953 Robin
Year
Mean 2.57 ±s.d.3.05
2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
№ of pairs 8 9 15 10 8 2 0 0 2 4
S T O N E C H A T Saxicola rubicola Scarce, population recovering / stable
A total of 15 breeding pairs were recorded, the same number as in 2020. This figure is 67% greater than
the ten-year mean (9.00±s.d.3.77) and 173.10% higher than the long-term 1953-2021 average (5.49
±s.d.4.24). A minimum of 39 juveniles fledged, giving a productivity figure of 2.60, 48.33% lower than the
ten-year mean (5.03±s.e.0.41).
Juvenile Stonechat © Steven Stansfield
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 5.49 ±s.d.4.24
160 REP. BARDSEY BIRD FLD OBS.
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Juvenile Robin © Steven Stansfield 161
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№ of pairs 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
№ of young 7 5 7 10 10 7 4 10 15 15
Productivity 44 28 43 49 52 26 27 52 59 39
4.90 5.2 5.20 3.93 2.60
6.29 5.60 6.14 3.71 6.75
W H E A T E A R Oenanthe oenanthe Scarce/uncommon, population recovering
The first breeding behaviour was recorded on 10 May and the first juvenile bird was seen on 3 June. A
total of 27 pairs nested, five fewer than last year but 2.66% above the ten-year mean (26.30±s.d.8.87)
and 78.56% greater than the long-term 1953-2021 average (15.12±s.d.9.79). A minimum of 26 young
fledged this year, giving a productivity figure of 0.96 young per pair; this was 31.91% lower than 2020 and
47.22% below the ten-year mean (1.82±s.e.0.23).
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 15.12 ±s.d.9.79
№ of pairs 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
№ of young 11 17 34 35 28 38 18 23 32 27
Productivity 28 55 65 54 31 51 46 36 45 26
2.55 3.24 1.91 1.54 1.11 1.34 2.56 1.57 1.41 0.96
Wheatear © Steven Stansfield REP. BARDSEY BIRD FLD OBS.
162
BREEDING BIRDS
D U N N O C K Prunella modularis Uncommon, population fluctuating
This year 15 males held territory, the same number as in 2020, but 28.23% below the ten-year mean
(20.90±s.d.14.28). As seen 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 6.91% above the long-term 1953-2021 average
(14.03±s.d.8.16). A minimum of 27 juveniles was recorded this year, the majority coming from the
Mountainside, with at least three broods at both Cristin and in the Withies.
Number of pairs 60
50
40 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
30 Dunnock Year
20 Mean 14.03 ±s.d.8.16
10
0
1953
2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
№ of males 49 45 20 20 11 15 9 10 15 15
P I E D W A G T A I L Motacilla alba yarelli Scarce, population stable
Ten pairs nested, three more than last year, 9.70% above the ten-year mean (9.20±2.39), and 120%
greater than the long-term 1953-2021 average (4.55 ±s.d.3.48). A minimum of 14 juveniles was recorded,
giving a productivity figure of 1.40, which is 58.16% below the ten-year mean (3.35±s.e.0.30).
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 4.55 ±s.d.3.48
№ of pairs 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
№ of young 10 10 10 14 10 9 6 6 7 10
Productivity 31 48 46 40 29 30 17 21 25 14
3.10 4.80 4.60 2.86 2.90 1.40
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 declined this year to 51 singing males, 50% fewer than 2020 and 38.7% lower than the
ten-year mean (83.20±s.d.24.08) and 4.91% lower than the long-term 1953-2021 average (53.63.
±s.d.26.93). The first fledged birds were recorded on the West Coast on 8 June and many more were
noted through the rest of the month and into July; productivity was considered to be reasonable, with a
good number of young seen throughout the summer months.
65: 138-167 163
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Number of pairs 140
120
100 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
Meadow Pipit Year
80 Mean 53.63 ±s.d.26.93
60
40
20
0
1953
2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
№ of pairs 66 72 97 118 93 106 81 46 102 51
R O C K P I P I T Anthus petrosus Uncommon, population increasing
This year’s total of 34 pairs is ten fewer than last year, 24.44% below the ten-year mean (45.00±s.d.7.04),
but 1.7% greater than the long-term 1953-2021 average (33.43±s.d.10.21). There was a conservative
minimum of 39 juveniles recorded along the coastline, giving a productivity figure of 1.15 young per pair.
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 33.43 ±s.d.10.21
20
10
0
1953
2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
№ of pairs 43 47 50 59 40 51 40 42 44 34
C H A F F I N C H Fringilla coelebs Scarce, population declining
One pair nested at Nant and a female was present at Cristin for much of the summer. However, no
young were seen and breeding was unsuccessful for the first time since 2000.
Number of pairs 14
12
10 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
Chaffinch Year
8 Mean 1.69 ±s.d.3.16
6
4
2
0
1953
2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
№ of pairs 11 11 12 11 4 2 2 3 5 1
164 REP. BARDSEY BIRD FLD OBS.
BREEDING BIRDS
L I N N E T Linaria cannabina Uncommon, population declining
This year 19 pairs were recorded, three fewer last year and 33.33% lower than the ten-year mean
(28.50±s.d.8.25) and 20.19% lower than the long-term 1953-2021 average (23.81 ±s.d.15.82). Nesting took
place across the island; 58 juveniles were recorded, giving a productivity figure of 3.05 young per pair,
19.28% above the ten-year mean (2.56±s.e.0.35).
80
Number of pairs 60
40
20
0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953 Linnet
Year
Mean 23.81 ±s.d.15.82
№ of pairs 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
№ of young 27 31 25 41 30 41 23 23 22 19
Productivity 58 148 68 39 38 114 62 80 39 58
2.15 4.77 2.72 0.95 1.27 2.71 2.70 3.48 1.77 3.05
L E S S E R R E D P O L L Acanthis cabaret Scarce
A female that was first ringed on 4 June was re-trapped on 2 July with a brood patch, and it is presumed
that a pair attempted to breed on the Mountain above Cristin, however no juvenile birds were recorded.
This is the first attempt to nest by Lesser Redpolls on Bardsey since 2012 when three pairs bred.
Number of pairs 4
3
3 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
2 Redpoll Year
2 Mean 0.18 ±s.d.0.49
1
1
0
1953
G O L D F I N C H Carduelis carduelis Scarce
Goldfinches have nested on Bardsey in 15 out of the last 20 years. Three pairs nested this year: one at
Nant, one in Cristin garden and one at Tŷ Bach. This is one fewer than in 2020 but still 25% above the ten-
year mean (2.40±s.d.1.58). All three pairs were successful, with the pair at Cristin rearing two broods. In
total, a minimum of nine juveniles fledged across the island. A productivity figure of 3.00 was calculated
which is 16.92% lower than the ten-year mean (3.61±s.e.0.74).
№ of pairs 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
№ of young
Productivity 1 4 4 4 1 2 1 0 4 3
5 11 31 21 0 9 1 0 13 9
5.00 2.75 7.75 5.25 0 4.50 1 0 3.25 3.00
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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 Goldfinch Year
Mean 0.49 ±s.d.1.11
Juvenile Goldfinch © Steven Stansfield
4 SUMMARY
The number of Razorbill pairs is still decreasing from a peak of 1321 in 2018, only 817 pairs were recorded
this year, 35.33% lower than the ten-year mean. Guillemots did better, with the breeding population
experiencing an increase to 1432 AOLs, 11.92% above the ten-year mean. This means the upward trend
that has been experienced since the late 1990s is still clear to see. The Puffin colony continues to grow
on Bardsey: the count of AOBs increased to 202. This could be due to immigration from the nearby colony
on Ynys Gwylan Fawr.
It was a record year for Kittiwakes, 57.53% above the 2012-2021 mean and with a productivity figure of
1.48. This is 115.51% higher than the ten-year mean (0.59±s.e.0.09). There were 59 Shag nests on Bardsey,
an increase of 20 from 2020, and 133 chicks fledged, giving a productivity of 2.25, 24.24% above the ten-
year mean. There were falls in the number of pairs of both Herring Gulls and Lesser Black-backed Gulls.
The number of Herring Gull pairs was 10.06% lower than the 2012-2021 mean (374.70±s.d.37.50) and Lesser
Black-backed Gulls, 44.41% lower than the ten-year mean(183.50±s.d.53.85). The productivity of a sample
plot of 248 Herring Gull AONs gave a figure of 0.48, 23.95 less than the ten-year mean (0.63±s.e.0.04). The
productivity of a sample plot of 69 Lesser Black-backed Gull nests at 0.54 was 27.89% higher than the ten-
year mean (0.42±s.e.0.05). Two pairs of Great Black-backed Gulls nested again, 53.49% below the ten-
year mean.
166 REP. BARDSEY BIRD FLD OBS.
BREEDING BIRDS
There were 21 AONs of Fulmars, 17.32% above the ten-year mean (17.90±s.d.5.61). A productivity figure
of 0.57 was calculated. Manx Shearwater productivity was 0.65 chicks per breeding pair; a decrease of
12.16% on last year’s productivity, 3.27% lower than the ten-year mean (0.67 ±s.e.0.02).
After being unable to monitor the Gwylan islands in 2020 due to the due to the SARS-CoV- 2 pandemic,
a boat trip to Ynys Gwylan Fawr was undertaken on 13 June 2021. A single Razorbill chick was found with
only 12 AOLs. 103 Guillemot AOLs were recorded, the highest total in the last ten years and 68.85% above
the ten-year average (61.00±s.d.24.41). The number of Puffin AOBs was 48.43% lower than the ten-year
average (676.80±s.d.203.47). The Great Black-backed Gull population appears to have stabilized after
dropping by almost 50% in the last eight years, with 51 AONs being one more than the 2020 total but still
16.78% lower than the ten-year mean (61.29±s.d.20.10). Herring Gull numbers appear to be decreasing
on the Gwylans with the total number of AONs being 41.5% below the ten-year average
(97.43±s.d.45.03). A total of 57 AON was found on Ynys Gwylan Fawr. Seven Cormorant AONs were
recorded on Ynys Gwylan Fawr, the first time they have bred on the larger of the two islands since 2013.
Away from seabirds, a pair of Cuckoos was recorded regularly at Nant and Cristin during May and June,
however no juveniles were seen. Eight Mallard broods were found, 27.93% lower than the 2012-2021
mean (11.10±s.d.3.84). Of these, a brood of five and another single chick fledged. The number of
Moorhen pairs was two higher than in 2020. Oystercatchers decreased to 54 pairs, 34.47% below the ten-
year mean (82.40±s.d.8.63). This year was the first since 2014 that Ringed Plovers have bred on Bardsey
with two young successfully fledging. Little Owls held three territories, although no breeding was
confirmed. The pair of Peregrines again successfully fledged two chicks with the youngsters first being
seen around the East Side on 29 June. Two pairs of Ravens were seen but only one pair successfully
fledged four chicks.
On the whole, there were increases in the number of breeding pairs of passerines on the island. Great
Tits bred again with four pairs across the island, one higher than last year and the most pairs since 1953.
Only one pair each of Chiffchaffs and Willow Warblers bred this year, however the Chiffchaffs managed
to raise five chicks to fledging age. Sedge Warblers had an excellent year with nine pairs. The number
of Whitethroat pairs decreased slightly but Blackcap numbers remained stable with three pairs breeding,
two of which fledged young. Wrens decreased slightly. Having only returned to Bardsey as a breeding
species three years ago, five pairs of Song Thrushes were recorded. Robins held four territories this year,
two more than in 2020 but still 28.57% below the ten-year average (5.60±s.d.4.29). Fifteen pairs of
Stonechats bred with the majority breeding within the gorse across the island. Wheatear pairs were five
fewer than 2020 but still 2.66% above the ten-year mean (26.30±s.d.10.09). After a slight dip in the previous
three years, Pied Wagtails were back to their normal levels this decade. Rock Pipits decreased slightly
with the total being ten less than last year. Meadow Pipits are fluctuating dramatically over the past few
years, with 51 fewer pairs breeding than the 102 pairs in 2020. Although one pair of Chaffinches
attempted to breed, they did not rear young this year, the first time since 2000. Lesser Redpolls attempted
to breed for the first time since 2012, though unsuccessfully. Three pairs of Goldfinches nested, one fewer
than last year.
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. & 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
65: 138-167 167
Manx Shearwater chick MANX SHEARWATER COSSEY & STANSFIELD
© Steven Stansfield Productivity, ringing and chick growth rates
168 Stuart Cossey and Steven Stansfield
REP. BARDSEY BIRD FLD OBS.
MANX SHEARWATER PRODUCTIVITY, RINGING & GROWTH
65: 168-181 169
COSSEY & STANSFIELD
1 INTRODUCTION
The Manx Shearwater Puffinus puffinus (Brünnich, 1764) is a truly pelagic bird, only returning to land to
breed on a small number of select offshore North Atlantic islands. Bardsey is considered to be the fourth
largest of only five major Manx Shearwater colonies in the world and is the third largest of the three major
colonies in Wales, following Skomer and Skokholm. 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). 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.
As a significant proportion of the world’s breeding population of Manx Shearwaters is found on Bardsey
it is well-studied on the island. Its wintering quarters are known through ringing recoveries and the
breeding population is known. 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 years one to three of the current 2019-2022 census shows a
further increase of about 25%, 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.
The average productivity (number of young fledged per pair) since 1998 is known from a sample of up
to 150 burrows monitored annually. 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 is 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 Manx Shearwater productivity monitoring programme and 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 population changes.
3 METHODS
3.1 PRODUCTIVITY MONITORING
A sample of Manx Shearwater burrows was selected over several years 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 while 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.
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Accessible burrows containing Manx 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 run the British and Irish Ringing Scheme on behalf of the UK government and issue rings for the
birds.
3.2.1 RINGING ADULT BIRDS
Adult Manx Shearwaters generally come ashore after dark during the new moon period. Accordingly,
they are trapped for ringing during this time, at night, 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.
This information is 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 which
are a depth of around 30cm or so beneath the surface allowing easy access by the ringer. However,
burrows of more than arm’s length may be accessed using a rubber-coated leg hook. The chick is
removed by 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, the principal being 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.
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 released. These data are then entered onto the BTO’s database as re-
traps. The database can match up all 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 illegible, so worn rings are carefully removed
with circlip pliers and replaced with a new ring. 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 salt
water for the majority of its life.
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3.3 GROWTH RATE OF CHICKS
In addition to monitoring the productivity, six 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.
4 RESULTS
4.1 PRODUCTIVITY
This season 165 burrows were located and monitored; 28 of the burrows were never occupied, and in 13
cases the marker post was lost and the burrow removed from the study. The remaining 124 burrows
containing an adult bird incubating an egg were monitored to conclusion.
Of these 124 nests, 40 (32%) failed at the egg stage, four (3%) failed at the chick stage and the remaining
80 pairs successfully reared young, giving a productivity figure of 0.65 chicks per breeding pair. This
represents a decrease of 12.16% on the 2020 productivity figure (0.74), and is 3.27% lower than the ten-
year mean (0.67 ±s.e.0.02) and 10.96% lower than the mean since 1998 (0.73. ±s.e.0.02).
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 2021
Year Mean 0.73 ±s.e.0.02
Productivity
Fig.3 Manx Shearwater productivity 1998-2021
Table 1 Productivity 2011 – 2021
Year 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
Productivity 0.72 0.61 0.70 0.64 0.64 0.65 0.60 0.77 0.74 0.65
Sample size 148 132 101 119 125 127 125 122 114 124
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Table 2 Area-specific nest and productivity details
Area Incubating Eggs Fledged Eggs hatched Productivity
(per pair) (chicks per
adults hatched/young 8
8 0.67 pair)
South End 12 8 7 0.47 0.67
Cristin 17 8 1 0.67 0.47
12 8 3 0.40 0.58
Pen Cristin 5 2 3 0.60 0.20
West Coast 5 3 5 0.75 0.60
Mountain Top 4 3 10 0.75 0.75
Mountain North 8 6 11 0.77 0.63
Mnt North East 13 10 2 0.85 0.77
North Stream 13 11 5 1.00 0.85
North West Fields 3 3 2 0.71 0.67
7 5 5 0.67 0.71
Barcut 3 2 0 0.56 0.67
Nant Valley East 9 5 10 0.00 0.56
Nant Valley West 1 0 80 0.83 0.00
12 10 0.68 0.83
Ty Pellaf 124 84 0.65
Treath Fynnon
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, 1213 individual full-grown birds were handled by Observatory staff, several more than once
during the season, with a total of 1326 handlings. Of these, 723 new birds were fitted with a ring, the
remaining birds were recorded as re-captures (see below). In addition to these, 138 young birds were
ringed in the nest, giving a grand total of 861 new birds ringed compared to 1474 last year.
4.3 RE-CAPTURING RINGED BIRDS
In addition to the 861 new birds ringed, 538 individual birds were recorded that were already wearing
rings, including 48 same-year recaptures and some were captured multiple times, with a total of 603
handlings during the year.
4.4 CHICK GROWTH RATES
Each week during the summer months, a visit was made to ten 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, the same date as last year; the final measurement was taken on 3 September,
although the first had fledged by 26 August, a week later than 2020. A single nest failed at chick stage
during the season and one nest was lost. The following results are based on the remaining eight chicks
from the original sample size of ten. The maximum wing-chord to the nearest 1mm was taken, though
this is not an easy measurement to take when the chicks have first hatched, as the tip of the wing is fluffy.
The birds were weighed using a 1kg electronic balance.
65: 168-181 173
Wing LengthCOSSEY & STANSFIELD
4.5 WING-LENGTHS
Most chicks had a wing-length of between 24 and 45mm. The mean first measurement was
34.25mm±s.d.7.48. The chicks continued to grow and fledged with an average wing-length of
229.63mm±s.d.8.14, 4.43%, shorter than an average adult bird from Bardsey at 240.27mm±s.d.4.75
(n2359).
250
200
150
100
50
0
Date
Fig. 4 Growth rate of chicks – wing-length by date
4.6 WEIGHTS
The mean weight of the sample in week 1 was 79.75±s.d.16.23. The heaviest weight recorded of all the
chicks measured was 670g; this was a chick that was first measured on 1 July and on that date weighed
46g. The final weight recorded before it fledged was 499g.
Most of the chicks reached their peak weights in early to mid-August; the mean peak weight was
587.00±s.d.42.97, 6.09% lighter than last year’s mean peak weight of 625.08g±s.d.78.75. The minimum
peak weight of any of the chicks was 531g from a chick whose fledging weight was eventually 408g,
amounting to a loss in weight of at least 123g, 23.16% of its peak mass.
800
700
600
500
400
300
200
100
0
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
Weight
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
Date
Fig. 5 Growth rate of chicks – weight by date
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Fig. 6 Manx Shearwater from nest P4 - photos taken weekly from mid-July to mid-September
© Stuart Cossey
4.7 FLEDGING
Once the shearwaters reached a size where their wing-lengths were c190-195mm (c85% of their fledging
wing-length) in week eight, they began to slim down and lose weight prior to fledging. The mean fledging
65: 168-181 175
COSSEY & STANSFIELD
wing-length was 229.63mm±s.d.8.14 (232.70mm±s.d.10.54 in 2020), with the smallest fledgling having a
wing-length of 218mm and the largest 240mm.
The mean fledging weight was 460.50g±s.d.36.94, 9.1g (1.90%) lighter than 2020 (469.40g ±s.d.50.45), the
lightest being 408g and the heaviest being 510g.
The average weight loss, from peak weight to fledging weight, was 126.50±s.d.23.27, 23.07% more than
the 2020 mean weight loss of 155.68±s.d.49.71. The largest weight loss was 171g from 670g to 499g (25.52%
of the chick’s mass).
800Weight
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
560Weight
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 2021 was
229.63mm±s.d.8.14, 4.43% shorter than the average wing length for adult birds on Bardsey).
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 are visited once a week from when the chicks first
hatched until they have 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
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only birds that fledged were included in the results. The sample size varies 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, 31 in 2020 and eight in 2021.
5.2 WING-LENGTH
When the first measurements were taken, chicks were up to 2.5 week in age and had a wing-length of
between 24 and 45mm, with an average of 34.25mm±s.d.7.48; thereafter wing-length grew rapidly for
the next five weeks.
Fig. 9 Five-day old chick © Steven Stansfield
Growth per week was variable through the season, with wing-lengths reaching a peak mean growth of
34.38mm per week three weeks in; however, some individuals grew as much as 44mm in a single week.
250
200
Mean wing length 150
100
50
0 9 10
12345678
Week
Fig. 10 Mean wing lengths of Manx Shearwater chicks per week in 2021
65: 168-181 177
COSSEY & STANSFIELD
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 week. The mean weekly growth rate was 23.17mm±s.e.0.84 per
week over the season.
Weekly growth 2021
Growth (mm) 50
45
40 8-9 9-10
35
30
25
20
15
10
5
0
1-2 2-3 3-4 4-5 5-6 6-7 7-8
Week
Mean Max Min
Fig. 11 Average weekly growth of wing-length in 2021 (mm/week)
Fig. 12 Three to four- week old chick © Steven Stansfield
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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 9-10
Mean 16.38 26.25 34.38 30.38 28.00 23.75 19.25 15.14 15.00
8.18 10.81 6.78 5.21 6.32 3.11 2.92 6.23 4.24
s.d.±
Max 28 40 44 39 38 30 23 24 18
Min 5 9 24 23 19 20 15 7 12
5.3 WEIGHTS
In their first week, the chicks had a mean weight of 79.75g±s.d.16.23. In the initial weeks, both adults
returned to shore most nights to feed their chick, which rapidly gained weight, with an average increase
of 105.27g±s.d.35.11 in the first two weeks. Over the first four weeks the chicks increased on average by
92.60g±s.d.40.95 per week. However, by the fifth week adults returned to the nest less frequently and
chicks grew by a mean weight of just 64.14g±s.d.42.71.
Mean weight 700
600
500
400
300
200
100
0
1 2 3 4 5 6 7 8 9 10
Week
Fig. 13 Mean weights of Manx Shearwater chicks per week in 2021
Most chicks reached their peak weight eight weeks after hatching, at a maximum mean weight of
575.50±s.d.50.57. One bird reached a peak weight of 670g whilst another significantly smaller bird
peaked at just 531g. 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 114g in a single week before fledging, which constituted 25.96% of its
fledging weight of 439g.
Table 4 Weight difference (±g/week)
Week 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9
Mean 93.29 107.62 93.14 66.71 64.14 40.57 8.00 -24.43
40.30 26.80 31.58 52.18 42.71 26.91 38.18 35.58
s.d.± 141.25 168.00 144.00 155.00 104.00 72.00 83.00 27.00
Max 28.25 81.00 53.00 23.00 -25.00 -2.00 -32.00 -67.00
Min
65: 168-181 179
Weight difference (±g/week)COSSEY & STANSFIELD
150
100
50
0
-50
-100
1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10
Week
Fig. 14 Average weekly weight difference (±g/week)
5.4 FLEDGING
Having reached their peak weight, the shearwaters began to lose weight in the final weeks before
fledging. The mean fledging wing length was 229.63mm±s.d.8.14, but one individual fledged with a wing
as short as 218mm, 9.17% below that of an average adult Manx Shearwater on Bardsey; another
individual fledged with a wing of 240mm.
The mean fledging weight was 460.50g±s.d.36.94, with the smallest fledgling leaving the nest with a
weight of just 408g and the largest with a weight of 510g. In the final weeks before fledging, chicks lost a
substantial amount of weight as they converted their fat reserves into muscle and feathers.
One bird lost 171g, 11.46% more than the average weight loss of 126.50g±s.d.23.27, having reached its
peak weight of 670g in the eighth week. It fell to 499g in two weeks, having lost 25.52% of its mass before
fledging.
6 DISCUSSION
The productivity of Manx Shearwaters on Bardsey in 2021 (0.65) was 10.96% lower than the long-term
mean 1998-2021 (0.73 ±s.e.0.02). Over the years the productivity has ranged from 0.88 in 2003 to 0.60 in
2018. Whilst the productivity has fluctuated dramatically, 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 appearing to indicate
a further 25% increase, with an estimate of 25-30,000 pairs. This steady rise in population will likely result in
more immature and inexperienced pairs breeding on the island, which could in turn lead to a lower
productivity.
Much like 2020, there was no clear pattern in terms of productivity in relation to habitat or location;
productivity simply varied across the island. The burrows above Cristin showed a high failure at egg stage,
possibly due to a higher level of predation. The Mountain Top and West Coast burrows also had a poor
productivity this year but there is only a small sample size in this area. The North West Fields burrows had
the highest productivity. The failures mostly occurred at the egg stage; once the chicks hatched, all but
four reached the fledgling stage.
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7 SUMMARY
In total 124 burrows containing birds incubating eggs were monitored to completion. Productivity was
calculated and a total of 80 chicks fledged. This gave a productivity of 0.65 chicks per pair, a decrease
of 12.16% on last year’s productivity (0.74), and 3.27% lower than the ten-year mean (0.67 ±s.e.0.02). This
is 10.96% lower than the mean since 1998 (0.73 ±s.e.0.02) when Bardsey Bird Observatory began
monitoring productivity.
Ringing and re-trapping birds was again successful, with 1350 individual birds handled during the year,
and of these 723 new full-grown birds were ringed. A further 538 unique birds were recaptured a total of
603 times. One individual captured in 2021 was originally ringed in 1985. The bird was ringed as an adult,
so it is at least 36 years old, since juvenile birds do not return to breed until they are three to four years
old. In addition to the 723 full grown birds, a further 138 young birds were ringed in their nests, giving a
grand total of 861 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 229.63mm±s.d.8.14, the smallest being 218mm and the
largest 240mm. The mean fledging weight was 460.50g±s.d.36.94, the lightest being 408g and the
heaviest 510g. The mean weight-loss was 126.50±s.d.23.27, with a maximum loss of 171g from 670g to
499g.
8 ACKNOWLEDGEMENTS
Thanks are due to Natural Resources Wales for providing funding for the productivity monitoring to be
completed. Thanks also to Ed Betteridge, Megan Hollywood-Harry, Emma Stansfield, Louis Driver and
others who helped with monitoring the burrows and measuring and ringing the chicks. We would also
like to thank Emma Stansfield for her comments on early drafts of this report.
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.
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MANX SHEARWATER
Population census 2019-2022 – year 3
Steven Stansfield
Manx Shearwaters REP. BARDSEY BIRD FLD OBS.
© Steven Stansfield
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65: 168-181 183
STANSFIELD
1 INTRODUCTION
The Manx Shearwater Puffinus puffinus (Brünnich 1764) is a pelagic seabird in the family Procellariidae,
more commonly known as tube-noses (Brooke, M. 2013). It was first described in 1764 by Danish zoologist
Morten Thrane Brünnich who first named it Procellaria puffinus. It was commonly called the Manks Puffin
in the 17th century, the current English name was first recorded in 1835 and these refer to the former
nesting colony of Manx Shearwaters on the Isle of Man. Puffinus derives from ‘Puffin’ and variants, such
as Poffin, Pophyn and Puffing, all Anglo-Norman words which referred to a former delicacy: the cured
carcass of the fat nestling of the shearwater. The original usage dates from at least 1337, but since 1678
the term came to be used for the Atlantic Puffin.
The Manx Shearwater 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 of five major Manx
Shearwater colonies in the world and the third largest colony in Wales, with 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). 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).
Owing to Bardsey’s significance for Manx Shearwaters, it is important that the colony is monitored on a
regular basis. It is currently the only one of the five major global colonies to have a complete population
census undertaken that involves a count of every burrow, rather than obtaining an estimate
extrapolated from sample plots. This census has been carried out every six years since 1996. Prior to that,
censuses and population estimates were undertaken by BBFO in 1954 and in 1981. These show that over
the last two decades the Manx Shearwater population on Bardsey has undergone a significant increase,
from fewer than 7,000 apparently occupied burrows (AOBs) in 1996 to over 21,000 in 2016 (Stansfield &
Carter 2017). The last complete census took place over three years in the 2014-2016 breeding seasons
(Stansfield & Carter 2017), however, the population increase means that subsequent censuses will now
take four seasons to complete; the current one from 2019-2022 (see Fig. 2).
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 at regular intervals
using consistent methodology. This enables comparable, long-term monitoring of the population and
detection of any population changes.
3 METHODS
In previous censuses, the island has been divided into three sections, with one section surveyed per year.
However, the expected population increase for the current census is such that it is now necessary to
further divide the island, by sub-dividing the East Side section in to two. This is both the most densely
populated area, representing 75% of the island’s population, and the most challenging in terms of terrain
and accessibility.
The south-western section of the island (Area A) was surveyed in 2019, the west side of the Mountain and
North End (Area B) was surveyed in 2020, the northern half of the East Side (Area C) was surveyed in 2021
and the southern half of the East Side (Area D) is to be completed in 2022 (see Fig. 1 and Fig. 2).
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The methods used in the current census follow those used in previous censuses, and also 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
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 change to the method was adopted for the current census.
Fig. 1. Some of the Manx Shearwater census area in 2021 185
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Fig. 2 Aproximate area of the 2021 survey area on the
East Side of Bardsey © Andy Purcell
65: 182-197 187
STANSFIELD Area C
Area D
Area B
Area A
Fig. 3. Manx Shearwater census areas 2019 to 2022
R. Brown and C. M. Perrins (pers. comm.) also advised that the low response rates could have been
compounded by low sound quality on the playback equipment used. Therefore, an EasyAcc Mini
Bluetooth Speaker – the same as the ones used on Skokholm (R. Brown pers. comm.) - was purchased
and has been used since, and MP3 dual-sex playback calls, the same as those used by Perkins et al.
(2017), have been used on Bardsey since 2019.
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The 2021 census was conducted between 19 May and 7 June. This period was chosen to coincide with
the peak incubation period as per Perkins et al. (2017). All surveys were conducted during the day, as
fewer non-breeding or prospecting birds will be present in the burrows at this time (Walsh et al., 1995). All
Manx Shearwater burrows within the census area were surveyed, and three separate counts were made
simultaneously.
3.1 COUNT 1: The total number of burrows
Every burrow encountered was counted. Each burrow was checked to make sure it was complete (at
least 30cm deep), and not just the start of a burrow, or a complex of burrows, or a burrow with multiple
entrances.
3.2 COUNT 2: Apparently-Occupied Burrows (AOB)
Any suitably sized burrow away from the main Puffin sites is likely to be that of a Manx Shearwater; rabbits
have been extinct on Bardsey since late 1996, and although there is now an increasing population of
Puffins, there are relatively few breeding on the island (~200 pairs).
An examination is made of all burrows to determine whether they are ‘apparently occupied’: this is done
visually by looking for signs of use, including fresh digging, parallel grooves made by the birds' feet,
smooth compacted soil at the entrance, and droppings. Checks were also made for a suitable diameter
to the burrow, and an appropriate tunnel length. However, this method can be subjective; most
occupied burrows show clear signs of use, but some can show little or none, such as burrow entrances
that lie between stones in walls, are hidden by grass or gorse, or are on the tops of walls and mounds.
Fig. 4 Apparently Occupied Burrow on the north east side of Bardsey © Steven Stansfield
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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, 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 to shallow excavations only 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 multiple entrances were clearly
linked to one burrow, only one was counted. Similarly, a response from two or more entrances was only
counted once if it was clearly from the same bird. In areas of boulders and 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 gorse bushes (tunnels in
vegetation under bushes and droppings); 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 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.
Fig. 5 Surveyor playing call to shearwater nest © Ollie King
Most of the habitat in the 2021 census was clear ground and steep vegetated slopes. Some areas
contained dense vegetation including European Gorse, tall grass, and bracken, which made it difficult
to count burrows and AOBs accurately. Some areas at sea level contained rocky boulders. Some burrow
entrances are still distinguishable in these latter 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;
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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 AOBs.
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 2021 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 2021 census was predominantly completed by two members of Observatory staff, with occasional
assistance from up to two other members of staff. Census fieldwork was always carried out during the
day, and in favourable weather conditions. However, 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 190 hours
was spent undertaking the field work in 2021.
4 RESULTS
A total of 11,501 burrows was counted within the 2021 census area, of which 10,627 (92.40%) were
considered to be apparently occupied. Responses to voice playback were recorded from 1438 (13.53%)
of Apparently-Occupied Burrows (AOBs).
Fig. 6. An Apparently-Occupied Burrow © Steven Stansfield 191
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Table 1: Results of the Bardsey 2021 Manx Shearwater population census.
Area C total Number of Apparently % of burrows Playback % of responses
burrows Occupied occupied responses from AOBs
(from AOBs)
11,501 Burrows 92.40 13.53
10,627 1438
In the 2009 and 2015 censuses, the whole of the East Side was counted in a single year (Areas C and D
on fig 3.). In 2021 only the northern half of the section (Area C) was counted, up to the 218-easting
gridline (see fig. 3).
In order to make the data comparable, we have used the data from Area C only (the northern sector
of the East Side), from both of the previous two censuses in 2009 and 2015, and not the combined census
areas of C+D from those years.
Table 2: Comparison of Area C in the 2021, 2015 and 2009 Manx Shearwater surveys.
N.B. Insufficient data were collected to enable playback analysis in 2009.
Area C total 2021 2015 2009 % change
Number of 11,501 10,279 7424 2015 – 2021 +11.89%
burrows 2009 – 2015 +38.46%
10,627 9624 6625 2009 – 2021 +54.92%
AOBs
1165 N/A 2015 – 2021 +10.42%
Playback 1438 2009 – 2015 +45.27%
responses 2009 – 2021 +60.41%
2015 – 2021 +23.43%
2009 – 2015 N/A
2009 – 2021 N/A
Fig. 7. An adult shearwater just before entering its burrow © Steven Stansfield
192 REP. BARDSEY BIRD FLD OBS.