Manx Shearwater and Storm Petrel Studies

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Shearwater and Petrel Studies
A supplement for the BBFO
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g birds Manx Shearwater studies

Abstract GPS tracking Manx Shearwaters

Individual specialisations in foraging behaviour have been Ben Porter and Steven Stansfield
discovered in a growing number of animal taxa and have
important impacts on the ecology and evolution of Photo – Manx Shearwaters © Ephraim Perfect
species. Whilst individuality in feeding strategies is
common in marine predators, its role in shaping foraging
distributions among many species remains unclear. In this
study, we use Global Positioning System (GPS) loggers to
study spatial patterns of marine habitat use in a range-
restricted seabird, the Manx Shearwater Puffinus puffinus,
from the world’s fourth largest colony, Bardsey, during the
2017 breeding season. Foraging activity during incubation
appeared concentrated around the productive Irish Sea
Front, consistent with findings from other colonies, whilst a
mixture of local feeding and long-distance trips to western
Scotland was discovered during chick-rearing. Adults
demonstrated strong individual foraging site fidelity within
the chick-rearing period, returning to similar distal points
over repeat feeding trips, suggesting a potential driver of
spatial and dietary segregation in this species.

Porter and Stansfield

2 Introduction

The Manx Shearwater Puffinus puffinus is a 400-450g, long-lived pelagic seabird that breeds in dense
colonies along the coasts of the North Atlantic and spends the winter off South America (Brooke 1990).
Although classed as Least Concern by the IUCN, a significant proportion of the world’s population
(approx. 80%) breed at just a handful of coastal sites around Britain and Ireland (Mitchell et al. 2004).
Manx Shearwaters demonstrate a variety of consistent behaviours typical of many seabirds: they form a
strong, monogamous pair bond with their mate, show long-term fidelity to their nesting burrow (Brooke
1990) and are highly consistent in both the timing of their migration and the locations of their
overwintering sites off South America over multiple seasons (Freeman et al. 2013). Whether individual
consistencies in foraging or diet exist within the breeding season is unknown. A wealth of tracking studies
have taken place on this species since 2008, particularly focussing on some of the larger colonies such
as Skomer in Pembrokeshire (e.g. Guilford et al. 2008; Dean et al. 2015; Shoji et al. 2015; Fayet et al. 2015;
Tyson et al. 2017). However, whilst this species is very well- tudied, there are still many gaps in our
knowledge of its multi-colony use of marine habitats and important behaviours such as individual
specialisations in foraging behaviour and diet.

The purpose of this study was to provide detailed information on the at-sea distribution of Manx
Shearwaters breeding on Bardsey – the fourth largest colony in the world – and place this in the context
of existing information on the marine habitat-use of the other large colonies bordering the Irish and Celtic
Seas, including Skomer, Lundy, Copeland and Rhúm (Guilford et al. 2008; Freeman et al. 2011; Dean et
al. 2015). We also provide the first assessment of individual foraging site fidelity in this species by
comparing foraging sites (distal points of trips), foraging routes (nearest neighbour distance) and
foraging effort (distance travelled) over repeat trips. By using GPS loggers to track the foraging
movements of breeders, we will be able to identify potentially important feeding areas for this large
colony and test the hypothesis that individual foraging and site fidelity exists in this species.

2 Methods
2.1 Study site

The study was carried out on Bardsey between June - August 2017. The island holds approximately 21,000
breeding pairs of Manx Shearwaters (Carter & Stansfield 2016), representing the fourth largest colony in
Britain and Ireland after those of Rhúm (approx. 120,000 pairs), Skomer (approx. 316,000 pairs) and
Skokholm (approx. 46, 000) (Mitchell et al. 2004; Perrins et al. 2012). Although the movements from these
latter colonies have been well-studied in recent years, knowledge of Bardsey’s population – which has
grown by approximately 6000 pairs since 2010 (Carter & Stansfield 2016), is limited to a small number of
incubating adults tracked in June 2013 by Spivey et al. (2014).

Shearwaters nest in underground burrows, where parents take turns to incubate their single egg in May
and June, before sharing the duty of chick rearing from July to September (Brooke 1990). At the start of
the breeding season 33 burrows were selected, which were already occupied by incubating breeders,
permitted easy access to the nest chamber (i.e. within arm’s reach), and were located within a short
distance of each other to allow regular checks during the night when adults return to the island. Burrows
were marked with small numbered pegs to allow relocation and continued monitoring over the breeding
season.

2.2 GPS tracking

Ringing and tagging of wild animals and birds within the UK is strictly prohibited under the Wildlife and
Countryside act (1981), except for certain circumstances which have to be licenced. Under a Special
Methods Licence granted by the British Trust for Ornithology (administrators of the British Ringing Scheme)

Manx Shearwater tracking

Manx Shearwaters were fitted with global positioning system (GPS) data logger devices, as well as the
usual individually identifiable numbered metal rings. The tags deployed were modified i-gotU GT-120
devices (Mobile Action Technology), sealed in a heat-shrink tube to make them waterproof (Fig. 1b) and
were programmed to take locational fixes every 5 minutes. This interval was increased to 10 and 15
minutes on longer incubation trips to increase battery lifespan and maximize the proportion of the
foraging journey recorded. Devices weighed 17.0g once sealed in the waterproof heat-shrink plastic,
the overall weight representing 3.3 – 4.7% of adult body mass (411g  33.4). The tags were secured to a
small number of feathers on the mid- and lower back of the shearwaters using three strips of Tesa tape
(Fig. 1c). In total, 36 individuals were fitted with GPS devices between 1 June and 16 August 2017, with
12 of these birds tracked twice: once during the incubation stage and then again during the chick
rearing stage. This totalled 48 deployments over the entire period.

a

b cd

Figure 1. Deployment of GPS data loggers on adult Manx Shearwaters. The devices were
modified i-gotU GT-120 GPS data loggers (Mobile Action Technology) weighing ~17g.
Loggers were deconstructed and sealed in heat-shrink plastic (b) and then attached to small
number of feathers on the mid-back of birds using three strips of marine Tesa tape (c & d),
processing of the bird and attachment took on average eight minutes.

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Incubating adult shearwaters were captured by hand within their burrows during the daytime, fitted with
a GPS device and then returned. Devices were retrieved by checking study burrows daily until the focal
bird returned from a foraging trip. During chick rearing, tracking began once chicks exceeded a weight
of 200g, a size recommended by studies on Skomer to avoid the most sensitive development stage of
chicks (Dean et al. 2015). Adults were captured as they returned to the colony during the night; small
pegs were placed vertically in study burrow entrances, which were knocked over when birds entered
and 20-30-minute inspection visits then allowed returning birds to be detected. Burrows were blocked for
30 minutes after the parent’s return to allow the adult to feed its chick, before removing the adult and
fitting the GPS logger device.

2.2.1 Testing for device effects

GPS tagging has on occasion been shown to have negative impacts on the survival and reproduction
of animals such as birds (Bodey et al. 2017), but we found that breeding success in our 33 study burrows
(0.79 chicks/pair) was 14% greater in comparison to 127 control burrows (0.65 chicks per pair) where no
tracking was carried out. Similarly, we found no significant difference in the mean growth rate (g/day-1)
of 24 chicks whose parents were tracked (4.90 s.e.10.62) to 21 control chicks (4.83 s.e.8.11) where no
tracking was carried out (2 sample t-test: t (336)=0.94, p=0.35). Overall, there was no evidence to suggest
that the tagging procedure caused adverse impacts to chick condition or breeding success.

2.3 Analysis of tracking data

All data-processing and statistical analyses were conducted using the statistical package R 3.2.3 (R Core
Team, 2015). Data were first filtered to remove erroneous fixes based on unrealistic locations and flight
speeds (>90 km/h; Guilford et al. 2008). Foraging trips were split into complete and partial tracks, as the
battery of some GPS loggers ran out before the end of foraging trips in some deployments. For each
complete GPS track, we calculated: (a) overall track length (km), (b) furthest location from the colony
(km), (c) terminal latitude and longitude of foraging trips, and (d) duration of foraging trips (days). We
visually examined tracks to identify likely foraging areas (where GPS tracks were highly contorted) and
commuting routes to and from Bardsey.

2.3.1 Inter-colony comparison of at-sea movements

Using the trip metrics outlined above and visual estimation of foraging areas, we were able to compare
the at-sea behaviour of Bardsey’s shearwaters to those from other colonies across its core breeding
range. In particular, we present our results alongside those from an inter-colony tracking study by Dean
et al. (2015), which tracked shearwaters from Skomer, Copeland, Rhúm and Lundy between 2009–2011.
This study filtered data using a similar methodology and calculated the same measures for quantifying
foraging movements, enabling a robust comparison.

2.3.2 Individuality of foraging behaviour

To examine individual consistencies in foraging behaviour, we focused on tracking data from the chick-
rearing stage, where we acquired multiple foraging trips (2-5 repeat trips per bird) from 13 adults. We
excluded long foraging trips (maximum distance >200km; trip duration >3 days) from the analysis, as
these long trips are often carried out by adults to recover their own body condition, and so birds may
use different areas, habitats and even food types to those when provisioning chicks (Shoji et al. 2015;
Tyson et al. 2017). GPS fixes within a 3km radius of the colony were removed, as birds often congregate
in non-foraging rafts close to the island at dusk prior to returning to their nest (Brooke. 1990; Guilford et
al. 2008; Tyson et al. 2011). The resulting dataset included a total of 38 repeat tracks, from which we
calculated three measures to assess individuality in foraging behaviour: (i) foraging route fidelity, (ii)
foraging site fidelity, and (iii) foraging effort.

Manx Shearwater tracking

2.3.2.1 Foraging route fidelity

Individual route fidelity was calculated using nearest neighbour distance (NND). This analytical
technique produces a quantitative estimate for the similarity between two GPS tracks. For any pair of
tracks, it calculates the distance (in km) from a point on one track to a neighbouring point on a
comparison track. This process is then carried out for all points on the pair of tracks, and then an average
is produced to quantify the NND for the two trips: a high NND value indicates dissimilar routes, whilst a
low NND indicates similar routes.

We calculated the average NND for all repeat foraging trips within individuals and among all individuals.
We then used linear mixed models (LMMs) to investigate whether fidelity of foraging routes was
significantly higher at an individual level than at the ‘population level’. We included pair as a random
effect and compared each model to a null model (intercept only) using likelihood-ratio tests (LRTs). NND
artificially increases if the difference in the trip length between two tracks varies widely, and so this
variable (difference in trip length) was included as a fixed effect. NND was square-root-transformed to
obtain normality for the analysis.

2.3.2.2 Foraging site fidelity and foraging effort

We used the terminal point of a foraging trip as a measure of foraging site fidelity. The latitude and
longitude of this point was estimated for each foraging trip, and then the similarity of these points for
repeat tracks was compared both within and among individuals. Visual analysis of the tracking data
revealed that, whilst foraging could occur at any point along a shearwater’s trip, feeding often took
place close to a bird’s furthest location from the colony.

Following Patrick et al. (2014) and Votier et al. (2017), we quantified foraging effort using three key
variables: (i) overall distance travelled (km); (ii) distance to the furthest point from the colony (km); and
(iii) total trip duration (days).

To examine whether individuals demonstrated any consistency in foraging sites or effort, we used the
repeatability package rptR in R (Nakagawa & Schielzeth, 2010). This package calculates the
repeatability (r) of each measure, its associated standard error and p-value by testing the null hypothesis
that within-individual variance equals among-individual variance (0 = low repeatability, 1 = high
repeatability). Track length and distance to distal point were log transformed to achieve normality, and
trip duration was natural-square-root-transformed.

3 Results
3.1 Foraging trips

We obtained data for a total of 58 foraging trips from 22 individuals during incubation and chick rearing
(1 June – 21 August 2017). A total of 14 birds returned without a GPS logger, and eight of the retrieved
GPS devices failed to record any data after running out of battery before incubating birds left their
burrows. Three birds tracked during the incubation period were ‘prospecting’ birds that did not lay an
egg, and tracks from these non-breeding birds were excluded from our main results. Most of the resulting
tracks (93%) were complete and were used to calculate trip metrics for each breeding stage (Table 1),
with tracks plotted individually in Fig. 2.

3.1.1 At-sea behaviour

Overall, foraging activity was concentrated in the Irish Sea to the northwest of Bardsey over the entire
period (Fig. 2a,b), in a region where significant inter-colony overlap in feeding is known to occur (Dean

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et al. 2015). The use of this region, the Irish Sea Front (ISF), was greater during incubation than chick
rearing, with more locally-concentrated foraging activity noted during this latter period (Fig. 2b; Table
1). Five birds during chick rearing also underwent long-range trips to western Scotland, reaching as far
west as the Outer Hebrides and the Isle of Rhúm to the north (Fig. 2b).

(aa)

Ireland

Irish
Sea

Wales

(bb)

Firth of Clyde

Ireland

Irish
Sea

Wales

Figure 2. (a & b) Raw GPS tracks of Manx Shearwater foraging movements from Bardsey
Island (marked with dark star) in the Irish Sea during the 2017 breeding season.

Six were incubating adults (a) and 14 were chick-rearing adults (b). Tracks are coloured individually, and the
approximate position of the Irish Sea Front is depicted by the curved dotted line.

Manx Shearwater tracking
c

d

Figure 2. (c & d) Raw GPS tracks of Manx Shearwater foraging movements from Bardsey
Island (marked with dark star) in the Irish Sea during the 2017 breeding season

The two longest and most surprising tracks of the whole project, where one bird (c) visited Morcombe Bay then
contiued its journey northward as far as Rhúm and the outer Hebredes. Another bird (d) went as far up the River
Clyde as Greenock and Dunoon and then spent several days off the Ayrshire coast around Troon.

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During the incubation tracking period (3 June – 2 July), foraging trips were relatively long, both in length
and duration (median track length 1163km; median duration 5 days), although were shorter than
recorded on Skomer (median track length 1517km) and marginally longer than Copeland (median track
length 1031km) (Table 1). This appears closely related to the distance of these colonies to the productive
Irish Sea Front region between Dundalk (Ireland) and the Isle of Man, which was visited on 100% of short
trips (<200km) from Bardsey, 64% of short trips from Copeland and 73% of longer trips (>200km) from
Skomer (Table 1). Additional evidence of foraging activity was noted around the Mull of Galloway,
between the Isle of Man and Morecambe, and to the north-east of Anglesey, with only one trip recorded
to the south of the island (Fig. 2a).

During chick rearing (1–22 August), 91% of trips were <200km from the colony, with shorter track lengths
(median 208km) and trip durations (median 1 day) than during incubation. Foraging activity appeared
concentrated locally to Bardsey, particularly in the waters south-west of Anglesey and in Liverpool Bay
off North Wales (Fig. 2b). This local foraging is consistent with findings from the four colonies studied by
Dean et al. (2015), where chick-rearing trips were similarly short in length and concentrated within 200km
of each colony (Table 1).

Five chick-rearing adults carried out long-distance trips to western Scotland, the longest covering 2000km
over nine days, rafting off the Isle of Rhúm for one night at a maximum distance of 502km from Bardsey
(Fig. 2b). During these long-range trips, concentrated feeding activity was observed in the Firth of Clyde
and the waters between the Scottish islands of Tiree and Islay (Fig. 2b), and 75% of these trips also visited
the ISF. Similar long-distance trips were recorded by Dean et al. (2015) from Skomer, Lundy and Rhúm
(Table 1), with feeding taking place almost exclusively around the ISF for birds from Lundy and Skomer
(Table 1). The foraging activity in Scotland we observed overlaps more with areas used by the colonies
of Copeland and Rhúm (see Dean et al. 2015), which are situated much closer geographically.

Table 1. Metrics of Manx Shearwater trips from Bardsey Island (2017) and four other colonies
in the core breeding range (tracked between 2009-2011 by Guilford et al, 2015).

Metrics are given for each stage of the breeding season and are pooled across years. Colony and approximate
distance to the Irish Sea Front (ISF); period (chick rearing [Chick-R] and incubation [Incub.]; % of trips that were short
distance (S, < 200km); % of both short and long distance trips (L, >200km) that were to the ISF area; and medians (with
interquartile ranges) for overall track lengths, maximum trip distances and trip durations are included.

Colony Period Short or % short trips % trips to ISF Track length (km) Max dist. (km) Trip dur. (d)
(distance to long trip
Incub. 64 1031 119 7
ISF, km) Chick-R S 95 (408 - 1552) (73 - 152) (5-9)
L
Copeland S 50 255 56 1
(120) L 44 (129 - 545) (27 - 93) (1 -3)
S 100
Bardsey Incub. L - 1163 161 5
(130) Chick-R S 100 (943-1370) (148-195) (4-7)
L 67
Skomer Incub. S 50 208 51 1
(215) Chick-R L 5 (116-352) (28-98) (1-2)
Chick-R S 91
Lundy Chick-R L 75 1517 254 8
(280) S 0 (925 - 2117) (176-295) (7-11)
Rhúm L 35
(375) S 73 297 86 1
0 (203 - 581) (61 - 134) (1-2)
L 85
100 305 49 2
0 (169 - 592) (29 - 104) (1 - 3)
82
83 184 35 1
0 (128 - 274) (29 - 73) (1 -1)
92 60

Manx Shearwater tracking

3.2 Individuality in foraging behaviour

We obtained 36 repeat trips from 13 adults between 1 - 21 August (Fig. 4), with a median of three repeat
trips per bird (range 2 – 4). Whilst there was variability in their spatial and directional consistency, some
shearwaters were highly consistent in their departure directions and foraging areas; other birds showed
little consistency in their routes or foraging sites, with GPS tracks for these birds showing less evidence of
foraging activity (Fig. 4).

3.2.1 Foraging route fidelity

Mean NND for repeat trips within individuals (20.4 s.e. 2.25) was very similar to mean NND amongst all
individuals (30.5 s.e. 0.08; fig. 2) and this was not statistically different (LRT: df = 4, χ2 = 0.69, p = 0.41).

Figure. 3. Variation in NND ( s.d. in km) within individual breeders was very similar to variation
in NND among all breeders.

This suggests no individual consistency in the foraging routes taken by adults during chick-provisioning trips from Bardsey.

3.2.2 Foraging site fidelity and repeatability of foraging effort

During repeat chick-provisioning trips, individuals were highly repeatable in their distal latitude and
longitudes (Table 2). Whilst distal points of foraging trips varied from one individual to the next (fig. 4),
some important shared areas were also apparent, including the waters around the ISF, the waters of
Liverpool Bay and the sea to the south-west of Anglesey (Fig. 4).

Foraging effort (total distance and distal point from colony) among the individuals was highly variable
and showed no significant repeatability, although duration of foraging trips was highly repeatable (Table
2).

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Table 2. Repeatability values (r  s.e., with 95% Cis in parentheses) for foraging site fidelity
(decimal degrees) and foraging effort (0 = low repeatability, 1 = high repeatability) of chick-
rearing Manx Shearwaters on Bardsey Island.

Birds show highly repeatable distal latitude and longitudes during chick-provisioning foraging trips. Foraging effort,
however, was highly variable and showed low repeatability. Trip duration was highly repeatable. Significantly
repeatable foraging behaviours are indicated in bold and asterisks indicate significance levels (** = p< 0.05, *** = p<
0.01).

Trip metric Chick-rearing adults (n = 13)

foraging site fidelity 0.33  0.18 (0, 0.64)**
longitude of distal point (DD) 0.50  0.17 (0.10, 0.75)***
latitude of distal point (DD)
0.24  0.16 (0, 0.56)
foraging effort 0.28  0.17 (0, 0.59)
total distance travelled (km) 0.35  0.17 (0, 0.65)**
distance to distal point (km)
duration of trip (hrs)

Figure. 4. Repeat foraging trips of chick-provisioning Manx Shearwaters (n = 12) breeding on
Bardsey (marked with black star) in summer 2017 (1 – 22 August).

Some breeders demonstrate a high degree of individual foraging site fidelity (e.g. a - d), as confirmed by significantly
repeatable distal trip locations (Table 2), and these birds also tended to follow consistent routes to their foraging sites.
Other birds show variable foraging routes and locations over repeat trips, but still appear to travel in a generally
consistent direction and broader-scale area to feed. Whilst most foraging activity occurred within 100km of the colony
(a – h), some birds foraged as far afield as the Firth of Clyde, Liverpool Bay and the Irish Sea Front (i – l), even during
chick-provisioning trips that were typically <2 days in duration (Table 2). Distal points are marked with a black dot, and
the number of repeat trips are shown for each bird.

a n=2 b n=2 c n=3 d n=2

e n=4 f n=2 g n=3 h n=2

i n=4 j n=3 k n=3 l n=3

Manx Shearwater tracking

4 Discussion

This study successfully tracked the movements of 24 Manx Shearwaters during the 2017 breeding season
on Bardsey (June-August), allowing a detailed insight into their at-sea behaviour and foraging activity.
Our results provide the first evidence of individual foraging site fidelity in breeding Manx Shearwaters,
with birds repeatedly returning to the same distal points over consecutive chick-provisioning feeding trips
(Fig. 4; Table 2). Birds showed little consistency in their foraging routes (Fig. 4), however, and low
repeatability in foraging effort (Table 2). Aside insights into individual-level behaviours, our results provide
valuable information on the colony’s population-level use of the pelagic environment: breeders during
the incubating stage foraged almost exclusively around the Irish Sea Front, while feeding activity of
chick-rearing adults was concentrated more locally (<200km) to the island (Fig. 2). Five birds underwent
striking long-distance trips to the Firth of Clyde and western Scotland during chick rearing, the longest
travelling over 2000km to the Outer Hebrides (Fig. 2b). The potential causes and consequences of the
behaviours we discovered are discussed below.

4.1 Foraging in relation to the Irish Sea Front

Recent studies have revealed the importance of the Irish Sea Front (ISF) as a feeding area for Manx
Shearwaters and other seabirds during the breeding season (Guilford et al. 2008; Dean et al. 2015; Fayet
et al. 2015), supporting its designation as a Special Protection Area (SPA) in 2017 (JNCC seabirds). A
tracking study by Dean et al. (2015) found that shearwaters from Lundy, Skomer, Copeland and Rhúm
overlapped in their use of this area, with 64% of short trips (<200km) from Copeland and 73% of long trips
(>200km) from Skomer visiting the ISF during incubation (Table 1). We found similar results from Bardsey’s
colony, with all short trips during incubation and 75% of long trips during chick rearing spending time here
(table 1), providing further evidence that concentrated inter-colony overlap in foraging activity occurs
within this highly productive area of the Irish Sea (Stone et al. 1994).

Whilst clearly an important resource for breeding shearwaters, there may be a trade-off between the
benefits of feeding here and the energetic costs of travelling to this spot, with Dean et al. (2015) finding
a decline in visits to the ISF with increasing colony distance. Given Bardsey’s position some 130km closer
to the ISF than Skomer and Skokholm, it might be expected that birds benefit from this proximity, for
example through increased breeding performance or adult condition. Investigating this trade-off further
amongst Celtic Sea colonies and those further afield would highlight the exact importance of this
restricted area.

4.2 At-sea movements during chick rearing

Foraging in the chick-rearing period took place much closer to the colony (median max. distance 51km;
median track length 208km), as is typical of central place foragers constrained by the need to regularly
return to their nest and feed their chick (Shoji et al. 2015). This local foraging activity highlights the
importance of the waters surrounding Bardsey for chick-provisioning adults, although we also recorded
striking long-distance journeys (1200km – 2000km) in five birds, which spent up to nine days at sea and
visited areas such as the Firth of Clyde in western Scotland (Fig. 1b). This finding is consistent with a
behaviour termed the dual-foraging strategy, employed by many species of Procellariformes, including
the Manx Shearwaters, Cory’s Shearwaters Calonectris diomidea and Sooty Shearwaters Puffinus griseus
(Weimerskirch 1998; Magalhães et al. 2008; Shoji et al. 2015). This strategy allows parents to combine short
foraging trips to local areas (shown to maximise feeding frequency to chicks) with longer trips to more
productive areas (allowing parents to recover their own body condition). Whilst it might be expected
that adults would carry out periods of self-fuelling locally, thus avoiding the energetic cost of flying
hundreds of kilometres further afield, areas close to the colony are likely to be highly exploited and offer
average-to-low rewards (Weimerskirch 2007). The long distances covered by self-feeding adults in our
study supports the hypothesis that these trips allow adults to visit more distant areas with potentially higher
productivity and allow access to higher quality prey items (Einoder et al. 2013; Shoji et al. 2015). In this

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study, the Firth of Clyde (fig. 1b) appeared to be the most important site for birds on self-feeding trips,
which contrasts to the findings from Skomer where most birds tended to visit the Irish Sea Front (Dean et
al. 2015; Shoji et al. 2015). It might be that there was higher productivity in the Firth of Clyde than the Irish
Sea Front in 2017, especially later in the season when this tidal front begins to dissipate (Stone et al. 1994).

4.3 Individual foraging site fidelity during chick rearing

Individual foraging site fidelity (IFSF), where individuals repeatedly use the same area for feeding, is a
form of spatial foraging consistency that might arise because of individual differences in preferred
foraging behaviour, habitat or prey (Patrick et al. 2014; Phillips et al. 2017). For colonial, central place
foragers such as Manx Shearwaters, this behaviour is thought to be important by reducing intraspecific
competition (Bolnick et al. 2002). During the chick rearing period, for example, adult shearwaters are
restricted to feeding close to the colony where productivity is low and the density of conspecifics is high
(Dean et al. 2015). By foraging in different locations, at different depths or on alternative prey to
conspecifics, birds can partition their niches and alleviate competition, as has been found in species
such as Gannets (Patrick et al. 2014) and King Cormorants Phalacrocorax atroceps (Kato et al. 2000).
Further work would be needed to examine the underlying mechanisms of this individual specialisation,
such as whether individuals specialise in their prey choice or foraging habitat.

It is hypothesised that the magnitude of individual foraging specialisations may be positively correlated
with the level of intraspecific competition (Svanback & Bolnick 2007; Araujo et al. 2011), and so it might
be predicted that the occurrence of IFSF in Manx Shearwaters is greater in colonies with higher breeding
populations, such as Rhúm (120,000 pairs) and Skomer (316,000 pairs) (Mitchell et al. 2004; Perrins et al.
2012). A study examining this relationship in seven Gannet colonies across the North Sea revealed no
relationship between adult IFSF and colony size, although intraspecific competition in this species is
thought to be less important in governing foraging distributions (Votier et al. 2017). Conducting a similar
inter-colony comparison across the shearwater’s core breeding range would present a novel insight into
the drivers of this individuality.

Although intraspecific competition may play a role in the individual foraging we observed, there are
other factors that might contribute to this behaviour. One explanation is linked to search behaviour:
short-term fidelity to feeding sites might represent birds employing a win-stay-lose-switch foraging
strategy (Davoren et al. 2003). For example, if a bird encountered a productive feeding area during a
foraging trip, it might return to this site until the prey aggregation disperses. Since the shearwaters in our
study were tracked over a very short time-period, this win-stay-lose-switch strategy might account for the
behaviour we observed. However, IFSF has been found to persist across breeding seasons in species such
as Gannets, indicating that birds might not just be responding to short-term feeding opportunities (Votier
et al. 2017). Furthermore, marine predators such as Manx Shearwaters feed on prey which is often
distributed in predictable patches associated with persistent oceanic fronts and upwellings
(Weimerskirch 2007; Patrick et al. 2014; Wakefield et al. 2015). It has been suggested that site familiarity
in such predictable environments is an alternate cause of IFSF: by foraging in a familiar location, birds
might acquire a competitive advantage by gaining information specific to that site, and this may be less
risky than searching for a new site that offers higher rewards but increases energetic costs (Yoder et al.
2004; Piper 2011; Wakefield et al. 2015). Tracking birds for longer periods across multiple breeding seasons
would allow us to reveal the exact nature of IFSF in this species.

Whilst we found significant consistency in the foraging locations used by chick-rearing adults, there was
low repeatability in their foraging routes and foraging effort (Table 2). This variability might reflect
differences in the weather conditions across the tracking period, particularly in wind speed and direction.
Procellariform seabirds often adopt a dynamic soaring flight style, making use of cross-winds to reduce
the energetic costs of flying at sea. They are therefore highly influenced by the strength and direction of
the wind, which might alter their trajectories according to the conditions (Spivey et al. 2014; Gibb et al.
2017). Lastly, we found significant repeatability in the duration of foraging trips within individuals (Table
2), which likely reflects the need for adults to return regularly to provision their chick (typically every 1-3

Manx Shearwater tracking

nights during the early and mid-chick-rearing period; Brooke 1990; Shoji et al. 2015). Furthermore,
shearwaters only return to the colony during the night to avoid predation (Brooke 1990), which would
result in relatively concentrated periods of return and departure from the colony, further explaining the
consistency we observed.

4.4 Wider implications

Seabirds are one of the most threatened groups of marine vertebrates (Croxall et al. 2012; Spatz et al.
2014), and so understanding their behaviour at sea is crucial for informing appropriate conservation
strategies. We found that breeders were particularly reliant on the ISF on feeding trips during incubation,
providing further evidence of the importance this area holds for a significant proportion of the species’
global breeding population (Dean et al. 2015). Conversely, the use of local areas during the chick-
provisioning period highlights the need for increased levels of protection in these areas, which are likely
to be used by other vulnerable seabirds breeding on Bardsey Island. Individual-level specialisations such
as IFSF have far-reaching consequences for the persistence of populations, as they can result in large
differences in how individuals interact with threats such as fisheries and changes to the marine
environment. However, variability in the use of foraging sites at a population level might present a level
of phenotypic plasticity that increases the ability of populations to buffer against the potentially
deleterious impacts of environmental change (Araujo et al. 2011).

5 Conclusions

In summary, the study found that Manx Shearwaters breeding on Bardsey utilised a broad variety of areas
within the marine environment during the 2017 breeding season. During incubation, off-duty adults
appeared particularly reliant on the waters associated with the Irish Sea Front (Fig. 2a), providing further
evidence for the importance of this area for several of the country’s largest colonies (Dean et al. 2015).
Chick-rearing adults combined a mixture of short foraging trips to local waters with long-distance trips to
areas in western Scotland (Fig. 2b). This behaviour is consistent with a dual foraging strategy, where
parents balance the need to regularly provision their chick with their own need to recover body
condition by accessing more productive feeding grounds further afield (Shoji et al. 2015). We also
discovered the first evidence of individual foraging site fidelity in this species, with chick-rearing adults
returning to similar distal points over consecutive feeding trips (Fig. 4). This strategy might arise as a means
of reducing high intraspecific competition in local waters (Araujo et al. 2011; Patrick et al. 2014), or might
be driven by the energetic benefits of maintaining familiarity with a feeding site (Piper 2011; Wakefield
et al. 2015). More work is needed to reveal exactly how important this behaviour is in driving the spatial
distribution discovered in this species and other marine predators.

6 Acknowledgments

We are immensely grateful to Mark Carter, Ephraim Perfect and Emma Stansfield for their invaluable help
in the field, and for the support of Bardsey Bird Observatory more generally with this project. We thank
the Bardsey Island Trust and Natural Resources Wales for permission to carry out the project on the island,
and to the Welsh Ornithological Society and British Birds for awarding grants to cover the purchase of
GPS loggers. We also extend many thanks to Charles Bishop and Richard Holland from Bangor University,
Kane Brides from the Wildfowl and Wetlands Trust (WWT) and Steve Votier from the University of Exeter for
their generous contribution of GPS devices for the study. In addition, we would like to thank Bethany
Clark, Sarah Bond, Holly Kirk and Annette Fayet for their invaluable advice and help with data analysis.

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Shoji A., Aris-Brosou, S., Fayet A., Padget, O., Perrins, C. and Guilford, T. 2015. Dual foraging and pair
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61: 174-189 169



Manx Shearwater studies 60: 153-158

Productivity, ringing and chick growth

Ephraim Perfect

Photo – Manx Shearwater chick © Ephraim Perfect

Perfect

1 Introduction

Bardsey is one of three major Manx Shearwater colonies in Wales and is the third largest, following Skomer
and Skokholm. It is also held to be the fourth largest colony in the world. Following a complete census
undertaken between 2014 and 2016, an estimated 21,000 Apparently-occupied Burrows (AOBs) were
counted (Stansfield and Carter 2016), showing an increase of almost 5000 AOBs (30.37%) from the
previous census in 2008 to 2010.

A sample of burrows have been monitored annually since 1997, to calculate productivity (number of
young fledged per pair) of Manx Shearwaters on Bardsey.

Data from this project is used by NRW to assess the health of the population; the data are also fed into
the Seabird Monitoring Programme (SMP) established in 1986. SMP is an ongoing annual programme to
monitor 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 Aims

Manx Shearwater is currently identified as a conservation priority in the following: 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).

A significant proportion of the world’s breeding population of Manx Shearwaters are found on Bardsey,
and it is therefore a well-studied bird. The breeding population is known, its wintering quarters are known
through ringing recoveries, and the average productivity since 1997 is known from a sample of up to 150
burrows. Because of the global importance of the colony on the island, the Manx Shearwater is one of
the avian features of the SSSI and NNR.

The aims of this project 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 number of re-traps will hopefully 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. The increased ringing of juveniles will provide more birds of known
age, which will hopefully give us a much better idea of the age of first breeding and juvenile survival, as
well as dispersal.

3 Methods
3.1 Productivity monitoring

A sample of Manx Shearwater burrows were selected 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 incubating. The
next visits were made whilst the adults were rearing their young, and the final visits were made during the
fledging period. The burrows that were selected for the survey had to be fairly shallow and easily
accessible so that the contents could be accurately monitored. Accessible burrows containing Manx
Shearwaters and eggs were located at a number of clustered sites across the island and provided a
broad range of habitats and areas. Each burrow was mapped, and marked with an individually

Manx Shearwater productivity, ringing & chick growth

numbered wooden stake and the grid reference taken to aid relocation both later in the season and
the next year. When the chicks were large enough, they were ringed and biometrics were taken (wing
length and weight). A final visit was then conducted to calculate how many chicks had successfully
fledged.

3.2 Ringing adult birds

Manx Shearwaters generally only come ashore in large numbers after dark during the new moon period.
Under licence from the BTO, ringers trap adult birds at night as they return to their burrows. Head torches
are used to dazzle the birds so they cannot see the ringer approaching. The birds are then picked up by
the ringer and, using special pliers, a uniquely numbered metal ring is fitted to the bird’s leg. The rings are
issued by the BTO which runs the UK Ringing Scheme. 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 then entered onto a
specially designed database created by the BTO and submitted to the BTO as part of the ringer’s
licensing requirements.

3.3 Ringing young birds

Many of the birds that nest on the island use nest chambers which are several metres underground and
are totally inaccessible. Some of the shorter burrows have chambers which are only 30cm or so beneath
the surface, whilst longer ones one metre or so deep, may be accessed by the ringer and the chick
removed by hooking the bird around its leg with a piece of bent fencing wire (which has been rubber
coated on the end) and the bird gently eased towards the surface. When the chicks are approximately
a month old they are taken from the burrow and the same ringing process used for adults is carried out.
During the night in late August and early September, some of the young are captured whilst out of the
burrows exercising 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 were aged as juveniles that were able to fledge (Euring age code 3, i.e. hatched during the
current calendar year). Birds with a body cover of >20% were aged as pullus (Euring age code 1, i.e. a
bird still in its nest). The birds aged as 3 were technically still pullus but an arbitrary figure of 20% down
cover was given to differentiate between birds that were capable of flight and ones that were not.

3.4 Growth rate of chicks

In addition to monitoring the productivity, a study plot of 31 of the ‘productivity burrows’ were chosen to
be part of a survey looking at the growth rate of the chicks from hatching through to fledging. Each
week the young from the selected nests were visited, carefully removed from their burrows, and
measurements of wing-length and weight were quickly taken before returning them. The young were
also ringed once they were large enough, usually at about two to three weeks old.

3.5 Re-trapping adult and young birds

When a ringer captures a bird, a preliminary check is made to identify whether the bird is already ringed.
If a ring is found, the physical state of the ring is examined and depending on level of wear, it is then
replaced - after 15 to 20 years, rings can become thin and numbers can become illegible, so worn rings
are carefully removed with circlip pliers. The numbers of both rings are then noted and later married-up
on the database, with the original ringing data 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 overwhelming majority of its life. For birds whose ring is in a satisfactory state, the number is
written down and double checked, the time, location, age and sex noted and the bird then released.
These data are then entered onto the database as re-traps. The database is sufficiently powerful to be

60: 190-201 173

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able to match up all the previous captures of any individual, including its original ringing data. The
database, when queried, can provide a complete capture history for each bird.

4 Results
4.1 Productivity

The first Manx Shearwaters to be logged in 2017 were two on 6 March passing the West Coast.
Subsequently, sightings became more frequent with adults reported on the island on 15 March. During
April prospecting adults were seen regularly across the island during the new moon period. The first
incubating adult was recorded on 17 May at Tŷ Pellaf.

This season 132 burrows containing an adult bird incubating an egg were found. These were marked
with a red-topped wooden post. Of these burrows, five were lost either because dense vegetation made
them impossible to re-find, or the marker posts had been knocked out of place by livestock, making
relocating the burrow impossible. These burrows were removed from the study and the productivity was
calculated from the remaining 127 burrows. Shearwaters from these remaining burrows were followed
through to fledging; four of these failed at the egg stage, 39 failed at egg or small chick stage, one failed
at a relatively developed stage and the remaining 83 pairs successfully reared young, giving a
productivity figure of 0.654 chicks per breeding pair; a minimal increase of 2.12% on last year, and 5.15%
lower than the ten-year mean (0.689 ±s.e.0.01). This is 11.68% lower than the mean since 1998 (0.74
±s.e.0.02) when we began monitoring productivity.

Manx Shearwaters © EFP

Manx Shearwater productivity, ringing & chick growth
Fig.1. Manx Shearwater productivity 1998-2017

0.85Productivity
0.75 1998
0.65 1999
0.55 2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017

Mean 0.74 ±s.e. 0.02 Year Mean
Productivity

Table 1 Productivity 2008 – 2017

Year 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017
Productivity 0.73 0.71 0.76 0.73 0.72 0.61 0.70 0.64 0.64 0.65
Sample size 102 100 90 122 148 132 101 119 125 127

60: 190-201 175

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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 1229 birds were handled by observatory staff. Of these, 799 full-grown birds were
ringed. In addition, 229 young birds were ringed giving a grand total of 1028, compared to 768 last year.

4.3 Re-trapping ringed birds

In addition to the 1028 new birds ringed, a further 201 recaptures of birds already wearing rings were
made, two of which were ringed at other sites (some details as yet unknown). One individual captured
in 2017 was originally ringed in 1984 and was not handled again until it was re-caught on 18 August 2017,
33 years and 80 days after being ringed. The bird was ringed as an ‘adult’ so it’s actual age when ringed
is unknown, other than it was probably more than four years old.

Table 2 A selection of the more interesting recovery details received from the BTO in 2016

FB17327 4 19.08.1993 Bardsey
XL 05.10.2016
Itapirubá Norte, Imbituba, Santa Catarin, Brazil
8448 days
9674km 206˚SSW

FC75988 4 31.07.1994 Bardsey
Praia Grande, São Paulo, Brazil
Sx 29.10.2016
9456km
8126 days 206˚SSW

FB35188 4 10.06.1999 Calf of Man, Isle of Man
R 23.04.2011 Bardsey
4335 days
145km 180˚S

FB17873 4 28.07.2004 Bardsey 206˚SSW
FB17781 XL 07.08.2017 Pinhal, Brazil 343˚NNW
FB18792 206˚SSW
FB20883 4758 days 10241km 197˚SSW
3 05.09.2004 Bardsey
XF 04.09.2017 Machir Bay, Argyll and Bute

4747 days 354km
4 14.06.2005 Bardsey
X 16.10.2017 Jardim do Éden, Tramandaí, Brazil

4507 days 10219km
4 08.06.2007 Bardsey
R 07.05.2016 Skomer Island, Pembrokeshire

3256 days 118km

FB29489 4 05.05.2008 Bardsey
X 10.10.2016 Balneário Barrancos, Pontal do Sul, Brazil
3080 days
9697km 207˚SSW

FB36539 4 09.06.2013 Bardsey 51˚NE
FB36591 XF 11.09.2017 Ainsdale, Southport, Merseyside 206˚SSW

1555 days 150km
4 11.06.2013 Bardsey
XF 05.10.2016 Praia da Vila, Laguna, Santa Catarina, Brazil

1212 days 9989km

4.4 Chick growth rates

Each week during the summer months, a visit was made to 31 of the most easily accessible shearwater
nests to weigh and measure the chicks to monitor their growth rate. The first chicks were measured on

Manx Shearwater productivity, ringing & chick growth

Fig. 2 Fully-grown Manx Shearwater chick (right) with a young chick (left)Wing Length

26 June, with the final six measurements being taken on 12 September, although the first had fledged by
4 September. Several young died during the season, and several burrows were ‘lost’. The following results
are based on the remaining 21 chicks that fledged. The maximum chord wing-length 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 1kg electronic balance.

4.5 Wing-lengths

Most chicks had a wing-length of around 30mm, but some were as large as 55mm or as small as 22mm
when first measured, with the mean first measurement being 32.73mm. The chicks continued to grow
and fledged with an average wing-length of 228.10mm, 5.07% shorter than an average adult bird from
Bardsey at 240.27mm ±s.d.4.75 (n2359).

Fig. 3 Growth rate of chicks – wing by date

250

200

150

100

50

0

24/06
26/06
28/06
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

Date

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4.6 Weights

Most chicks had an initial weight between 100 and 200g, with the mean initial weight being 136.77g
±s.d.67.50, 36.35g (36.20%) heavier when compared to last year’s mean initial weight of 100.42g. The
heaviest any of the chicks reached was 595g (9.85% lighter than last year’s heaviest birds at 660g); this
was a chick that was first checked on 10 July and weighed 190g. It reached its peak weight on 22 August,
a time when many were at their heaviest, and fledged weighing 464g shortly after 4 September having
shed 131g, over 22% of its mass. Most of the chicks reached their peak weights in mid-August; the mean
peak weight was 496.5g ±s.d.61.53, 9.59% lighter when compared to last year’s mean peak weight of
549.17g ±s.d.43.35. The minimum peak weight of any of the chicks was just 373g (compared to 476g last
year) from a chick whose fledging weight was eventually 311g, having shed 62g, just 16.62% of its mass.
All chicks this year were generally of a lighter weight than usual, with two individuals fledging at their
peak weights.
Fig. 4 Growth rate of chicks – weight by date

700
600
500
400
300
200
100

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Fig. 5 Manx Shearwater chick weighing 164g, only 3 weeks after hatching

© EFP
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Manx Shearwater productivity, ringing & chick growth

4.7 Fledging

Once the shearwaters reached a size where their wing-lengths were about 180-190mm (c¾ of theirWeight
fledging wing length) in mid-August, they began to slim down and lose weight prior to fledging. The
Weightmean fledging wing-length was 228.10mm ±s.d.6.67 (227.17mm ±s.d.7.76 in 2016), with the smallest
fledgling having a wing length of 214mm and the largest 240mm. The mean fledging weight was 420.48g
±s.d.58.19, 10.73% lighter than 2016’s 471.04g (±s.d.31.19), the lightest being 310g and the heaviest being
503g, 39g lighter than the heaviest of 2016 which was 542g. The average weight loss, from peak weight
to fledging weight, was 77.81g ±s.d.41.73, only 0.41% less than 2016’s mean weight loss of 78.13g
±s.d.42.42, with the largest weight loss being 168g (171g in 2016) from 563g to 395g (29.84% of the chick’s
mass).

Fig. 6 Growth rate of chicks – weight against wing-length

600

500

400

300

200

100

0
0 25 50 75 100 125 150 175 200 225 250 275
Wing Length

Fig. 7 Fledging size of chicks – weight against wing length

560
540
520
500
480
460
440
420
400

210 212 214 216 218 220 222 224 226 228 230 232 234 236 238 240 242
Wing

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 2017 was 12.18mm (5.1%)
shorter than that of average adult birds and 0.94mm longer than the mean of birds that fledged in 2016.

5 Discussion

With the Manx Shearwater productivity study now reaching its 20th anniversary, the data set is large
enough to begin to draw comprehensive conclusions from some of the trends. In the past 20 years, the
productivity of the population of Manx Shearwaters on Bardsey has fallen by 0.13 or 16.67%. The general
trend shows a decrease in productivity, with occasional fluctuations. One of the reasons for the worrying
fall in productivity could be the immense increase in the breeding population on Bardsey, with the 2014-
2016 census predicting 20,675 pairs from 15,859 only six years earlier. With this drastic rise in population

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there will obviously be more competition for food and space, and this will result in more new and
inexperienced pairs breeding on the island, which could in turn lead to a lower productivity. This
conclusion suggests the population size on Bardsey is reaching its maximum, and in time will begin to
plateau. Whilst, this is the most likely conclusion, the productivity has fluctuated between years,
depending on the sample size of the study, with generally the lower the sample size the higher the
productivity. Since the study began in 1998 the sample size has increased from 50 to nearly 150 pairs
since 2011; this may have caused some bias in the data. Therefore, sample size should remain constant
from year to year.
From censuses conducted during the past two decades, it is clear that the East Side of Bardsey holds the
majority of the Manx Shearwater population on the island. Yet the number of burrows surveyed on the
East Side is minimal compared to the western portion of the island, which is the result of a combination
of different factors, including ease of access and safety. However, the productivity on the East Side of
the island is far higher than the western portion of Bardsey. This is probably a product of a better drainage
system (burrows are less prone to flooding), the sub-colonies are bigger, the burrows are sheltered from
the prevailing westerly winds and suffer no disturbance. Therefore, a larger sample size from the East Side
should be adopted to better represent the productivity.
Fig. 8 The East Side of Bardsey

© EFP

Between 2000 and 2004 the Observatory’s historic archive data-set for Manx Shearwaters (from 1953-
1997) was digitised with funding from NRW and BBFO. In order for these ringing and re-trap data, which
are held at the Observatory, to be of any benefit for monitoring the island’s population, they need to be
analysed. In addition to this analysis, it will be necessary to continue ringing and re-trapping as many
shearwaters as time allows in the future. The number of birds wearing rings should increase over time and
the number of birds re-trapped may be even greater than the number ringed each year; this will give
the data even greater value.
It is suggested that the Observatory continues to trap shearwaters in good numbers and that extra effort
is put into trapping ringed birds. These data can then be used (using mark and recapture statistics) to
see if there are any fluctuations in the island’s population in the short-term, without having to carry out
time-consuming full censuses of the whole island every year. A full census will still need to be undertaken
every six years.

Manx Shearwater productivity, ringing & chick growth

© EFP

Fig. 9 Manx Shearwaters taking flight

6 Summary

In total 132 burrows containing birds incubating eggs were found and subsequently marked. Productivity
was calculated from 127 of these burrows and a total of 83 chicks fledged. This gave a productivity of
0.65 chicks per pair, a slight increase of 1.6% on 2016 (0.64) and 5.8% lower than the ten-year mean (0.69
±s.e.0.01); this is 12.2% lower than the mean since 1998 (0.74 ±s.e.0.02) when we began monitoring
productivity. Ringing and re-trapping birds was again successful, with two individuals of more than 30
years old. One individual captured in 2017 was originally ringed in 1984 and was not handled again until
it was re-caught on 18 August 2017, 33 years and 80 days after being ringed. The bird was ringed as an
‘adult’ so it’s actual age when ringed is unknown, other than it was probably more than four years old.

A sample of young were weighed and measured weekly during the nesting season, from hatching to
fledging. The mean fledging wing-length was 228.10mm, the smallest being 214mm and the largest
240mm. The mean fledging weight was 420.48g, 10.7% lighter than 2016’s 471.04g, the lightest being 310g
and the heaviest being 503g.

7 Acknowledgments

Thanks are due to Natural Resources Wales for providing funding for the productivity monitoring to be
completed.

8 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.

Stansfield. S.D. 1999. Breeding birds in 1998. Rep. Bardsey Bird Fld Obs. 42: 52-58.
Stansfield. S.D. 1998 - 2016. Reports to Natural Resources Wales. Unpublished.
Stansfield, S. and Carter, M. 2017. Manx Shearwater Studies: productivity, ringing and chick growth. Rep.

Bardsey Bird Fld. Obs. 60:159-169.
Walsh, P.M. et al. 1995. Seabird Monitoring Handbook for Britain and Ireland. JNCC/RSPB/ITE/Seabird

Group, Peterborough.

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Storm Petrel studies Perfect

2017 Storm Petrel population census

Ephraim Perfect

Photo – Storm Petrel © Ephraim Perfect

Chough breeding studies

1. Introduction

The colony of Storm Petrels on Bardsey remained undiscovered until relatively recently; the first evidence
of successful breeding was not proven until 2001 (Stansfield, 2001); however, eggs were noted on three
occasions previously. The number of breeding adults is minimal when compared to larger colonies in
Britain, but since we do not have a robust and up-to-date population estimate, most of our data is
conjecture. Storm Petrel is currently identified by the International Union for Conservation of Nature as
being of Least Concern due to its high total number. However, the Storm Petrel is an amber-listed species
in the fourth review of Birds of Conservation Concern (BoCC) due to concern about declining breeding
populations (Eaton et al., 2015). Therefore, an estimate of the breeding population on Bardsey is of vital
importance to seabird monitoring on both an island scale and a national one. Up until now a complete
census of the Bardsey Storm Petrel population has been almost on an ad-hoc basis, but hopefully
following this paper will become a yearly or biannual occurrence, to help us understand the fluctuations
of the population on Ynys Enlli.

The last thorough census was undertaken in 2004 when an estimated 111 pairs were counted (Stansfield,
2004), showing an increase compared to previous years. However, 2004 was the first year that concerted
efforts were put in and a correction factor actually applied to the final number of responses. The areas
surveyed this year were all on the East Side, bar one site, Bae Nant. The other sites surveyed were the
Scree Slope, Bau Felen, Seal Cave, south of Seal Cave, Briw Gerrig and Pen Cristin.

Data from this project is used by Natural Resources Wales (NRW) to assess the health of the population,
the data are also fed into the Seabird Monitoring Programme (SMP) established in 1986. SMP is an
ongoing annual programme to monitor 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 survey methodology laid out in this paper is largely based on the survey method used on Skokholm
and Skomer (Wood et al. 2017), to make the data as comparable as possible and to get a robust
population estimate.

2.1 Population census

The exact census techniques used for the previous Storm Petrel censuses on Bardsey was not outlined in
the earlier papers, only that diurnal visits were made to record responses to voice playback. The data
are therefore unfortunately not directly comparable to previous counts, but may be comparable to
different sites.

Every Storm Petrel site within the census area is counted, with two separate counts made simultaneously.

The survey has three main components:

1. Identify potential nest sites for Storm Petrels by i) locating known breeding sites from previous surveys,
and ii) mapping out suitable habitats (rock fall, boulder scree, rock walls, etc.)

2. Carry out playback survey of potential breeding sites. A single diurnal visit was made to all suitable
sites across the island, and a recording of a male purr call, recorded on Skokholm in 2010 by two
members of the British Wildlife Recording Society, was played at approximatively 75dB for 10 seconds at
each potential breeding crevice (Sutcliffe and Vaughan, 2010) using an iPod (iLuv iSP110 Speaker, mp3
track on iPod). The same call was used, as call variation is known to have an influence on the probability
of response to playback in Storm Petrels).

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3. These two components provided a total number of responses of Storm Petrel to voice playback, which
were extrapolated to the number of Apparently Occupied Sites (AOSs) by the application of a
correction factor, based on the calibration of rockfall habitat on Skokholm and Skomer (Wood et al.
2017).

Count 1 – Responses to voice playback.

Using speakers and an iPod, recorded calls of Storm Petrels were played into every crevice of each site
and the number of responses across each site was tallied.

Count 2 – The number Minimum Active Sites (MASs).

An examination of the burrow was made to determine whether it was apparently occupied and active;
this included looking for any signs of presence of droppings in and around the burrow, eggs and a strong
unique aroma emitted by Storm-petrels. If any of these signs were detected, the burrow was considered
to be apparently active, and was added on to the number of responses to get the MASs.

2.2 Correction factors

The breeding population of Storm Petrels on Bardsey is too small to be able to calibrate our own response
rate to voice playback at this stage. For this reason, the national response rate between 1.3 and 1.5
(JNCC) was used in the 2004 population census. However, following the results of the Skokholm and
Skomer census in 2016 (Wood et al. 2017) the use of a correction factor as low as 1.4 is thought to be
predicting a vast underestimate of the actual breeding population. Therefore, for this and hopefully
future population censuses a response rate calculated from the mean of Skokholm and Skomer’s rockfall
calibration sites was used (4.55, 2.44), giving a correction factor of 3.495 (Wood et al. 2017). For the
purpose of comparing to past population censuses, a figure based on the national correction factor was
also calculated in this paper (1.4).

2.3 Census time and dates

Matching the time of the Skokholm and Skomer census, the survey took place between 20 June and 10
July 2017 (Wood et al. 2017), which falls within the peak incubation period of Storm Petrels. Any areas
which would cause severe disturbance to auks and nesting seabirds were surveyed on or from 26 July
2017. So as to keep results as constant as possible, the surveying was done between 0600 and 1200
(Ratcliffe et al. 1998).

2.4 Census area

With a population of Storm Petrels as small as Bardsey’s, a total census of the entire island would be
wasted effort. Also, unlike Manx Shearwaters, they are not recorded to breed across the entire island.
Therefore, earlier in the season, before the census was to take place, visits were made to identify suitable
habitat for breeding Storm Petrels.

The areas surveyed in 2017 included sites on the East Side and Nant. The following sites were surveyed;
Bae Nant, the Scree Slope, Bau Felen, Seal Cave, south of Seal Cave, Briw Gerrig and Pen Cristin. The
surveyed area generally comprised of steep sloping ground with minimal vegetation and vertical cliff
edges. This terrain limited surveying to dry and calm days for safety reasons.

Chough breeding studies

3. Survey effort

The census for 2017 was completed in just under three weeks and took a total of 19 days to complete.
Most of the time two or more surveyors worked together to try and complete the survey in the time-frame
allowed for this census, and for safety purposes on the steeper slopes. Due to the topography of the
terrain, surveying could only take place on days of fair weather, without strong winds or rain.

4. Results
4.1 2017 Population census

In the 2017 census, the total number of responses to voice playback were 50 individuals exactly, and a
further 11 sites were deemed to be MASs. One bird responded in Bae Nant, 25 responded from the Scree
Slope with a further five MASs, ten responded from Bau Felen, 11 responded from Seal with a further three
MASs, three responded from the south of Seal Cave, only one MAS was noted from Briw Gerrig and
another at Pen Cristin. Concluding a total of 50 responses and 61 MASs.

Table 1. 2017 Storm Petrel census totals

Responses MASs % of Responses Responses
responses at x 3.495 x 1.4
MASs

Total 50 61 81.97 175 70

Table 2. Comparison of final results in the 2004 census with the 2017 census

Number of responses 2004 2017 % change
MASs 79 50 -36.71
79 61 -22.78
Playback responses (JNCC, -36.71
Stansfield 2004) 79(x1.4)=111 50(x1.4)=70
57.66
Playback responses (JNCC 79(x1.4)=111 50(x3.495)=175
and Wood et al. 2017) -36.71
79(x3.495)=276 50(x3.495)=175
Playback responses (Wood
et al. 2017)

4.2 Population census 2004-2017

Over the three-week period in the summer of 2017 all suitable areas for Storm Petrels were surveyed. The
total number of MASs counted in the 2017 season was 61. Of these 50 (81.97%) responded to voice
playback. Using the correction factor of 3.495 we get an estimation of 175 Apparently Occupied Sites
(AOSs). However, this figure is not comparable with previous estimates as a different correction factor
and different methodology was used. Instead we must compare the 2004 census, by extrapolating the
number of responses by the national correction factor between 1.3 and 1.5 (JNCC) used in the previous
census. When applying the correction factor of 1.4 (JNCC, Stansfield 2004) we get an estimated 70 AOSs,
which is a decrease of 36.94% since the last census in 2004, which predicted 111 AOSs. In comparison,
the three main sites of the 2004 population census saw a marked decrease in number. Bau Felen only
produced 10 responses compared to 29 (65.52%), Seal Cave produced 11 responses compared to 26
(57.69%), and no birds responded at Briw Gerrig compared to 24 in 2004! Even more worrying is the fact

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that the Scree Slope, which produced the largest number of responses in 2017 was not surveyed in 2004.
This alone suggests a clear decrease in numbers of breeding Storm Petrels on Bardsey.

However, because the methodology of the previous census differed in multiple aspects, the data is not
directly comparable. A mixture of variabilities such as duration, volume and origin of the playback
recording as well as the date and time that the census took place can easily produce large variations
in the final results (Ratcliffe et al. 1998). Not only this, but also the fact that multiple visits were made
during the 2004 census meant that clearly more AOSs would have been picked up, compared to the
single visit made to each site during the 2017 census. Therefore, a decrease in the Storm Petrel population
on Bardsey is not the definitive conclusion, but is perhaps the general deduction.

Table 3. Population change from 2001-2017

2001 total Responses Responses x 3.495 Responses x 1.4
2002 total 10 35 14
2003 total 26 91 36
2004 total 36 125 50
79 276 111

2017 total 50 175 70

2001-02 change 16 56 22
2002-03 change 10 34 14
2003-04 change 43 151 61
2004-17 change -29 -101 -41
2001-17 change 40 140 56

2001-02 % change 160.00 - -
2002-03 % change 38.46 - -
2003-04 % change 119.44 - -
2004-17 % change -36.71 - -
2001-17% change 400.00 - -

5. Discussion

Storm Petrels are a difficult species to census due to their erratic behaviour towards voice playback,
especially because response rates within the same colony are thought to differ from year to year.
Furthermore, response rates can differ drastically between colonies due to a host of different factors,
including: effects of the environment, colony location, playback equipment, etc. (Ratcliffe et al. 1998).
Though not proven, vocal learning could also cause problems, leading to localised variation with
different colonies thought to have ‘dialects’, which would potentially alter response rates between
colonies using the same recording.

In the latter part of the census, prolonged bad weather and a lack of manpower at times made
completing the census in the given time frame impossible. Unfortunately, the last visit to the Scree Slope
was not made until 13th July. However, this did not fall significantly outside the allotted time frame and
therefore should not have effected results in any major way.

Despite these potential shortcomings, the 2017 census is thought to be the most reliable estimate of
Bardsey’s breeding population of Storm Petrels, because previously unknown breeding sites were
located, all potential breeding sites were covered, and a more thorough census was undertaken with
more stringent guidelines set out, to make the data as accurate and comparable as possible. By

Chough breeding studies

adopting a correction factor developed on Skokholm and Skomer, a reasonably close colony, and using
the rockfall habitat-specific data, the response rate was far more relevant than using the national
correction factor which has probably given an underestimation of the breeding population of Bardsey
up until now.

Unfortunately, with a new census technique having been developed this year, the previous data
recorded on the island in 2001, 2002, 2003 and 2004 is no longer comparable. Future population censuses
will be directly comparable with the 2017 census, since the technique is now clearly laid out and all the
raw data is given.

From the censuses conducted during the past two decades, it is clear that the East Side of Bardsey holds
the overwhelming majority of the Storm Petrel population on the island, with only one response/colony
found on the western side of the island. The East Side contains sub-colonies which are all found by rockfall
sites, either in boulder fields or small rocky crags.

6. Summary

The 2017 season saw the first thorough population census of Storm Petrels on Bardsey. There was an
overall decrease in the population since the last survey carried out in 2004. The decrease in population
between the two surveys was 36.71%, in real terms this equates to 29 responses. The total number of
responses on the island this year stands at 50, compared to 79 13 years ago, a worrying decrease, but
perhaps not wholly reliable.

The new census technique and correction factor estimates the current breeding population of Storm
Petrels on Bardsey to be 175 AOSs, compared to 111 recorded in 2004, using a different method and
correction factor.

6. References

Eaton, M., Aebischer, N., Brown, A, Hearn, R., Lock, L., Musgrove, A., Noble, D., Stroud, D. and Gregory, R.
2015 Birds of Conservation Concern 4: the population status of birds in the UK, Channel Islands
and Isle of Man. British Birds 108 708–746

Ratcliffe, N. 1998. Storm-petrels. G. Gilbert, D. W. Gibbons, and J. Evans (editors). Bird Monitoring Methods:
a manual of techniques for UK species. RSPB, UK.

Ratcliffe, N., Vaughan, D., Whyte, C. and Shepherd, M. 1998. Development of playback census methods
for Storm-petrels Hydrobates pelagicus. Bird Study 45: 302-312.

Stansfield, S. 2001. Breeding birds Ynys Enlli and Ynysoedd Gwylan in 2000. Rep. Bardsey Bird Fld. Obs. 45:
48-54.

Stansfield, S. 2002. Breeding birds Ynys Enlli and Ynysoedd Gwylan in 2001. Rep. Bardsey Bird Fld. Obs. 46:
52-61.

Stansfield, S. 2003. Breeding birds Ynys Enlli and Ynysoedd Gwylan in 2003. Rep. Bardsey Bird Fld. Obs. 47:
59-70.

Stansfield, S. 2004. Breeding birds Ynys Enlli and Ynysoedd Gwylan in 2004. Rep. Bardsey Bird Fld. Obs. 48:
73-85-75.

Wood, M.J. et al. 2017. Repeat playback census of breeding Storm Petrels Hydrobates pelagicus on the
Skokholm and Skomer SPA in 2016. NRW Evidence Report 190.

8. Acknowledgments

A special thank you to all volunteers involved with this project for their outstanding efforts working in
challenging conditions to complete the survey. Thanks must also go to Natural Resources Wales for
providing funding for this year’s survey.

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