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Running head: Device and harness effects on seabirds
Contrasting effects of GPS device and harness attachment on adult survival
in the Lesser Black-backed Gull Larus fuscus and Great Skua Stercorarius
skua
CHRIS B. THAXTER1*, VIOLA H. ROSS-SMITH1, JACQUIE A. CLARK1, NIGEL A. CLARK1,
GREG J. CONWAY1, ELIZABETH A. MASDEN2, HELEN M. WADE2,3, ELIZA H.K. LEAT4,
SHEILA C. GEAR5, MIKE MARSH6, CHRIS BOOTH7, ROBERT W. FURNESS2, STEVE C.
VOTIER8 & NIALL H.K. BURTON1
1
British Trust for Ornithology, The Nunnery, Thetford, Norfolk IP24 2PU, UK.
Environmental Research Institute, North Highland College, University of the Highlands and Islands,
Ormlie Road, Thurso, KW14 7EE, Scotland.
3
MacArthur Green, 95 South Woodside Road, Glasgow G20 6NT, UK.
4
College of Medical, Veterinary and Life Sciences, Graham Kerr Building, University of Glasgow,
Glasgow G12 8QQ, UK.
5
Magdala, Foula, Shetland ZE2 9PN.
6
Landguard Bird Observatory, View Point Road, Felixstowe, Suffolk, IP11 3TW. UK.
7
34 High Street, Kirkwall, Orkney KW15 1AZ.
8
Environment and Sustainability Institute, University of Exeter, Penryn Campus, Penryn, Cornwall
TR10 9EZ, UK
2
*Corresponding author:
Email: chris.thaxter@bto.org
The use of telemetry has become an important means for studying the biology and ecology of
animals. However, the impact of tracking devices and their method of attachment on different
species across multiple temporal scales has seldom been considered. We compared the
behavioural and demographic responses of two species of seabird, Lesser Black-backed Gull
Larus fuscus and Great Skua Stercorarius skua, to a GPS device attached using a crossover
wing harness. We used telemetry information and monitoring of breeding colonies to
compare birds equipped with a device and harness (hereafter ‘tagged’ birds) and control birds
without an attachment. We investigated whether tagged birds could have, in the short-term,
lower breeding productivity and, in the longer term, lower over-winter return rates (indicative
of over-winter survival) than controls. For Great Skua, we also tested whether territory
attendance within the breeding season differed between tagged and control birds. In
accordance with previous studies on Lesser Black-backed Gull, we found no short-term
impacts on breeding productivity or long-term impacts on over-winter return rates. In
contrast, for Great Skua, there was no evidence for impacts of the device and harness on
territory attendance or breeding productivity, but there was strong evidence for reduced overwinter return rates. Results of a previous study on Great Skua using a different (body) harness
design were in accordance with these findings. Consequently, a device attached using a wing
harness was considered suitable for long-term deployment on Lesser Black-backed Gull, but
not for Great Skua.
Keywords: Breeding productivity, foraging behaviour, return rate, seabird, telemetry, wing
harness
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53
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57
58
59
60
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62
WORD COUNT
Main text: 7 716
Summary and key-words: 255
Introduction: 881
Methods: 1 673
Results: 570
Discussion: 2 223
Acknowledgements and references: 1 855
Tables/Figures: 518
Appendices: 2 870
2
63
Attaching devices to animals may affect their behaviour, physiology, reproduction and
64
survival (Murray & Fuller 2000, Wilson & McMahon 2006, Walker et al. 2012). Among
65
birds, the most serious deleterious effects include deterioration in body condition and
66
foraging behaviour, compromised energetics and direct physical injury, which can then lead
67
to reduced nesting success (e.g. productivity and propensity to breed) and lower adult
68
survival (see reviews by Barron et al. 2010; Vandanabeele et al. 2011). Impacts may arise
69
from initial discomfort, subsequent abrasion, compromised plumage insulation, or the overall
70
inability to accommodate any increased weight or drag leading to reduced manoeuvrability
71
(Calvo & Furness 1992; Vandanabeele et al. 2011). Individual species can vary considerably
72
in their responses to device shape and weight, the attachment method used and the length of
73
deployment (Barron et al. 2010, Vandenabeele et al. 2011, Bridge et al. 2013). Therefore,
74
while some studies have reported effects, many others have found no such deleterious
75
impacts (Vandanabeele et al. 2011; see for example Davis et al. 2008, Lislevand & Hahn
76
2013). Based on the relatively limited number of comparative studies and the wide range of
77
metrics and effects reported, it is therefore difficult to make broad generalisations about the
78
potential impact of a device and its attachment.
79
Bird-borne telemetry has been extensively used to study the ecology of bird species
80
during their breeding seasons. For such short-term studies, telemetry devices may be
81
attached temporarily to feathers (Wilson et al. 1997). However, to study birds outside the
82
breeding season, the device must remain in place through periods of feather moult, dictating a
83
need for alternative attachments (Kenward 1987). As a result, monitoring species across their
84
full annual cycle remains a key research priority for many species (Marra et al. 2015). Long-
85
term attachments can be achieved through the use of leg ring attachments, e.g. for geolocators
86
(Bridge et al. 2013, Bustnes et al. 2013), harness mounting, or surgical implantation (Meyers
87
et al. 1998, White et al. 2013). Harnesses have been used previously on waterbirds (e.g. Pietz
3
88
et al. 1993), seabirds (e.g. Falk & Møller 1995, Nicholls et al 2002, Manosa et al. 2004,
89
Shamoun-Baranes et al. 2011, Klaassen et al. 2012) and birds of prey (e.g. Britten et al.
90
1999, Peniche et al. 2011, Steenhof et al. 2011, Sergio et al. 2015), as well as on much
91
smaller migrant birds (e.g. Bridge et al. 2013). However, the reported effects of harnesses on
92
different species have also varied widely (e.g. Peniche et al. 2011, Sergio et al. 2015).
93
Consequently, there is a pressing need to assess the suitability of particular harnesses and
94
devices for different species across different time scales.
95
In this study, we compare the responses of two species of seabird, Lesser Black-backed
96
Gull Larus fuscus and Great Skua Stercorarius skua, to the attachment of the same
97
type/model of GPS device using a Teflon crossover wing harness, within the breeding season,
98
and across years. The combined effects of the GPS device and the wing harness are assessed.
99
We compared information from birds equipped with a GPS device and wing harness
100
(hereafter ‘tagged birds’), with information from groups of control birds without any
101
attachment. We investigated whether in comparison to control birds within the breeding
102
season (short-term) tagged birds exhibited lower breeding success, and between consecutive
103
breeding seasons (long-term) tagged birds may have lower ‘return rates’. In addition, for
104
Great Skua, we tested whether impacts on foraging ability within the breeding season could
105
lead to alterations in the territory attendance of tagged birds in comparison to controls.
106
Recent studies on Lesser Black-backed and Herring Gull Larus argentatus have used either
107
the same device and type of wing harness as in this study (e.g. Ens et al. 2008, Camphuysen
108
2011; Shamoun-Baranes et al. 2011), or a slightly heavier device and similar harness
109
(Klaassen et al. 2012) with no apparent adverse effects recorded for breeding success
110
(Camphuysen 2011), or for over-winter return rates, which are indicative of over-winter
111
survival (Camphuysen 2011, Klaassen et al. 2012). Given that the device type and wing
112
harness attachment in this study was equivalent to previous studies, we predicted no effects
4
113
on Lesser Black-backed Gull on these short- or long-term measures. Previous studies on
114
Great Skuas found no detrimental short-term effects on the duration of foraging trips during
115
breeding when a radio-tracking device (10 g) was attached to a central pair of tail feathers
116
(Votier et al. 2004, 2006). In a separate study using a full body (‘back-pack’) harness to
117
attach platform terminal transmitters to Great Skuas in Shetland, there were no short-term
118
effects recorded on nest survival, territory attendance, body condition or foraging trip
119
duration within the breeding season of marking (Crane 2006, Furness et al. 2006). However,
120
no tagged birds returned and successfully bred the following season, suggesting their
121
condition may have been compromised (Furness et al. 2006). The full body harness uses a
122
central breast strap and sometimes a neck loop, whereas the wing harness uses two loops
123
under the wings, with no central breast strap. The wing harness therefore has a smaller
124
amount of skin contact than the body harness, giving a non-constricting fit that better
125
accommodates changes in body size (Thaxter et al. 2014). However, the effects of using a
126
wing harness to attach a tracking device to Great Skuas have not previously been evaluated.
127
128
METHODS
129
Study sites and periods
130
Lesser Black-backed Gulls were studied between 2010 and 2013 at Orford Ness, part of the
131
Alde-Ore Special Protection Area (SPA), Suffolk, UK (52°06’N, 1°35’E), a declining colony
132
(23 000 Apparently Occupied Territories AOTs in 2000, 550-640 AOTs between 2010 and
133
2012, JNCC Seabird Monitoring Programme database). Great Skuas were studied between
134
2011 and 2013 at (i) Foula SPA, Shetland, UK (60°08’N, 2°05’W), a declining colony (2 293
135
AOTs in 2000, 1 657 AOTs in 2007; JNCC Seabird Monitoring Programme database), and
136
(ii) Hoy SPA, Orkney, UK (58°52’N, 3°24’W), also a declining colony (1 973 AOTs in 2000,
137
1 346 AOTs in 2010; JNCC Seabird Monitoring Programme database). Both species were
5
138
studied during the breeding season. This was defined as the period from the first return of
139
individuals to the colony during pre-breeding, to when chicks fledged, the latter either
140
observed or estimated using species’ breeding durations (Robinson 2005) and hatching dates
141
(Lesser Black-backed Gull, 15 February to 1 August; Great Skua, 10 April to 15 August).
142
143
Catching and harness attachment methods
144
Adult birds were captured at the nest during incubation using a wire mesh walk-in cage trap
145
(Bub 1991) for Lesser Black-backed Gulls and a remote-controlled nest-snare trap for Great
146
Skuas. In 2010, solar-powered data storage GPS devices (weighing 19 g, Bouten et al. 2013)
147
were attached to four Lesser Black-backed Gulls at Orford Ness using a crossover Teflon®
148
wing harness (see Thaxter et al. 2014 for more details). A small piece of neoprene was
149
attached underneath the tag to provide additional comfort to the bird (Thaxter et al. 2014). In
150
2011, GPS devices of the same type and model were attached to a further 14 Lesser Black-
151
backed Gulls and 20 Great Skuas (ten each in Foula and Hoy) using the same wing harness
152
design.
153
As recommended by Phillips et al. (2003) the device and harness was <3% body mass
154
(max. 2.9% Lesser Black-backed Gulls and 1.8% Great Skuas). Upon catching, all birds were
155
fitted with uniquely identifiable colour rings to enable subsequent field identification. Lesser
156
Black-backed Gulls were sexed using head and bill length measurements (Coulson et al.
157
1983, Camphuysen 2011) recorded along with body mass on capture. Great Skuas were sexed
158
using DNA from feathers (Griffiths et al. 1993), previous copulation behaviour, or within-
159
pair relational size from simultaneous viewing of both pair members. Sample sizes of male
160
and female Lesser Black-backed Gulls were sufficient to allow comparative analyses, but too
161
few Great Skuas were sexed to provide meaningful assessment (Appendix S1). Two ringing
162
teams undertook the work (one for Orford Ness and Foula, and one for Hoy) with procedures
6
163
standardised across teams. The mean bird handling time was 26±10 SD mins (range 15–46
164
mins) for Lesser Black-backed Gull and 25±4 SD mins (range 19–32 mins) for Great Skua.
165
166
Control groups
167
To evaluate effects of the device and wing harness, a separate group of birds was also
168
monitored during the 2011 breeding season. These ‘control’ birds had no device and harness
169
and were captured at the nest in the same vicinity of colonies as tagged birds using the
170
techniques described above. Each bird was then fitted with a colour ring to enable them to be
171
individually identified in the field (Lesser Black-backed Gulls, n = six and 47 in 2010 and
172
2011 respectively; Great Skuas, Foula: n = 10, Hoy: n = 10). In addition, the nests of separate
173
groups of unmarked birds (Lesser Black-backed Gulls, 21 nests; Great Skuas at Foula, 37
174
nests) also located in the same vicinity as those nests of other marked birds were followed
175
(‘other’ nests in Table 1). The choice of control group is an important aspect to consider in
176
any tagging study (Authier et al. 2013), and we therefore attempted to minimise any
177
locational bias, through consideration of nesting density and possible behavioural
178
differences. However, the nests of unmarked birds were located across a slightly wider area,
179
with some nests potentially around colony edges or in sub-optimal locations, which may have
180
contributed to a lower observed breeding productivity compared to the colour-ringed controls
181
(Table 1, Appendix S1). Nevertheless, these nests were subsequently pooled with the colour-
182
ringed controls to give a larger control group sample size (Appendix S1).
183
184
Assessment of device and harness effects
185
Breeding productivity
186
During 2011 we examined whether (i) the number of eggs hatched, (ii) the number of chicks
187
present up to mid-July (Lesser Black-backed Gull, 9 July; Great Skua, 15 July), and (iii) the
7
188
number of fledglings per nest (for Great Skua at Hoy only) differed between tagged and
189
control birds. Monitoring of Lesser Black-backed Gull nests was undertaken weekly (5 May–
190
9 July 2011). However, it was not possible to monitor the number of chicks present later in
191
the season or in turn the number that fledged as the origins of many chicks could not be
192
determined, due to their greater mobility with increasing age. Consequently, 9 July
193
represented the last point in the season when breeding productivity could be assessed for the
194
species. At this stage, mean chick age across nests was between 6±8 d (range, 7-35 d) and
195
14±9 d (range 1-26 d) after hatching (lower and upper values reflecting the uncertainty in
196
estimated hatching dates from the above nest monitoring protocol). The nests of Great Skuas
197
were monitored on Foula with daily visits (10 June–15 July 2011) and on Hoy with weekly
198
visits (11 June–15 August 2011). Where gaps in the monitoring of nests were greater than a
199
single day, breeding productivity was expressed by mean minimum and mean maximum
200
estimates of numbers of eggs or chicks surviving to provide a level of error (see Table 1). For
201
Great Skuas at Foula, nest monitoring allowed a final estimate to be made of the number of
202
chicks present up to 15 July (when mean chick age was 20±9 d, range 1-35 d). However,
203
nests at Hoy were followed for a longer period into chick-rearing (mean chick age lower
204
estimate, 26±22 d range 1-64 d, upper estimate, 32±21 d range 7-64 d), allowing fledging
205
success to be assessed. Hatching dates and the duration of monitoring were no different
206
between tagged and control groups (Appendix S2).
207
208
Over-winter return rates
209
Comparison of the over-winter return rates of tagged and control birds was based on re-
210
sightings of marked birds. For Lesser Black-backed Gulls, return rates are indicative of over-
211
winter survival (Camphuysen 2011, Klaassen et al. 2012). Similarly, Great Skuas are highly
212
faithful to their breeding colonies and normally return to their nest site within the colony
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213
(Klomp & Furness 1992, Catry et al. 1997), hence permanent emigration is considered
214
negligible and return rates are likely to reflect true survival rates. Return rates for the four and
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14 tagged Lesser Black-backed Gulls were compared with those of six and 47 colour-ringed
216
control birds in 2010 and 2011, respectively. Similarly, return rates for the 20 tagged Great
217
Skuas (10 at each of Foula and Hoy) were compared with those of the equivalent number of
218
colour-ringed controls at each colony. Re-sighting effort was conducted during the breeding
219
season in 2011, 2012 and 2013. A second estimate of over-winter return rates incorporating
220
information received through the tracking system (Bouten et al. 2013) was also made. This
221
combined measure, as it increased re-sighting probabilities, provided a more accurate
222
estimate for comparisons with previous estimates of annual survival (e.g. Wanless et al.
223
1996). In each year, for both species, re-sighting effort included searches of breeding
224
territories, bathing locations, and gatherings of birds at the colony, for example at open
225
shingle loafing areas for gulls (Marsh 2013) and non-breeding club-sites for Great Skua. The
226
respective tracking systems at each of the sites were also used to check for returning tagged
227
birds.
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229
Breeding season territory attendance
230
For Great Skuas, the presence/absence of tagged and control birds in breeding territories was
231
monitored at Foula in 2011 through spot-checks conducted from vantage points as a measure
232
of foraging effort (Catry & Furness 1999). These data were then used to compare the
233
probability of tagged and control birds being present on their territories. Watches were
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conducted 2-3 times per day between 3 May and 15 July in a randomised pattern covering
235
morning (05:01–11:00 BST), midday (11:01–17:00 BST) and late afternoon/evening (17:01–
236
23:00 BST) periods. Different vantage points were used to cover the breeding territories
237
(average watch duration, 2.0±0.9 h). If birds could not be identified (e.g. due to obstructed
9
238
views), then these data were excluded from analysis. Similar monitoring of Lesser Black-
239
backed Gulls was not possible as there were no vantage points that gave clear views due to
240
tall vegetation.
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Statistical analysis
243
To test whether breeding productivity and territory attendance were different between tagged
244
and control birds, we used general linear models (GLMs), with Poisson and binomial errors,
245
respectively. Response variables of the number of eggs hatched, the number of chicks present
246
per nest and presence or absence in the territory were investigated as separate analyses, with
247
fixed effects for ‘group’ (tagged and control), ‘sex’ and ‘time of day’ also included. For Great
248
Skuas, a pooled analysis for Foula and Hoy was conducted including ‘colony’ as a fixed
249
effect to control for potential site differences. However, in the case of fledging success, this
250
could only be investigated for birds at Hoy. Differences in over-winter return rates between
251
tagged and control birds (fixed effect of ‘group’) were examined using binomial
252
(presence/absence response) general linear models (GLMs, Great Skua), or GLMMs (Lesser
253
Black-backed Gull). GLMMs included bird ID as a random effect to account for repeat
254
measurements for individuals in different years, controlling for additional fixed effects of
255
‘sex’ and ‘year’. Significant terms in all GLMs and GLMMs were selected through
256
backwards step-wise deletion. Analyses were conducted using R v 2.5.11 (R Core Team
257
2013). We conducted a power analysis for all tests, presenting the effect size (Cohen’s d) for
258
each test of ‘group’. Appendix S1 includes full discussions of power, effect size, and sample
259
size for all measures presented from these analyses, from which we draw conclusions
260
regarding the suitability of initial sample sizes.
261
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RESULTS
264
Breeding productivity
265
For Lesser Black-backed Gull in 2011, there were no differences between tagged and control
266
groups in the number of eggs hatched (dDev = -1.59, df = 1, P = 0.207, d = 0.07) and number
267
of chicks present per nest up to 9 July (dDev = -0.28, df = 1, P = 0.598, d = 0.68, see Table 1,
268
Appendix S1). For Great Skua in 2011, there were no differences between Foula and Hoy in
269
either the number of eggs hatched (dDev = -0.13, df = 1, P = 0.715) or number of chicks
270
present per nest up to 15 July (dDev = -0.01, df = 1, P = 0.910). Across colonies there were
271
no differences between tagged and control groups in the number of eggs hatched (dDev = -
272
0.27, df = 1, P = 0.601, d = 0.22) or number of chicks present per nest up to 15 July (dDev =
273
-0.40, df = 1, P = 0.530, d = 0.19). On Hoy, there were no differences in the number of chicks
274
fledged per nest between tagged and control nests (dDev = -0.20, df = 1, P = 0.654, d = 0.22;
275
Table 1).
276
277
Breeding season territory attendance
278
For Great Skuas at Foula in 2011, spot checks of nests showed that there was no difference in
279
territory attendance between tagged and control birds (dDev = -1.32, df = 1, P = 0.250, d =
280
0.06; probability of presence: control, 74.0±44.0%, tagged, 71.1±45.4%).
281
282
Over-winter return rates
283
For Lesser Black-backed Gull, there were no differences in over-winter return rates between
284
tagged and control groups (χ21 = 2.00, P = 0.157, d = 0.25), after controlling for potential
285
differences between years (χ21 = 1.70, P = 0.427) and sexes (χ21 = 0.09, P = 0.754). The
286
return rates for birds tagged in 2011 were 79% (11/14 birds) for 2011–12 and 71% for 2012–
287
13 (10/14 birds), compared to return rates of 53% (25/47 birds) and 64% (16/25 birds) for
11
288
control birds over the same periods respectively (Table 2). Including tagged Lesser Black-
289
backed Gulls that were recorded through the GPS system but not re-sighted, gave more
290
complete return rates of 14/14 birds (100%) and 13/14 birds (93%) for the 2011 cohort for
291
periods 2011–12 and 2012–13 respectively, and a measure of 94% (17/18 birds) combining
292
the 2010 and 2011 cohorts for the 2011-12 period (Table 2). Two tagged Lesser Black-
293
backed Gulls and five control birds were recovered dead at the colony in 2013 due to fox
294
predation, and another tagged bird was recovered outside the breeding season in Portugal
295
(Appendix S3).
296
By contrast, there was a highly significant difference between tagged and control group
297
return rates for Great Skua (χ21 = 44.69, P < 0.001, d = 0.70, Appendix S1). Return rates in
298
2012 of birds tagged in 2011 were just 10% (1/10 birds) for birds from Foula and 0% (0/10
299
birds) for those from Hoy, while the one tagged bird that returned in 2012 was not seen in
300
2013 (Table 2). For control birds, return rates over 2011-2012 and 2012-2013 were 100%
301
(10/10) and 56% (5/9) for Foula, and 80% (8/10) and 75% (6/8) for Hoy respectively (Table
302
2). Two tagged Great Skuas (one from Foula and Hoy respectively) were recovered dead
303
outside the breeding season in Germany and Portugal and another was seen on migration off
304
the Devonshire coast (see Appendix S3).
305
306
DISCUSSION
307
For both Lesser Black-backed Gull and Great Skua, results indicated that the GPS device
308
attached using a wing harness had no deleterious effects on breeding productivity within the
309
season of capture. Similar findings have previously been recorded for Lesser Black-backed
310
Gull in the Netherlands when using the same GPS and harness attachment (Camphuysen
311
2011). From a separate study of Great Skuas in Shetland using devices attached with a body
312
harness, nest survival rates in the breeding season after marking were no different between
12
313
tagged and control nests (Crane 2006; Furness et al. 2006). Furthermore, two South Polar
314
Skuas Stercorarius maccormicki, a close relative to the Great Skua, have also been studied
315
for up to 45 days during the breeding season using a leg-loop harness with no detrimental
316
impacts on breeding success (Mallory & Gilbert 2008). The GPS device and wing harness
317
used in this study also had no apparent deleterious effects on territory attendance of Great
318
Skuas at Foula. Similarly, at St. Kilda, UK, Votier et al. (2006) found that breeding Great
319
Skuas tagged using a GPS device attached to feathers spent on average 69% of their time at
320
the colony, this value being comparable to the probability of tagged (71%) and control birds
321
(74%) being present on their territories in our study. Crane (2006) also found that the territory
322
attendance of Great Skuas tagged using a body harness was no different to control birds.
323
For Lesser Black-backed Gulls, the GPS device and attachment had no apparent effect
324
on return rates, and thus their over-winter survival. The apparent annual survival rate of
325
Lesser Black-backed Gulls at Orford Ness (as derived from colour-ring sightings) varies
326
between years (from less than 50% to over 90%: Marsh 2013). However, the more complete
327
return rates of tagged birds to Orford Ness derived by also incorporating information received
328
through the tracking system (Table 2) were similar to previously reported annual survival
329
rates from sites elsewhere (91.3%, Wanless et al. 1996; 91%, Camphuysen & Gronert 2012).
330
Similar findings have been reported for Lesser Black-backed Gull at other colonies using the
331
same GPS and harness attachment (Camphuysen 2011). By contrast, the return rate of tagged
332
Great Skuas was much lower than that of controls, which was similar to a previously
333
estimated return rate of 88.8% (Ratcliffe et al. 2002). Although individuals may skip
334
breeding attempts, the recovery of two dead tagged Great Skuas and birds’ failure to return to
335
the colonies during the two years following the attachment of the device, suggests that
336
mortality was the most likely outcome. The migration routes of the Great Skua found dead in
337
Portugal and the additional bird seen off the Devonshire coast (Appendix S3) matched the
13
338
routes recorded previously from other studies using geolocation (Furness et al. 2006,
339
Magnusdottir et al. 2012). These results suggest that tagged Great Skuas had attempted
340
migration but encountered difficulties during this period. One bird spent the winter in the Bay
341
of Biscay region and made no attempt to migrate back to the colony, before its recovery the
342
following summer (Appendix S3). There have been no other published findings of the
343
specific effects of the wing harness on Great Skuas. However, the study of Great Skuas using
344
devices attached to Great Skuas with a body harness, found that no tagged birds returned and
345
successfully bred the following season (Crane 2006, Furness et al. 2006), which also suggests
346
a long-term effect for that attachment method.
347
In accordance with previous studies on Lesser Black-backed Gull, we thus found no
348
short-term impacts on breeding productivity or long-term impacts on over-winter return rates
349
and the device and wing harness were thus considered suitable for deployment across the
350
year. However, for Great Skua, although no deleterious impacts were apparent from the
351
device and wing harness during the breeding season, effects on apparent adult survival over
352
migration and wintering periods were catastrophic. Thus, deployment of the GPS device
353
using a wing harness was not suitable for long-term deployment for Great Skuas.
354
355
Limitations and considerations
356
Identifying suitable groups of control birds and adequate sample sizes of tagged birds are
357
important aspects to consider when assessing device and attachment effects. The chosen
358
control group may be unsuitable for causal inference, and small sample sizes cannot replicate
359
all characteristics of the total population (Baron et al. 2010, Authier et al. 2013). In this
360
study, the nests of unmarked control birds had lower productivity compared to colour-ringed
361
controls (see Appendix S1). This highlights that caution is needed when defining a
362
meaningful control group. Although our conclusions regarding the impacts of devices and
14
363
harnesses were no different no matter what combination of the two control groups was used
364
(Appendix S1), the wider group of unmarked nests may have potentially been derived from
365
more sub-optimal locations. Quantitative methods to define control groups are therefore
366
preferable to rule out such issues (Authier et al. 2013). Common to other tagging studies, we
367
also had restricted sample sizes of tagged birds and their nests to compare to controls.
368
However, power analyses revealed the observed effect sizes in nearly all statistical tests (with
369
the exception of Great Skua return rate) to be so small as to have little biological meaning.
370
While these analyses do not indicate there were no effects, they demonstrate how a very large
371
sample size (sometimes more than the number of birds available in the population, Appendix
372
S1) would be required to detect the observed effect.
373
374
Comparison of species ecology and behaviour
375
The reasons for the difference in over-winter return rates between Lesser Black-backed Gulls
376
and Great Skuas are unclear, but potentially may be due to species differences in the extent of
377
offshore habitat use during migration and winter, the potential compromise of insulation due
378
to tag impact on plumage, and/or differences in foraging costs, aggressive behaviour and/or
379
piracy. Lesser Black-backed Gulls use a range of terrestrial stopover sites outside the
380
breeding season (Klaassen et al. 2012), whereas Great Skuas are thought to remain at sea
381
(Furness 1987). Great Skuas may therefore be in flight for longer periods in maritime areas
382
than Lesser Black-backed Gulls. This may exacerbate any effects that the device and wing
383
harness may have on birds, and may have also increased the feather wear from the
384
attachment, possibly compromising insulation and foraging efficiency. Great Skuas roost at
385
night on the water, and this may be crucial in determining consequences of a reduction in
386
insulation.
15
387
For the two gulls predated by foxes, there was no sign of damage to the underlying skin
388
or contour feathers; there was some minor removal of down under the harness straps, but
389
otherwise the feather layer was completely intact. There was some evidence of feather
390
removal directly under the neoprene pad supporting the device, which created a bare patch
391
(56 x 25, and 47 x 24 mm, for the two birds respectively), but otherwise feather growth was
392
normal. Consequently, the use of this neoprene pad has since been discontinued. It is possible
393
that this may have compromised insulation, either through direct exposure of skin to the air or
394
penetration of water under plumage. However, the neoprene pad itself is likely to have
395
provided some level of insulation.
396
Although survival estimates do not indicate an impact of the device and harness on
397
Lesser Black-backed Gulls, any compromise of insulation may potentially be more
398
problematic for Great Skuas that use the pelagic environment outside the breeding season.
399
We had no information from the recovered Great Skuas to determine impacts on insulation.
400
However, one Great Skua returned to the colony without its device or harness. This bird was
401
re-trapped and showed no sign of any feather abrasion where the device had been. It is
402
unknown how long the device remained attached to the bird, and it was unclear how the
403
harness and device had been lost; the GPS device and harness remained on the bird at least
404
until 29 August 2011 when the last data transmission from the device was received. It is
405
clear, however, that this bird did not lose its attachment during the breeding season, with its
406
departure from the colony being similar to other tagged birds, including the two birds
407
recovered (Appendix S3). This further suggests that the effects of the device and harness for
408
Great Skua occurred during the non-breeding seasons.
409
In addition to the mass of devices and their attachments relative to body mass, wing-
410
loading is also a potentially important consideration in using bird-borne telemetry. Larger
411
birds with higher wing-loadings may not so easily accommodate extra mass within their
16
412
normal flight power requirements (Vandanebeele et al. 2011, 2012). Wing area was not
413
recorded in our study, but using available estimates from other studies (Lesser Black-backed
414
Gull, 0.195 m2 mean of males and females, Camphuysen 1995; Great Skua, 0.214 m2,
415
Pennycuick et al. 1990), alongside our observed masses of tagged birds (Lesser Black-backed
416
Gull, 0.851±0.085 kg, and Great Skua, 1.346±0.101 kg), we estimated that wing loadings
417
were 44% higher for Great Skua compared to Lesser Black-backed Gull (6.29 kg/m2 and 4.36
418
kg/m2, respectively). Furness & Tasker (2000) also considered Great Skuas (on a scale of 1 to
419
4) to have a higher foraging cost per unit of time (score of 3) compared to Lesser Black-
420
backed Gull (score of 2). Therefore, an increase in per unit foraging cost associated with the
421
attachment of a device could have had a greater relative impact on the energetics of Great
422
Skua compared to Lesser Black-backed Gull.
423
Great Skuas have also more powerful beaks than Lesser Black-backed Gulls, and may
424
be more likely to try and remove a device they are carrying, and perhaps even injure
425
themselves in the process. However, there was no evidence of any beak marks on the two
426
devices that were recovered. Great Skuas are also known to be mobbed frequently by other
427
birds, and fight between themselves. The GPS device could therefore be seen by other birds
428
as a target, which may then provoke aggressive attacks; the Great Skua recovered in
429
Germany was seen being mobbed by other birds before it died. Great Skuas are also more
430
reliant on piracy (kleptoparasitism) for feeding than most other seabirds (Furness 1987). If a
431
reduction in aerial agility or foraging efficiency affected piracy more than other foraging
432
tactics such as surface feeding or use of discards, then this could have more detrimental
433
impacts on Great Skuas than Lesser Black-backed Gulls. It is also possible that the impacts of
434
a device and harness could also differ within a species between colonies or years with
435
different ecological conditions. For instance, birds exposed to extremes of food shortage or
436
weather may be more susceptible to impacts that may not be evident under benign conditions.
17
437
The colony productivity of Great Skuas at both Foula and Hoy was low in comparison to
438
previous years (see Wade et al. 2014). However, Lesser Black-backed Gull productivity was
439
relatively low and variable between years over the period of study (Thaxter et al. 2015), and
440
for Great Skuas, no impacts were recorded during the breeding season.
441
442
Implications for further work
443
Advances in remote detachment methods may provide future solutions for studying Great
444
Skuas, allowing devices to fall off safely after one or two months; for example using weak
445
points in the attachment between the harness and the device or the use of harness materials
446
that will break under influence of UV-radiation. The attachment of a device for short-term
447
study during the breeding season is currently possible by attachment to feathers on the back
448
or tail. The device then falls off when the feathers are shed and so is less likely to have long-
449
term impacts. Such solutions could also be valuable in other species, where issues identified
450
at the outset may prevent longer term study. Until more suitable deployment methods are
451
developed, the most practical approach for studying the migration and longer-term
452
movements of Great Skuas and many other species is through a geolocator attached to a leg
453
ring (e.g. Magnusdottir et al. 2012).
454
This study demonstrates how species’ responses to harness attachments should not be
455
expected to be the same just because they share similar traits. For example, some species
456
exhibit marked changes in body shape throughout their annual cycle (e.g. Portugal et al.
457
2007), or have comparatively higher foraging costs (Vandanebeele et al. 2012). Behaviour at
458
particular life history stages, e.g. the relative use of different habitats, also needs to be
459
scrutinised closely, but equally, other aspects such as migration strategy or preferred feeding
460
tactics could be highly influential. The ecology, lifestyle, morphology and physiology of the
461
species therefore all need consideration before decisions on the shape and weight (Bowlin et
18
462
al. 2010), positioning (Thaxter et al. 2014, Vandenabeele et al. 2014) and attachment
463
methods of devices (e.g. this study) are taken. Consideration of any single species-specific
464
aspect in isolation or extrapolation based on particular comparable aspects of other species is
465
therefore discouraged. These decisions are not straightforward, but are particularly important
466
within the marine environment as the greater conductivity of water than air means any
467
compromised plumage insulation and heat loss will be magnified, and the higher viscosity of
468
water will increase drag underwater more so than in air (Vandenabeele et al. 2011). It is
469
hoped the findings in this study will help inform the planning required for future tracking
470
studies, and also highlight that the absence of breeding season effects does not mean that
471
longer term effects can be discounted.
472
473
474
This study was funded by the Department of Energy and Climate Change (DECC), and the
475
Marine Renewable Energy and the Environment (MaREE) project (funded by Highlands and
476
Islands Enterprise, the European Regional Development Fund, and the Scottish Funding
477
Council). We thank John Hartley (Hartley Anderson), Emma Cole, Mandy King, Sophie
478
Thomas and James Burt (DECC) and Mark Rehfisch (formerly of BTO) for their support of
479
the work on Foula and at Orford Ness. Thanks to the National Trust, Scottish Natural
480
Heritage, RSPB and Natural England for site permissions and thanks to Aileen Adam
481
(University of Glasgow) for sexing of Great Skuas. We are very grateful to all who have
482
helped with discussions and fieldwork. We thank four anonymous reviewers, the Editor
483
Ruedi Nager and Associate Editors Francis Daunt and Mark Bolton for their suggestions on
484
previous versions of the manuscript.
485
486
19
487
SUPPORTING INFORMATION
488
Additional Supporting Information may be found in the online version of this article:
489
Appendix S1. Sample sizes and consideration of power of data presented.
490
Appendix S2. Information on duration and timing of breeding periods.
491
Appendix S3. Additional information on recovered birds.
492
493
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26
654
Table 1. Mean metrics of breeding productivity (±1 SD) of (a) Lesser Black-backed Gulls at
655
Orford Ness and (b) Great Skuas on Foula and Hoy during 2011 for nests of tagged birds,
656
nests of colour-ringed controls, and other control nests. Uncertainty in measures is expressed
657
as worst case (minimum) and best case (maximum) scenarios (see text for more details).
658
Control birds
(a)
Colony Year
Orford
Ness
2011
Measure
Tagged
birds
Colour-ringed
Other
No. nests
13a
26b
21
47
Eggs hatched/nest
(min, max)
Chicks present/nestc
(min, max)
1.8±1.12.6±1.0
0.8±1.11.9±1.0
1.2±1.02.6±0.5
0.7±1.02.1±0.9
1.1±1.11.4±1.2
0.3±0.51.2±1.1
1.2±1.02.0±1.1
0.5±0.81.7±1.1
No. nests
Eggs hatched/nest
10
1.5±0.7
10
1.7±0.5
37
1.3±0.7
47
1.4±0.7
Chicks present/nestd
(min, max)
0.8±0.91.0±0.9
0.9±0.71.5±0.8
0.8±0.80.9±0.8
0.8±0.70.9±0.9
All
(b)
Foula
No. nests
10
10
Eggs hatched/nest
1.1±0.71.1±0.7(min, max)
1.6±0.8
1.5±0.7
d
Chicks present/nest
0.4±0.70.5±0.5(min, max)
0.6±1.0
0.8±0.8
e
Chicks fledged/nest
0.2±0.4
0.3±0.5
a
Two tagged birds were from the same nest (14 birds monitored at 13 nests).
Hoy
659
2011
2011
660
b
661
members of the pair were marked (i.e. 28 birds were monitored at 26 nests);
662
c
663
d
664
e
Of the total sample of 47 colour-ringed birds, the nests of 19 were not followed, and for two nests, both
Up to 9 July 2011;
Up to 15 July;
Up to 14 August;
665
27
666
Table 2. Over-winter return (survival) rates of (a) Lesser Black-backed Gull, and (b) Great
667
Skua between consecutive breeding seasons, based on (i) observations of colour-ringed birds,
668
and (ii) also including additional records obtained through the GPS system. n = the number of
669
marked birds at the end of the preceding breeding season.
(a)
Colony
Year
Group
marked
Orford
2010
Ness
2011
n
2010-11
No. reNo. resighted /
sighted
n
recorded by
(%)
GPS (%)
2011-12
No. reNo. resighted /
sighted
recorded by
(%)
GPS (%)
2012-13
No. reNo. resighted /
n sighted
recorded by
(%)
GPS (%)
3
2 (67%)
3
1 (33%)
4
3 (75%)
3
2 (67%)
Tagged
4
1 (25%)
Control
6
4 (67%)
Tagged
14 11 (79%)
Control
47 25 (53%)
Hoy
2011
14 (100%)
14 10 (71%)
25
1 (33%)
13 (93%)a
16
(64%)b
2012-13
No. reNo. resighted /
sighted
recorded by
(%)
GPS (%)
0 (0%)
0 (0%)
Tagged
Control
10 10 (100%)
Tagged
10
0 (0%)
0 (0%)
0e
0 (0%)
0 (0%
Control
10
8 (80%)
8 (80%)
8
6 (75%)
6 (75%)
Year
Group
marked
Foula 2011
3 (100%)
2011-12
No. reNo. resighted /
n sighted
n
recorded by
(%)
GPS (%)
10 1 (10%)
1 (10%)
1c
(b)
Colony
3 (75%)
10 (100%)
9d 5 (56%)
670
a
671
one further tagged Lesser Black-backed Gull (483) found dead in Portugal on 26 November 2013.
672
b
673
c
674
in 2012 but without its tag and harness;
675
d
676
e
5 (56%)
Two tagged Lesser Black-backed Gulls (459 and 492) subsequently found dead at the breeding colony in 2013;
Five colour-ringed control Lesser Black-backed Gulls subsequently found dead at the breeding colony in 2013;
One tagged Great Skua (419) found dead in Germany 15 October 2011, one Great Skua (450) returned to breed
One colour-ringed control Great Skua found dead at the breeding colony in 2012;
One Great Skua (467) found dead in Portugal 13 August 2012.
677
678
679
28
680
Appendix S1. Sample sizes and consideration of power of data presented
681
682
Breeding productivity of control birds
683
In this study, two types of controls were included for comparing breeding productivity with
684
nests of tagged birds – nests where adult birds were colour-ringed, and additional nests that
685
were also followed. For both Lesser Black-backed Gull and Great Skua, these additional
686
nests were in the same vicinity as the nests of both the colour-ringed controls and the tagged
687
birds. However, for Lesser Black-backed Gulls in 2011, the maximum number of eggs
688
hatched per nest and chicks present per nest up to 9 July 2011 for these additional nests were
689
both significantly lower (dDev = -6.81, df = 1, P = 0.009; dDev = -6.03, df = 1, P = 0.014,
690
respectively) than for the nests of colour-ringed controls; minimum numbers of eggs and
691
chicks were not significantly different (P > 0.05 in both cases). For Great Skuas at Foula, the
692
maximum number of chicks present up to 15 July was lower for these additional nests than
693
for the nests of colour-ringed controls (Table 1 in main paper) but was not significantly
694
different (dDev = -2.33, df = 1, P = 0.127). For both species, the nests of the sample of
695
unmarked birds included nests from the wider area, including the periphery of both colonies,
696
and so were likely to have included some sub-optimal nesting sites, presumably resulting in
697
lower productivity compared to other colour-ringed control birds. Note, however, there was
698
no significant difference between the nests of colour-ringed control birds alone and those of
699
tagged birds for any measures of breeding productivity (P > 0.05 in all cases). To provide as
700
wide as possible a sample of breeding productivity of the nests of control birds to compare to
701
tagged birds, we therefore pooled both groups of control nests together. Tests for breeding
702
productivity in the main paper were conducted on both minimum and maximum scenarios
703
(see Table 1), but provided equivalent results in all instances, therefore for simplicity, results
704
from the tests of maximum scenarios are presented.
29
705
Sample sizes of sexed birds
706
In this study, birds of both species were sexed to allow further investigations into potential
707
differences in device and harness attachment effects. For Lesser Black-backed Gulls at
708
Orford Ness, tagged and control birds were sexed using head and bill length measurements
709
(Coulson et al. 1983; Camphuysen 2011). Out of 18 tagged birds, ten were males and seven
710
were females, with the remaining bird not assigned sex due to uncertainty. Out of the 28
711
control birds, 17 were males and 11 were females. A sufficient number of males and female
712
Lesser Black-backed Gulls from tagged and control groups were therefore available to
713
investigate device and harness effects on breeding productivity and over-winter return rates.
714
For Great Skuas, tagged and control birds were sexed using either DNA from feather
715
samples (Griffiths et al. 1993), previous known sex from ringing data or within-pair
716
relational size from simultaneous viewing of both pair members. For tagged birds at Foula
717
and Hoy, out of a total of ten birds at each site, three and five females and two and three
718
males were identified, respectively, with the remaining birds not assigned sex due to
719
uncertainty; for control birds at both sites, five females and zero males were identified.
720
Therefore for Great Skua, given the small sample sizes and particularly the lack of male
721
control birds, sex differences in the effects of device and harness on breeding productivity
722
and over-winter return rates were not investigated.
723
724
Power of the data presented
725
The power of all tests presented in this study was assessed for the presence of type I or II
726
errors that may have occurred with limited sample sizes and low statistical power. Effect
727
sizes here are presented as Cohen’s d, and sample sizes are presented that would be needed to
728
reach significance at an alpha level of 0.05 and a power of 0.8. It should be noted that the
729
number of birds tagged and monitored as control groups was relatively high for this study.
30
730
Breeding productivity
731
Tests for differences in the breeding productivity of tagged and control groups of Lesser
732
Black-backed Gulls and Great Skuas were non-significant, and in most cases associated with
733
low effect sizes. For Lesser Black-backed Gull, the effect size for number of eggs hatched
734
was 0.07, with a power of 0.05, and the sample size needed for a significant difference to
735
have been observed between tagged and control birds was 3,651 nests. This compares to a
736
colony size of 540–620 pairs at Orford Ness (Marsh 2013). The mean differences between
737
values for tagged and control birds in all cases were very small and standard deviations and
738
overlaps in the distributions of variables were very high (see Table 1). Therefore, it is very
739
unlikely that Type II errors occurred. For Lesser Black-backed Gull, there was a non-
740
significant tendency for nests of tagged birds to have more chicks present up to mid-July in
741
2011 than control nests (Table 1), which was opposite to our expectation had tags and the
742
harness been deleterious to foraging behaviour. In this case, the effect size was 0.68, and the
743
power was also quite high at 0.6, and the sample size needed for a significant difference to
744
have been observed between tagged and control birds was just 35 nests. However, given the
745
direction of the effect, it is quite unlikely that a Type II error occurred here.
746
For Great Skua at Foula and Hoy respectively, the effect size for eggs hatched per
747
nest was 0.17 and 0.26, with a power of 0.07 and 0.08, and the sample size needed for a
748
significant difference to have been observed between tagged and control birds was 576 and
749
236 nests. Likewise, the effect size for number of chicks present per nest up to 15 July was
750
0.19, with a power of 0.11, and the sample size needed for a significant difference was 432
751
nests. For Hoy, the effect size for number of chicks fledged per nest was 0.22, with a power
752
of 0.08, and the sample size needed for a significant difference was 324 birds. This compares
753
to colony sizes of 1 657 AOTs at Foula and 1 346 AOTs at Hoy (JNCC Seabird Monitoring
754
Programme database, last accessed 6 November 2014).
31
755
Over-winter return rates
756
For Lesser Black-backed Gull, the effect size for return rates was 0.25, resulting in a low
757
power of 0.15 and thus a high sample size of 251 birds needed for a significant difference to
758
have been observed between tagged and control birds. The difference between mean return
759
rates (across all years) was 0.13 (see Table 2 for return rate estimates). These results suggest
760
the non-significant result for Lesser Black-backed Gulls was very robust, and not subject to
761
type II errors. For Great Skua, the difference in mean return rates (over 2012 and 2013)
762
between tagged and control birds was 0.70. This resulted in a very high effect size of 2.10
763
and a very high power of 0.99. For this difference, a sample size of just five birds would have
764
been needed for a significant difference to have been observed between tagged and control
765
birds. As such it is conclusive that sample sizes were sufficiently high to detect the effect
766
recorded in differences in over-winter return rates between tagged and control groups of
767
Great Skuas, and unlikely a type I error occurred.
768
769
Breeding season territory attendance
770
For Great Skua at Foula, the effect size for territory attendance was 0.06, with a power of
771
0.05, and the sample size needed for a significant difference to have been observed between
772
tagged and control birds was 3 952 predicted. This value is more than twice the number of
773
AOTs at Foula (see above). The very similar mean estimates and large overlaps in standard
774
deviations (control = 74.0±44.0%, tagged = 71.1±45.4%) show that it is highly unlikely that
775
any effect was missed using the sample sizes obtained.
776
777
REFERENCES
778
Camphuysen, C.J. 2011. Lesser black-backed gulls nesting at Texel Foraging distribution,
779
diet, survival, recruitment and breeding biology of birds carrying advanced GPS
780
loggers, Royal Netherlands Institute for Sea Research, Texel, NIOZ-Report 2011-05.
32
781
782
783
784
785
786
Coulson J.C., Thomas, C.S., Butterfield, J.E.L., Duncan, N. & Monaghan, P.C. 1983.
The use of head and bill length to sex live gulls Laridae. Ibis 125: 549-557.
Griffiths, R., Double, M. C., Orr, K. and Dawson, R. J. G. 1998. A DNA test to sex most
birds. Mol. Ecol. 7: 1071_1075.
Marsh, M. 2013. Here’s one I made earlier: the Landguard gull RAS, BTO RAS News 13:
10-11.
787
33
788
Appendix S2. Information on duration and timing of breeding periods
789
To assess the effects of the GPS device and harness on Lesser Black-backed Gull and Great
790
Skua, a comparison of breeding productivity was made between nests of tagged birds (with a
791
GPS device and harness) and control birds (birds without a device and harness). Clutch size
792
was not investigated as nests were discovered at different times during incubation, hence the
793
initial clutch size was not always known. However, the number of eggs hatched, the number
794
of chicks present up to mid-July, and the number of chicks fledged per nest were investigated
795
between control and tagged groups. It is important therefore that the two groups were
796
followed for similar periods (and thus that the risks of failures were the same) to ensure that
797
correct conclusions were reached.
798
For Lesser Black-backed Gull, the earliest and latest estimates of mean hatch dates in
799
2011 were 5 and 14 June for tagged nests and 8 and 16 June for control nests. The number of
800
chicks present at nests was recorded in the early stages of chick-rearing. The lengths of time
801
that nests of tagged Lesser Black-backed Gulls were monitored after hatching during the
802
chick-rearing period ranged from a minimum of 7±8 d to a maximum of 15±8 d. For control
803
birds, nests were monitored from 6±8 to 13±9 d. The ranges for minimum and maximum
804
estimates were 1-26 d and 7-35 d for the respective groups. The standard deviations indicate
805
variability across nests monitored, while the lower and upper values represent uncertainty in
806
estimated hatching dates due to the frequency of nest monitoring. For Great Skua, nests were
807
monitored daily at Foula until 15 July 2011. The mean first egg hatching dates were 18 June
808
for tagged nests and 23 June for controls. The length of time that nests were monitored during
809
the chick-rearing period was consequently 22±9 d for tagged birds and 20±10 d for control
810
birds. At Hoy, nests were monitored weekly in 2011. Mean earliest and latest estimates of
811
first egg hatching dates for nests of tagged birds were 17–21 June, and for nests of control
812
birds, 17–25 June. The mean length of time that nests were monitored for during the chick-
34
813
rearing period when birds still had eggs or chicks was 20±25 d (range, 1–65 d) for tagged
814
birds and only slightly longer 26±19 d (range, 3–64 d) for control birds. At Hoy, two nests of
815
tagged birds and three nests of control birds successfully fledged chicks. For neither species
816
were there significant differences in the hatching dates of nests or the ages of chicks
817
monitored between the nests of tagged and control birds (GLMs of ‘chick age’ and Julian
818
date ‘laying date’, considering minimum and maximum estimates in separate analyses,
819
testing a fixed effect for ‘group’ (tagged and control): P > 0.05 in all cases for each species).
820
Hence, any differences in the number of chicks present between the nests of tagged and
821
control birds would not have been due to the relative length of time that nests were monitored
822
during the chick-rearing period.
823
35
824
Appendix S3. Additional information on recovered birds
825
Lesser Black-backed Gull
826
Two tagged Lesser Black-backed Gulls from 2011 were subsequently found dead due to fox
827
predation within the breeding colony during 2013, and another was recovered dead at
828
Corroios, Seixal, Portugal (38°38’ N, 9°8’ W) on 26 November 2013. Five colour-ringed
829
control birds were also recovered dead due to fox predation in June 2013. Although sample
830
sizes were small, it is therefore unlikely that birds wearing a device and harness were more
831
vulnerable to fox predation.
832
833
Great Skua
834
One control Great Skua on Foula was recovered dead within its breeding territory on 12 May
835
2012. Another Great Skua was reported off the coast of south Devon on 12 October 20111
836
with its GPS device and wing harness in place. Two further birds were recovered dead with
837
their device and harness in place outside the breeding season. One from Foula was found near
838
Norddorf, Germany (54°42’ N; 8°20’ E), on 18 October 2011. The GPS data showed that this
839
bird departed the colony on 25 August 2011, remaining in the North Sea, reaching the
840
Lincolnshire coast, UK on 16 September 2011 before venturing back out to sea prior to its
841
subsequent recovery. A tagged Great Skua from Hoy was recovered at Praia do Meco,
842
Setubal, Portugal (38°28’ N; 9°12’ W) on 13 August 2012. The GPS data showed that this
843
bird left the colony on 10 September 2011 migrated along the west coast of Scotland and
844
Ireland, reaching the Bay of Biscay on 17 September 2011, where the bird spent the 2011/12
845
winter and following months before its recovery. Movements of these birds are shown in
846
Figure S3.1.
1
http://marinelife-charm3.blogspot.co.uk/2011/10/12th-october-2011-english-channel-20nm.html [last accessed
26 May 2015]
36
847
Only one Great Skua tagged in 2011 returned in 2012 (Table 2), but without its device and
848
harness. This bird was tagged at Foula and was present at the colony until 29 August 2011
849
(last recorded GPS fix). This date was similar to other tagged Great Skuas at Foula in 2011
850
(mean 22 August ±17 days, max 18 September). The two birds recovered with their device
851
and harness intact left the breeding colony on 25 August and 10 September 2011. This
852
strongly suggested that Great Skuas departed with their device and harness intact. In 2012 the
853
previously-tagged bird at Foula laid two eggs (2011, same clutch size), and weighed 1 305 g
854
(5 June 2011 09:10 h BST) upon re-trapping. This was heavier than its weight upon initial
855
trapping in 2011 (1 220 g, 5 June 10:24 BST) and both weights were similar to breeding birds
856
on Foula during 2011 (range 1 180–1 490 g, mean 1 324±100 g, n = 20 birds).
857
37
858
Figure S3.1. Map showing the GPS locations of two Great Skuas after they departed their
859
breeding colonies prior to final recoveries, and a re-sighting of another bird off the
860
Devonshire coast (see text in Appendix S3 for details).
861
38