Mar Biol (2013) 160:1239–1248 DOI 10.1007/s00227-013-2175-y
ORIGINAL PAPER
Environmental variability and fledging body mass of Common Guillemot Uria aalge chicks Robert T. Barrett • Kjell Einar Erikstad
Received: 29 August 2012 / Accepted: 9 January 2013 / Published online: 22 January 2013 Ó Springer-Verlag Berlin Heidelberg 2013
Abstract To gain a better understanding of population processes and in the light of the critically endangered status of the Common Guillemot Uria aalge in Norway, we investigate which environmental factors might affect the fitness of guillemot chicks as they depart from the nest site over a 16-year period on a colony in NE Norway. Although prey composition did not seem to influence the fledging body mass of the chicks, there were significant relationships between the yearly variations in chick body mass and abundance of two important prey species (1-group herring Clupea harengus that is an important chick food item and 0-group cod Gadus morhua that is an important adult food item), population size and the sea surface temperature around the colony. The positive influence of young herring and cod on Common Guillemot chick mass occurred during a period of warming in the Barents Sea such that future recruitment into the population will depend partly on the long-term changes in ocean climate in the region.
Communicated by S. Garthe.
Electronic supplementary material The online version of this article (doi:10.1007/s00227-013-2175-y) contains supplementary material, which is available to authorized users. R. T. Barrett (&) Department of Natural Sciences, Tromsø University Museum, 9037 Tromsø, Norway e-mail:
[email protected] K. E. Erikstad Norwegian Institute for Nature Research, FRAM––High North Research Centre for Climate and the Environment, 9296 Tromsø, Norway
Introduction There is growing evidence that, among seabirds, chick development and condition at colony departure, through effects on subsequent survival, may be important in population dynamics. Many studies of long-lived seabirds document large yearly variation in breeding conditions (e.g. Weimerskirch 2002). This has led to the need to account for yearly variation in the quality of offspring in discussions of population dynamics and demography since both chick growth and body condition may have long-term consequences on fitness (reviewed in Cam et al. 2003; Cam and Aubry 2011; Monticelli and Ramos 2012). The Common Guillemot Uria aalge is a long-living, circumpolar, boreal and low Arctic seabird. Although numbers in the North Atlantic increased over much of the twentieth century (to nearly 3 million breeding pairs at the turn of the millennium), the Norwegian population declined severely during the last half of the century from 120,000 to 160,000 pairs in the 1960s to ca. 15,000 pairs in 2005 (Brun 1969; Mitchell et al. 2004; Barrett et al. 2006). This decline was initially a result of hunting, egging, drowning in fishing gear and, at times, food shortage, but subsequent breakdown of social structure and harassment from white-tailed eagles Haliaeetus albicilla has recently exacerbated the situation resulting in several Norwegian colonies now being in danger of extinction (Brun 1979; Strann et al. 1991; Erikstad et al. 2007; Hipfner et al. 2012). The Common Guillemot is consequently classified as critically endangered in the Norwegian Red List (Ka˚la˚s et al. 2010) and high on the list of management priorities. To improve the biological certainty on which to implement measures concerning the management of Common Guillemots in Norway, it is therefore important to understand in detail all the mechanisms behind this decline.
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The survival rates of immature Common Guillemots to breeding age (4–5 years) range from 27 to 43 % (Crespin et al. 2006 and references therein) and are especially low during the first year post-fledging. For example, Harris et al. (2007) showed that the mean survival rate of 20 cohorts of Common Guillemots was 56 % (range 30–91 %) during the first year with a peak after the chick had become independent of its accompanying parent in December–February. Although there is little evidence of guillemot chick body condition affecting post-departure survival (Hatch 1983; Harris et al. 2007), other seabird studies have shown that chicks that suffer nutritional stress and slow growth may exhibit a higher post-fledging mortality than well-fed chicks or, if they survive, develop into lower-quality adults (Metcalfe and Monaghan 2001; Kitaysky et al. 2006 and references therein; Morrison et al. 2009; Noguera et al. 2012). The body condition of a fledging Common Guillemot chicks thus not only reflects its nutritional state and hence the effort required by the parents to provision them, but possibly also of its quality as an adult when entering the breeding population. Chick growth depends largely on the quality and quantity of food supplied by the adults and, because Common Guillemots are often considered to be specialized and/or limited in their choice of food for their chicks, they are sensitive to changes in prey availability within foraging distance of the colony during the chick-rearing period (e.g. ¨ sterblom et al. 2001; Wanless et al. 2005; Burke and O Montevecchi 2008). The distribution of the guillemots’ main prey in the Barents Sea is, in turn, partly driven by the marine climate (e.g. Hjermann et al. 2004), and any changes in temperature or other oceanographic parameters may cause constraints on food-searching adults through changes in marine food webs and/or a mismatch between food availability and the chick-rearing period (e.g. Irons et al. 2008; Shultz et al. 2009; Watanuki et al. 2009). Important Common Guillemot chick food in the Barents Sea region is the capelin Mallotus villosus and 1-group (i.e. second year) herring Clupea harengus whose distribution is restricted to the SW Barents Sea (Loeng 1989; Barrett et al. 1997; Barrett 2002). Sandeels Ammodytes sp. may also make up significant proportions of fish brought to chicks at Hornøya. Early studies of Common Guillemots in the region have shown that capelin is also an important constituent of the adult diet in the pre-breeding season, and it has long been assumed to have been important in other parts of the year (Erikstad and Vader 1989; Bugge et al. 2011 and refs therein). Recent studies have, however, shown that 0-group (i.e. first year) gadids, most probably cod Gadus morhua that, like the one-group herring, has a predominantly coastal distribution (Vikebø et al. 2011), are an important constituent of the diet of adults during the chick-rearing period (Bugge et al. 2011) and may be a key
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constituent driving the population dynamics of the species in the region (Erikstad et al. in press). In their study, Erikstad et al. (in press) showed that most important singleprey species driving the population fluctuation in one North Norwegian Common Guillemot population was the amount of 0-group cod near the colony during the breeding season. Erikstad et al. (in press) suggested that the availability of the youngest age classes of cod may be critical for the presence or absence of adult guillemots in the colony, either because the adults choose not to breed or because they increase their effort by spending longer time searching for food during years of low cod abundance. They also found a lagged effect of the abundance of 0-group cod and capelin 4–6 years earlier suggesting a positive effect on the breeding success and hence the number of recruits entering the population at maturity. To gain a better understanding of population processes, the trade-off between parental investment and chick survival and the possibility that post-fledging survival and recruitment age and quality may be influenced by fledgling fitness, and in the light of the species’ status as critically endangered in Norway, it is important to address further what environmental or intrinsic factors affect chick size of guillemot chicks when they depart from the colony. Such factors will primarily include the availability, quality and quantity of food brought to the chicks, but also that eaten by the adults as they will influence the fitness of the latter during the costly chick-rearing phase. Because there is also evidence of a negative correlation between colony size and nest-site density and chick fledging mass (Gaston et al. 1983; Ashbrook et al. 2010), we here examine the relationships between chick diet, abundance of fish prey, population size and sea surface temperature (SST, a proxy of local marine climate) and the fledging mass of Common Guillemot chicks on a colony in NE Norway based on 16 years of data. We test which of these parameters, or combinations thereof, affect the departure mass of the chicks most and discuss the results in the perspective of possible climate change in the Barents Sea.
Methods The study was carried out between 1996 and 2011 at Hornøya (72°220 N, 31°100 E), East Finnmark. The numbers of Common Guillemots breeding on Hornøya were estimated to ca. 10,000 pairs in 2010 based on a single total count made in 1989, and subsequent changes documented in annual counts of monitoring plots that were considered representative of the colony. Full details are given in Barrett (2001) and Erikstad et al. (in press). Because chicks lose body mass during the last few days before departing
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from the colony (Barrett 2010), only chicks caught on their way from the nest site to the sea were used in this study. They were weighed (±2.5 g), measured (wing length, from the carpal joint to the tip of the longest primary covert, ±0.5 mm) and ringed. To compare measurements of chicks leaving the colony in different seasons and to avoid effects of seasonal variation in the measurements (Hedgren 1979), only data collected within 5 days of the start of the main fledging period were used. In nine of the 16 seasons, chicks were caught on 2–3 different nights, and the remainder were caught on one night. Sample sizes varied between 47 and 201 chicks. Fuller details are given in Barrett (2010). As a proxy of chick condition, we restricted our analyses to chick mass as we consider it inappropriate to estimate body condition of chicks correcting for body mass in a regression analyses. We did not know the age of the chicks at departure, and data from the present study colony show that chicks lose some weight before they depart (Barrett 2010). We therefore use the raw data of chick body mass in all analyses. Body mass and wing length of chicks at departure were also closely correlated (r2 = 0.75), and using the wing length in the analyses gave the same top rank models as for chick body mass (not shown). Chick food data were collected by direct observation of food items carried by adults into the colony using 10 9 50 binoculars at distances of 5–20 m. Observations lasting about 1 h were made once a day on 4–12 days prior to the first night of chick weighing. When possible, all food items were identified to species and counted, and at the end of the season, the proportions of each taxon were estimated by mass. To determine the mass of items carried by Common Guillemots, they were categorized as small, medium, large and very large in relation to bill length. To check identification and length estimates, control samples were collected from adults caught using a noose-pole. The masses of individual items not collected were then integrated from estimated lengths based on length/mass relationships determined from the control samples (RTB unpubl. data).
Analyses Multivariate linear regression models were used to examine the relationship between body mass and diet of chicks in relation to estimates of fish abundances (acoustic and trawl surveys) in the Barents Sea. We confined the regression analyses in relation to the total stock of capelin and stocks of 1-group herring and 0- and 1-group cod. Sandeels had to be excluded from the analyses as there are no abundance data for the species in the Barents Sea. Data on fish abundance were downloaded from http://www.imr. no/sjomil/index.html and from the ICES AFWG 2011
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report (ICES 2011). All estimates of fish abundances were log transformed before analyses. Since an increased influx of warm Atlantic water since the late 1990s led to a general warming of the Barents Sea with a distinct culmination in 2006 (Boitsov et al. 2012; Johannesen et al. 2012, Fig. 1a), we also used the sea surface temperature (SST) around Hornøya in July (the chickrearing period of guillemot) as a covariate. SST data were obtained from http://iridl.ldeo.columbia.edu/SOURCES/. NOAA/.NCDC/.ERSST/.version3b/.sst choosing grid cells [spanning 1° latitude 9 1° longitude that covers the approximate foraging range (often \100 km) of chickfeeding adult guillemots (Gaston and Jones 1998)] northeast of Hornøya. As the first step we tested for any temporal linear trends in the body mass of chicks, their diet and the environmental covariates that may complicate the analyses. Such trends were found in the July SST around Hornøya and the guillemot population size (Fig. 1a f, Table S.1). We detrended the SST data by using the 1st order differencing technique (SAS 2008) (estimating the change in temperature from 1 year to another) (Table S.1). The population size increased steeply over the years with an extraordinary good linear fit (r2 = 0.99, Table S.1). There was therefore no need to detrend these data. The nearly perfect linear fit also precluded separating any trend in chick body mass over years from that of any density-dependent effect of population size. As a second step, we examined the relationship between chick mass and the prey composition of the diet. Prior to the analyses, all percentage values were arcsine transformed towards normality. Since the different prey fish species were highly correlated (see Table 1), we first carried out separate univariate regression analyses for each prey type. We then ran multivariate models (PROC REG) on the relationships between chick body mass and all covariates considered, including both abundance estimates of fish, SST around Hornøya and the population size of guillemots. Finally, we used the top rank model from these analyses and entered the fish species in the diet 1 by 1 to examine whether the model could be improved. Different models were evaluated using Akaike information criteria corrected for small sample size, AICc (Burnham and Anderson 2002). Models within 2 AICC of each other were considered to be equally well supported. All analyses were carried out in SAS version 9.3 (SAS 2008).
Results Chick body mass and diet There was a large and significant variation in the annual mean body mass (ANOVA F15,1849 = 58.2, P \ 0.001)
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Fig. 1 Annual variation in the covariates used to model the variations the body mass of Common Guillemot chicks leaving the colony at Hornøya, NE Norway (1996–2011)
Table 1 Correlation matrix for different covariates used to estimate the yearly variation in mass of Common Guillemot chicks on Hornøya, NE Norway
Herring (1) abundance Cod (0) abundance
Herring (1) abundance
Cod (0) abundance
Cod (1) abundance
Capelin
1
0.43
-0.39
-0.39
1
Cod (1) abundance Capelin abundance SST July Herring in diet Capelin in diet Sandeel in diet Pop. size Numbers in parentheses are age-groups of fish * P = 0.05, ** P = 0.005
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0.21 1
SST July
Herring in diet
Capelin in diet
0.36
0.39
-0.33
-0.34
-0.24
0.30
-0.20
-0.24
0.25
-0.10
1
-0.13
-0.44
0.006
1
0.28 1
0.50* -0.18 **
Sandeel in diet 0.007
Pop size -0.17
-0.06
0.33
-0.49*
0.04
-0.36
0.20
-0.28
0.01
-0.92
-0.12
0.41
1
-0.26
-0.41
1
0.06 1
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(a)
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300
Capelin
Herring
Other
100
290 280
80
270
Frequency (%)
Chick body mass (g)
Sandeel
260 250 240 230
60
40
220 1996 1998 2000 2002 2004 2006 2008 2010
20
Year
(b)
88
0
Chick wing length (mm)
86
1996
133
82 80 78
92 87
99 90 81
102
47 119
76 139 201
74
2000
2002
2004
2006
2008
2010
Fig. 3 Annual variation (by mass) in the composition of Common Guillemot chick diet at Hornøya, NE Norway (1996–2011). Yearly sample sizes range from 390 to 1,655 observations of adults with single fish loads for chicks
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110 134
1998
Year
114
84
185
72 70 1996 1998 2000 2002 2004 2006 2008 2010
Year
Fig. 2 Annual variation in mean mass and wing length (±1 SE) of Common Guillemot chicks leaving the colony at Hornøya, NE Norway, 1996–2011. Sample sizes are given for each year
different prey species and the body mass of chicks at fledging (capelin, r2 = 0.06, df = 1,15, n.s.; herring, r2 = 0.13, df = 1.15, n.s.; sandeel r2 = 0.04, df = 1.15, n.s.). Nor was there any relationship between the mean annual chick body mass and the mean annual mass of the fish delivered to the chick (r2 = 0.02, df = 1.15, n.s.). Chick diet and fish abundance in the Barents Sea
and wing length (ANOVA F15,1850 = 33.2, P \ 0.001) of chicks as they left the colony over the study period (Fig. 2). There was a slight but insignificant decrease in mean body mass over the years (r2 = 0.23, P = 0.06, Table S.1). The chick diet consisted of three main fish species, capelin, herring and sandeel that made up 56.9, 28.9 and 13.6 % of the diet, respectively, for the whole time period. A small fraction of the diet consisted of gadoids (mainly 0- and 1-year-old cod or saithe Pollachius virens (pers. obs.)), and a very small fraction (0.1 %) was unidentified (Fig. 3). The diet composition varied considerably between years (v2 = 2976, df = 45, P \ 0.0001), but there was no overall trend. In 2005 and 2006 when capelin stocks were low (Fig. 1d), however, the fraction of capelin fed to chicks was low (18.5, 10.4 %, respectively) and that for the herring correspondingly high (71.5, 63.8 %, respectively). Overall, the fractions of capelin and herring in the diet were strongly negatively correlated (r = -0.92, df = 15, P \ 0.0001), but there was no clear correlation between the fractions of herring and sandeel (r = -0.12, df = 15, n.s.) and capelin and sandeel (r = -0.26, df = 15, n.s.) (Table 1). Although the diet varied considerably between years, there was no apparent relationship between the fraction of
Although prey composition did not seem to influence the body mass of chicks, there was a positive correlation between the fraction of capelin in the diet and the capelin stock in the Barents Sea, but only when using SST in July as a covariate (Table 2) with chicks being fed more capelin when the stocks were high and the July temperature low. There was a similar, albeit insignificant, relationship between the fraction of herring in the diet and abundance of 1-group herring when controlling for SST in July. The slope was negative for the SST when controlling for capelin and positive when controlling for herring (Table 2). Chick body mass, fish prey abundance and SST As a final step, we used multiple regression models to examine the relationship between the yearly variations in chick body mass and abundance of prey species important for chicks (1-group herring, capelin) and adults (0- and 1-group cod) and the SST around the colony. We also entered population size as a covariate. For an overview of the annual variation in all the covariates used, see Fig. 1. The best model included two parameters (1-group herring and population size) with an explained variance of 64 %
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Table 2 The relationships between the fraction of capelin and herring in the diet of Common Guillemot chicks and the total stock size of capelin and 1-group herring in the Barents Sea
Parameter
Estimate (SE)
Partial r2
Model r2
F
P
Capelin Intercept
2.94 (2.0)
SST July
-0.57 (0.20)
0.28
0.28
7.80
0.02
0.18 (0.20)
0.25
0.53
7.04
0.02
Capelin Herring Sea surface temperature (SST) in July around the colony is used as a covariate
Intercept
-3.60 (1.52)
SST July
0.46 (0.19)
0.32
0.32
5.75
0.02
Herring (1)
0.06 (0.05)
0.07
0.40
1.57
0.23
Table 3 Comparison of candidate models describing the variance in body condition of Common Guillemots at fledging Rank
Models
AICC
DAICc
ML
r2 0.64
1
Herring (1) ? Pop. size
82.48
0.00
1.00
2
Herring (1)
83.69
1.21
0.55
0.54
3
Herring (1) ? SST July ? Pop. size
84.02
1.54
0.46
0.67
4
Herring (1) ? Cod (0) ? Pop. size
84.60
2.12
0.35
0.66
5
Herring (1) ? SST July
84.73
2.25
0.32
0.59
6
Herring (1) ? Cod (1) ? Pop. size
84.77
2.29
0.32
0.66
7
Herring (1) ? Capelin ? Pop. size
85.14
2.66
0.26
0.65
8
Herring (1) ? Capelin
85.61
3.13
0.21
0.56
9
Herring (1) ? Cod (1)
85.78
3.31
0.19
0.56
10
Herring (1) ? Cod (0)
86.25
3.77
0.15
0.55
27
Capelin ? Cod (1)
90.75
8.28
0.02
0.40
The fish species considered are 1-group herring, 0-group cod, 1-group cod and mature capelin. The population size (pop. size) of Common Guillemots and the SST around the colony in July are also used as covariates. Models are sorted by ascending DAICc. For each model, we also give model likelihood [ML = exp(-0.5 9 DAiCc)] and r2. The 10 best models are shown, all of which included herring. The first model without herring ranked 27 and included two parameters (1-group cod and capelin). For a list of all models, see Table S.2 Table 4 Estimated slopes, explained variance (partial and for the model) and variance inflation factor (VIF) for the abundance of herring (age of fish) and population size that best explained the annual variation in the body mass of Common Guillemot chicks at fledging Parameter
Estimate (±SE)
Intercept Herring (1)
343.55 (49.38) 11.90 (2.44)
Pop. size
-14.18 (5.61)
t 6.96 4.87 -2.53
P
Partial r2
Model r2
VIF
0.0003
0.54
0.54
1.03
0.02
0.10
0.64
1.03
Estimates are from the top rank model in (Table 3)
(Table 3). The body mass of chicks increased with the densities of 1-group herring and decreased with the population size (Table 4). The variance inflation factor was low (1.03), suggesting no autocorrelation between the two covariates. The top rank model had a DAICc that was 1.21 units lower than the second ranked model including 1-group herring alone with an explained variance of 54 % (Table 3). The two next models, 1-group herring ? SST ? population size and 1-group herring ? 0-group cod ? population size, were also well supported with a DAICc of only 1.7 and 2.1 units higher than the top rank model. SST in July had a negative estimate, and 0-group cod had a positive estimate.
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This suggests that the warming of the sea may (indirectly, through the prey) negatively affect body mass of chicks at departure while an increase in density of 0-group cod had a positive effect. Herring was included in all the 10 highest ranked models, and the first model without herring (1-group cod and mature capelin) was ranked 27 and had a DAICc 8.3 units higher than the top rank model. However, the effect of capelin density on fledging mass was negative. Although we consecutively entered the fraction of prey species in the diet as covariates to the top rank model, this never improved the top rank model (results not shown). A list of all models tested is shown in Table S.2.
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(a)
(b)
Fig. 4 a Annual variation in Common Guillemot chick body mass at fledging and the fitted values from the top rank model (with 95 % CL, shaded area) best describing the variation in Common Guillemot chick body condition (Tables 1 and 2) at Hornøya, NE Norway. b The same data as in (a) but predicted values are plotted against the observed values
Finally, we tested for any heteroscedastic OLS residuals (variance in error structure) with four lags for the top rank model using the archtest in the PROC AUTOREG procedure. These analyses revealed no variance in error structure over time (range in P values from 0.58 to 0.90 for the first four lags), suggesting no need for any correction of estimates. Predicted values of the body mass of chicks from the top rank models could be closely fitted to the observed variance in body mass of chicks (Fig. 4).
Discussion As chick food, capelin, sandeels and 1-group herring are the most important fish species for many seabird species in the southern Barents Sea (reviewed in Fauchald et al. 2011). The situation at Hornøya is no exception where they constitute most of the diet of tens of thousands of chicks of Black-legged Kittiwake Rissa tridactyla, Atlantic Puffin Fratercula arctica, Common Guillemots, Bru¨nnich’s Guillemots U. lomvia and Razorbill Alca torda (Barrett et al. 1997; Barrett 2002, 2003, 2007; pers. obs.). Capelin, sandeels and 1-group herring are high-quality, lipid- and
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energy-rich and easily digested fish (Hilton et al. 2000), and their dominance in guillemot chick diet in this study corroborates earlier studies that have shown that Common Guillemots, that often feed their chicks on a few fish species (Gaston and Jones 1998), are relatively insensitive to changes in available prey when alternative prey sources are available (Burger and Piatt 1990; Zador and Piatt 1999). It was, nevertheless, surprising that the models did not reveal any effect of diet composition on chick mass. This may be partly due to the lack of data on the body condition of the prey fish to which Common Guillemots, being single-prey provisioners, are particularly sensitive and which ¨ sterblom is also known to influence chick fledging mass (O et al. 2001; Wanless et al. 2005; Burke and Montevecchi 2008). Nor were there data on the feeding frequency or activity budgets of the adult Common Guillemots at Hornøya, both of which might compensate for differences in prey quality and affect chick growth and/or the condition of the adult birds in years of different chick food availability (e.g. Wilhelm and Storey 2004; Kadin et al. 2012). Furthermore, the analyses were restricted to chick data collected early in the fledging period and thus biased towards higher-quality chicks (Hedgren 1979, but see Hatch 1983) and possibly masking larger variations in chick body mass that may occur later in the season. Although variable, the overall mean masses of fledged Common Guillemot chicks on Hornøya were high and ranged between 22 and 27 % of the mean adult body mass, well within the range of other studies (Barrett 2010 and refs. therein). The positive relationship between the fraction of capelin in the diet and the total stock size of mature capelin in the Barents Sea when using the July SST as a covariate (Table 1) and a similar, albeit insignificant relationship between the fraction of herring in the diet and the abundance of 1-group herring has interesting climate implications (Johannesen et al. 2012). That the slope was negative for SST when controlling for capelin and positive when controlling for herring suggests that capelin may respond to local temperature regimes [as demonstrated in Newfoundland (Davoren 2012)] and were more available to guillemots around the colony during years with low SST and that herring were more available during years with high SST. In both cases, however, a relatively poor relationship between chick diet and fish abundance may be due to spatial and temporal incoherencies between the foraging area of adult guillemots and the fish surveys. That being said, it is in accordance with the central place foraging theory (Orians and Pearson 1979) and the optimization of energy provisioning to chicks by adults choosing large, high-quality herring, capelin or sandeel even when prey availability is variable and/or while adults themselves feed more on poorer quality fish such as the youngest stages of cod or haddock Melanogrammus aeglefinus (Burke and Montevecchi
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2009; Bugge et al. 2011). The inclusion of 0-group cod in the top models further highlights the previously inadequately appraised importance of this component as adult food during the Common Guillemot chick-rearing periods in the region (Bugge et al. 2011; Erikstad et al. in press). With more young cod in the region, adults can spend less time searching for food for themselves and more time to find optimal food items for their chicks. The Common Guillemot population at Hornøya increased greatly during the study period (from ca. 3,000 to ca. 10,000 pairs) as a result of a combination of high adult survival rates, high breeding success and recruitment rates and probably also immigration from other colonies (Barrett 2001; Reiertsen et al. 2012; Kristensen et al. 2012). Plausible negative effects of this increase on chick body mass might be a depletion of prey species near the colony (Gaston et al. 1983; Kitaysky et al. 2000; Ashbrook et al. 2010) or increased crowding resulting in a decrease in breeding site quality and/or an increase in (anti)social interactions between breeding pairs (Potts et al. 1980; Gaston 2004). At Hornøya, however, the regression models showed only a small effect of population size (when alone it ranked 21, Table S.2), suggesting some of the positive effects (e.g. more synchronous breeding, higher vigilance and less predation) found by Birkhead (1977) may be counteracting any negative ones. One unknown factor is the possibility that light chicks leave the colony at the encouragement of the adults to escape any negative influences on the nesting shelves and/or to take advantage of the rich food supply in the sea at the outer range or beyond the normal adult foraging range (see also Ydenberg 1989). That the amount of young herring and 0-group cod in the waters around Hornøya seems to have a larger influence on the fledging mass of Common Guillemots at Hornøya than the amount of capelin is again at odds with the earlier belief that capelin is the dominant prey of seabirds in the region. Since early assumptions that capelin was the most important constituent of adult diet [e.g. as used in estimates of seabird consumption in the region (Furness and Barrett 1985; Erikstad and Vader 1989; Barrett et al. 2002)], other prey species such as young gadoids have been recognized as being equally or more important as adult food (Bugge et al. 2011; Erikstad et al. in press). This documentation that the presence of young herring (as chick food) and 0-group cod (as adult food) in the waters around Hornøya also favours the mass of departing Common Guillemot chicks, and hence, their future recruitment into the breeding population further corroborates Erikstad et al.’s (in press) findings of the importance of the youngest life stages of cod as a major driver of the guillemot population dynamics and again demotes capelin as the dominant forage fish species for seabirds in the region. The Barents Sea is influenced by multi-decadal cycles of warming and cooling (reviewed in Loeng et al. 2010), and
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since 1980, there has been a warming phase in the NE Atlantic that has had a positive influence on the stocks of cod and Norwegian spring-spawning herring and a negative one on the capelin stock both in their sizes, productivity and extent of their northerly distributions (e.g. Stenevik and Sundby 2007). The positive influence of herring availability on Common Guillemot chick body mass documented in this study came during this period when more young herring have entered the Barents Sea at the same time as the distribution of capelin has moved northwards and hence out of the foraging range of seabirds on the mainland. The recent deceleration of this warming phase and a possible entry into a 2–3 decade cooling phase in the Barents Sea (Klyashtorin et al. 2009; Loeng et al. 2010) will lead to fewer young herring in the Barents Sea and favour a temporary return of capelin to more southern waters. With the possible accompanying reduction in Common Guillemot chick body mass (as also mooted by Barrett 2010) and hence future recruitment into the breeding population at Hornøya, this is bad news for the critically endangered Common Guillemots in mainland Norway as Hornøya is the one remaining large and seemingly viable population in the country (Erikstad et al. 2007). How effects of a warmer climate in the Barents Sea in the longer term will affect its seabird community is still uncertain due to changes in and weakening of many of the long-established trophic interactions and links between temperature and biological parameters (Johannesen et al. 2012). Acknowledgments The Norwegian Coastal Administration is thanked for the use of the lighthouse at Hornøya as a base for the fieldwork. We are also grateful to Ha˚kon Dahlen (Tromsø Univ. Museum) and Thierry Boulinier (CNRS Montpellier) and his and many other co-workers over the years for their help in catching, weighing and measuring the chicks, to Tycho Anker-Nilssen and Svein-Ha˚kon Lorentsen (Norwegian Institute for Nature Research, NINA) for their comments on an early draft of the manuscript and to Tony Gaston (Environment Canada) and an anonymous referee for their comments on the submitted version. The study was financed by Tromsø University Museum, NINA, the Norwegian National Monitoring Programme for Seabirds and the Norwegian SEAPOP programme www.seapop.no).
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