Apparent survival on Nasaruvaalik Island
This study presents the first estimate of apparent survival (∼0.90) for a High Arctic colony of breeding Sabine’s gulls. Our estimate was in the upper range of those reported for other Arctic-breeding gull species (
Stenhouse et al. 2004;
Allard et al. 2010) and was similar to that reported for the Low Arctic colony of Sabine’s gulls at East Bay (
Stenhouse and Robertson 2005). Similar to other colonial nesting seabirds, Sabine’s gulls have strong fidelity to their breeding grounds, even if they experienced breeding failure in previous years (
Stenhouse and Robertson 2005). Furthermore, the High Arctic colony is relatively small (16–31 pairs), with nearly all banded birds resighted each year. Therefore, the high annual survival rate is not unexpected. We were not able to directly test for a colony effect on survival due to confounding effects of time. Our results indicate that survival was constant over time for the Nasaruvaalik Island colony, but it is difficult to detect temporal variability in short-term survival studies. Indeed, models with fewer parameters (e.g., constant models) tend to be better-supported (
Anderson et al. 1994).
Attachment of tracking devices, such as GLS, may influence survival probabilities and other demographic parameters for seabirds (
Igual et al. 2005;
Quillfeldt et al. 2012); however, we do not believe this was the case in our study. The birds in our study that received GLS in 2008, all returned in the following year, so the low survival estimate during 2008–2009 was unlikely a result of GLS attachment in 2008. The majority of the GLS were deployed in 2010 (
n = 23 birds, one of which was previously tagged in 2008) and 2011 (
n = 21 birds, 10 of which were previously tagged in 2010), and there were no differences in return rates or nest success between tagged and untagged birds in any year (S.E. Davis, unpublished data).
Effects of climate on survival
From the analysis combining both colonies, we found evidence of a negative relationship between anomalous winter climate and apparent survival of adult Sabine’s gulls. The best supported model suggested that annual survival probabilities were generally high, although they declined nonlinearly as TNH moved from negative to positive, and dropped in 2008/2009, in relation to a notably high TNH pattern in that year. A similar pattern was observed for adult razorbills (
Alca torda), which maintained high annual survival probabilities in all years except 1997/1998 when Labrador Current SSTs were particularly high (
Lavers et al. 2008). Likewise, annual survival of Cassin’s auklets (
Ptychoramphus aleuticus) was lowest during 1997/1998, coinciding with an El Niño event in the Pacific (
Bertram et al. 2005;
Morrison et al. 2011). These patterns suggest, for some long-lived seabirds, that the capacity to maintain consistently high survival probabilities is diminished during pronounced shifts in broadscale weather patterns.
Although adult mortality can be a direct consequence of extreme weather events (
Frederiksen et al. 2008;
Mallory et al. 2009), effects of regional climate on seabird population dynamics tend to manifest indirectly via changes in food availability (
Durant et al. 2004;
Sandvik et al. 2005;
Gaston 2011;
Hovinen et al. 2014). A reliable food source leading up to the breeding season is important for maintaining body condition, and thus it influences adult survival and reproductive effort (
Monaghan et al. 1989;
Oro et al. 2004;
Davis et al. 2005;
Harding et al. 2011). Sabine’s gulls are surface foragers that feed primarily on various zooplankton species (e.g., amphipods;
Duffy 1983;
Day et al. 2001). Climatic conditions may have negatively influenced Sabine’s gull survival indirectly by limiting access to or reducing their primary food source at some point during the nonbreeding season (see
Vincent et al. 2002;
Dorresteijn et al. 2012). Unfortunately, with our data we are unable to pinpoint where or when this might occur. For example, we found lower Sabine’s gull survival with higher TNH indices, opposite to what we predicted, because high TNH usually is associated with cooler sea surface temperature conditions in the Pacific (
Barnston et al. 1991), which should mean higher marine productivity (e.g.,
Hays et al. 2005). However, recent findings suggest that high, positive TNH is associated with stronger development of warm Pacific Blob conditions in the northeastern Pacific (
Liang et al. 2017), in the staging and migratory route of Sabine’s gulls in our study. The late arrival at the colony and below-average reproductive effort (clutch size) and hatching success in 2009 (
Mallory et al. 2012) would be consistent with this interpretation.
Spatial variability in climate regimes throughout the range of habitats encountered by long-distance migratory species presents obvious challenges for investigating influences of climate on adult survival (
Grosbois et al. 2008). Climate systems across such large areas are governed by interdependent atmospheric fluctuations; thus, multiple indices and other factors (e.g., time of year, small-scale weather variables;
Sandvik et al. 2005;
Lavers et al. 2008) likely contribute to overall effects of climate on Sabine’s gull survival. Potential variation in migratory routes among, and even within, colonies should also be acknowledged. For example, Sabine’s gulls returning north to Nasaruvaalik Island from their wintering habitat off the coast of Peru follow three routes: all Pacific coastal, Pacific coastal plus a terrestrial “short cut” across Alaska, or Pacific coastal plus a major terrestrial route across Canada to Hudson Bay and then north (
Davis et al. 2016). Exposure to food supplies, predation, and weather conditions should differ markedly on these routes. More tracking work is needed to determine the proportion of gulls going to each location, so we can be more confident in selecting climate indices to test.
Although the model with TNH was best-supported among the given set of models, it is possible that a different index, or a suite of indices and/or local climate variables, better explains variability in Sabine’s gull survival estimates. However, testing effects of interactions requires high statistical power that is beyond the present data. We also wanted to avoid finding spurious results by taking an “all possible models” approach (as described by
Anderson et al. 2001). Nevertheless, findings from this study add to growing evidence that variability in climate influences the survival of long-lived seabirds (e.g.,
Grosbois and Thompson 2005;
Sandvik et al. 2005;
Lavers et al. 2008;
Dorresteijn et al. 2012;
Sydeman et al. 2012;
Genovart et al. 2013;
Hovinen et al. 2014), and it highlights the role of global climate in affecting Arctic-breeding species, notably those that are long-distance migrants (
Both et al. 2010).