No difference in chick-provisioning by bridled and non-bridled Common Guillemots Uria aalge
Department of Natural Sciences, Tromsø University Museum, NO-9037 Tromsø, Norway.
Despite recent evidence that bridled Common Guillemots Uria aalge may depend more on cold-water prey species than non-bridled, there was no morph-specific difference in the species composition or size of fish caught for chicks by adults in a North Norwegian colony. This supports the recent suggestion that selection pressure on the morphs acts in the non-breeding season, rather than in summer.
Parental investment is a trade-off between the costs or benefits of resource allocation to offspring or to self-maintenance and own survival (Royle et al. 2004). An excess effort in either direction will lead to natural selection against the individuals in question. In birds, feeding chicks is costly and in most species both parents contribute to the effort, although not necessarily equally (Lack 1968; Lewis et al. 2002 and references therein). Many studies have documented sexual differences in foraging behaviour and food provisioning, even among bird taxa where sexual dimorphism is all but absent (e.g. Table 1 in Lewis et al. 2002). Among the auks (Alcidae), these include subtle differences in behaviour, time spent in the colony, the time of day spent foraging, dive depths or dive lengths but not necessarily in the choice of fish (e.g. Thaxter et al. 2009; Elliott et al. 2010; Paredes & Insley 2010). Among Uria guillemots, the behavioural differences may partly be due to a differential allocation of effort to maintain or enhance body condition in anticipation of the period of post-fledging paternal care and hence future survival (Thaxter et al. 2009; Elliott et al. 2010).
Whereas sexual dimorphism is minimal among guillemots, the Common Guillemot U. aalge is a clear colour-dimorphic species consisting of a bridled morph with a white eye-ring and auricular groove and a non-bridled morph with a black head and no such patterning. There are otherwise no differences in morphology and body measurements (e.g. wing, culmen, gonys, head+bill) of the two morphs (Table 1: 2-sample t-test, P > 0.05 for all measurements). The frequency of the bridled form, however, increases with latitude, suggesting a higher tolerance for cold than non-bridled birds (Birkhead 1984 and references therein). This was supported by Reiertsen et al. (2012) whose models clearly showed a differential survival between the two morphs at a breeding colony in North Norway, with that of the bridled morph being negatively correlated to the winter sea-surface temperature in the Barents Sea while that of the non-bridled morph was slightly positively correlated to the same parameter. They also suggested that there was no differential selection on the two morphs during the breeding season, but rather during the winter. This is in contrast to Birkhead (1984) who suggested the opposite pattern. Reiertsen et al.'s (2012) study also suggested that the bridled morph was more dependent on cold-water species such as Capelin Mallotus villosus as prey whereas the non- bridled birds foraged more on warm-water species such as young age-classes of Cod Gadus morhua and the Norwegian spring-spawning Herring Clupea harengus.
In North Norway, Common Guillemots feed their chicks on small, energy-rich pelagic fish such as Capelin, Herring and sandeels Ammodytes sp. (Barrett 2002). Capelin is a cold-water species that lives its entire life-cycle in arctic, subarctic and cool-temperate seas (Stergiou 1991). Reaching a maximum length of ~20 cm, it is an important prey for seabirds throughout its life cycle. Capelin is one of the dominant pelagic fish of the Barents Sea and in spring approaches the coast of North Norway where it spawns on the sea bed at depths of 20-75 m (but sometimes deeper) and temperatures of < 7 °C (Stergiou 1991; Gjøsæter 2009). Herring, on the other hand, is a warmer-water species, belonging to the Norwegian Sea ecosystem and entering the Barents Sea as 0- and I- group fish having drifted northeastwards as larvae. These prey remain in the Barents Sea...
I thank the Norwegian Coastal Administration for permission to use the lighthouse on Hornøya as a base for fieldwork, and Tone Reiertsen, Chris Thaxter and a second referee for comments on earlier drafts of the manuscript. The study was financed by the University of Tromsø and the Norwegian SEAPOP programme (www.seapop.no).
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