|Electronic supplementary material (ESM_1)
Time in water as a proxy of foraging time and foraging success
Time in water as a proxy of foraging time. European shags and other members of the family Phalacrocoracidae have a unique partially wettable plumage consisting of an outer section that becomes wet on immersion in water and a waterproof inner section (Grémillet et al. 1998; Grémillet et al. 2005). As a result, the insulating layer of air trapped in the plumage is thinner than in other aquatic birds that have homogeneous waterproof feather structure, resulting in higher energetic costs of diving and resting on the water (Enstipp et al. 2005; Enstipp et al. 2006). European shags therefore enter water principally to forage. Deployments of loggers during the breeding season (Daunt et al. 2006) record both time in the water and time spent actively foraging. These data demonstrate the highly significant relationship between time in water and foraging time (Fig. ESM_1.1). Time spent diving accounted on average for 84% of time in water, with no change in this proportion with increasing time in the water.
Time in water as a proxy of foraging success. Under many circumstances the time spent in the water is also an accurate reflection of foraging success. This is because of a second unusual feature of cormorant biology: they do not carry substantial body reserves at any time of the year, so they rely on their daily food intake to balance their daily energy requirements (Grémillet et al. 2003). As such, daily foraging time will be an accurate reflection of foraging success. This relationship will not hold in other species of aquatic bird that store substantial body reserves because birds may be operating on a daily basis in balance, net gain or net loss. For Phalacrocoracidae, exceptions to foraging time reflecting foraging success will occur when there are constraints placed on foraging time. We present two such constraints in this study:
Extrinsic: day length placed an upper limit on potential foraging time in midwinter such that individuals in both age classes used all of the available day length for foraging.
Intrinsic: In the two weeks prior to death, we recorded a linear decline in foraging time. Given the proximity to death, we believe that this reflects a weakening, increasingly moribund individual, rather than one that is showing a linear increase in foraging success. Thus, foraging time and foraging success would not be correlated under these circumstances.
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