We compared the composition and overlap in the chick diets of the two most abundant auks in the Northwest Atlantic, the Common Murre (Uria aalge; hereafter murres) and the Atlantic Puffin (Fratercula arctica; hereafter puffins). Our objective was to assess how auks with different foraging strategies (dietary specialization among murres vs. generalization among puffins) and physiological constraints imposed by differences in body size and prey-loading meet the energy demands of reproduction. Murres provisioned chicks with female capelin (>100 mm): 98.8% by number in 2004 and 95.5% in 2005. The relative contribution of gravid capelin to murre chick diets decreased by an order of magnitude between years (41.1 and 4.9% by number in 2004 and 2005, respectively) and was related to the timing of capelin spawning. Puffins provisioned chicks with high proportions of forage fish in both years with the focal species changing from sandlance Ammodytes spp. in 2004 (97.6% by index of relative importance, IRI) to capelin (>100 mm) in 2005 (92.3% IRI). Murre fledglings were significantly lighter in 2005 (203.0 ± 4.6 g) compared to 2004 (215.0 ± 3.9 g) whereas puffins chicks exhibited no such differences. These results suggest that generalized feeding is a viable strategy for puffins but that specialized feeding by murres can lead to poor offspring condition when there is low overlap between peak food availability (i.e., capelin spawning) and chick-rearing.
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Vol. 31 • No. 3