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Red-legged Kittiwakes (Rissa brevirostris) and Black-legged Kittiwakes (R. tridactyla) are morphologically similar, breed in mixed colonies, and nest at the same time, but they exhibit substantial differences in diet, rate of nestling provisioning, and foraging distribution. We cross-fostered nestlings of the two species to test the competing hypotheses that growth in mass of kittiwakes is constrained by diet (i.e. composition, provisioning rate, and quality) or by inherent species-specific physiology. Survival and body mass of cross-fostered nestlings at 30 to 32 days posthatching did not differ from those of conspecific controls. Black-legged Kittiwake fledglings had higher lean mass than Red-legged Kittiwake fledglings regardless of whether they were raised by foster or natural parents. However, nestlings of both species raised by Red-legged Kittiwakes were 50% fatter at 30 to 32 days posthatching than those raised by Black-legged Kittiwakes. Regurgitations from nestlings raised by Red-legged Kittiwakes consisted primarily of lanternfish and contained about twice the lipid (percent dry mass) as regurgitations from nestlings raised by Black-legged Kittiwakes. Consequently, growth rate of lean tissue was genetically and/or physiologically constrained, whereas rate of fat deposition was constrained by diet. We hypothesize that the adaptive significance of lanternfish in diets for Red-legged Kittiwake nestlings is manifest in higher prefledging and/or postfledging survival. Interspecific differences in energy density of food and food provisioning rates balanced each other so that rates of energy provisioning were similar.

Diet and Postnatal Growth in Red-Legged and Black-Legged Kittiwakes: An Interspecies Cross-Fostering Experiment