DOI 10.1007/s00442-010-1853-4PHYSIOLOGICAL ECOLOGY- ORIGINAL PAPER Do purely capital layers exist among Xying birds? Evidence of exogenous contribution to arctic-nesting common eider eggs

Abstract The strategy of relying extensively on stored resources for reproduction has been termed capital breeding and is in contrast to income breeding, where needs of repro-duction are satisWed by exogenous (dietary) resources. Most species likely fall somewhere between these two extremes, and the position of an organism along this gradi-ent can inXuence several key life-history traits. Common eiders (Somateria mollissima) are the only Xying birds that are still typically considered pure capital breeders, suggest-ing that they depend exclusively on endogenous reserves to form their eggs and incubate. We investigated the annual and seasonal variation in contributions of endogenous and exogenous resources to egg formation in eiders breeding at the East Bay colony in the Canadian Arctic. We collected prey items along with females and their eggs during various stages of breeding and used two complementary analytical approaches: body reserve dynamics and stable isotope [13C, 15N] mixing models. Indices of protein reserves remained stable from pre-laying to post-laying stages, while lipid reserves declined signiWcantly during laying. Similarly, stable isotope analyses indicated that (1) exoge-nous nutrients derived from marine invertebrates strongly contributed to the formation of lipid-free egg constituents, and (2) yolk lipids were constituted mostly from endoge-nous lipids. We also found evidence of seasonal variation in the use of body reserves, with early breeders using pro-portionally more exogenous proteins to form each egg than late breeders. Based on these results, we reject the hypothe-sis that eiders are pure capital layers. In these Xying birds, the Wtness costs of a strict capital breeding strategy, such as temporary loss of Xight capability and limitation of clutch and egg size, may outweigh beneWts such as a reduction in egg predation rate