Allocation of Nutrients to Reproduction at High Latitudes: Insights from Two Species of Sympatrically Nesting Geese

—Understanding factors that determine sources of nutrients for reproduction is vital to interpreting different lifehistory strategies used among species, within species, and among taxa. We used stable carbon isotope analysis (δ13C) to compare the relative use of endogenous and exogenous nutrients during egg synthesis between two species of geese, Canada Geese (Branta canadensis interior) and Lesser Snow Geese (Chen caerulescens caerulescens), nesting on Akimiski Island, Nunavut. We estimated that Canada Geese used 49 ± 1.1% (SE), 61 ± 1.3%, and 51 ± 1.1% endogenous nutrients for albumen, yolk protein, and yolk lipid, respectively. Lesser Snow Geese used significantly less endogenous nutrients during clutch formation, allocating only 25 ± 1.2%, 36 ± 1.5%, and 34 ± 1.7% endogenous nutrients for albumen, yolk protein, and yolk lipid, respectively. Although the proportion of endogenous nutrients allocated to eggs did not vary by year in Canada Geese, proportions varied significantly among years in Snow Geese. We discuss how access to exogenous nutrients appears to be an important factor in determining nutrient allocation strategies during egg production in geese and conclude that, although body size is likely an important ultimate factor in determining overall breeding strategies in birds, proximate factors that influence access to nutrients during egg production appear to be more important in shaping nutrient allocation to egg synthesis. Thus, interactions between physiological and morphological constraints and local environmental conditions can promote the use of flexible strategies in animals that migrate to breed. Received 5 February 2012, accepted 5 September 2012.