ARE GREATER SNOW GEESE CAPITAL BREEDERS? NEW EVIDENCE FROM A STABLE-ISOTOPE MODEL

Abstract

The strategy of relying extensively on stored nutrient reserves for reproduction (capital breeding) was thought to be common in large-bodied birds breeding in harsh environments, such as arctic-nesting geese, but this view has been challenged recently. Our objective was to model inputs to the eggs from stored reserves and from local food plants in Greater Snow Geese (Chen caerulescens atlantica) breeding in the high Arctic, using a new approach based on stable-isotope signatures. Snow Geese and their eggs were collected during laying from 1999 to 2001 (N = 66 females and 110 eggs). We analyzed the isotopic signature (δ13C and δ15N) of egg constituents (lipid-free yolk, yolk lipid, and albumen), goose tissues (lipid-free breast muscles, abdominal fat, and whole liver) and of the food plants eaten by laying geese in the Arctic (graminoids and forbs). We applied a two-isotope mixing model approach to delineate nutrient input to eggs quantitatively. Differences in the isotopic signature of endogenous reserves and arctic food plants were relatively large (5.3–8.0‰ for Δδ13C and 7.5‰ for Δδ15N) because reserves were accumulated in southern staging areas where geese feed in farmlands and estuarine habitats. The percentage of egg nutrients derived from exogenous sources (food consumed in the Arctic) was higher than from endogenous (body) reserves and varied little among the three years. Isotopic signatures indicated that endogenous reserves contributed 33% of lipid-free yolk nutrients, 27% of albumen, and 20% of yolk lipid, on average. Isotopic signatures of egg constituents of individual females were more strongly related to those of liver than endogenous sources (breast muscles or abdominal fat), indicating that the endogenous isotopic signature was diluted by a dietary input in the liver. We also found evidence of seasonal variation in the use of endogenous reserves. Late-laying females apparently invested proportionally more endogenous reserves in their eggs than did early layers, but not those laying larger clutches. We conclude that Greater Snow Geese use a mixed capital/income breeding strategy. Our study shows that isotopic composition of tissues can be used to infer the contribution of exogenous vs. endogenous sources of nutrients for egg formation where inputs differ isotopically.