Nutrient allocation to egg formation of lesser scaup
Cutting, Kyle Anthony
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Lesser Scaup (Aythya affinis) populations have declined for nearly three-decades. Recent evidence suggests that decreases in habitat quality and availability of spring staging areas may have resulted in a decline of recruitment. Recently, stable isotopes analysis has emerged as a powerful ecological tool to measure the degree of cross-seasonal effects of birds. In 2006 through 2008 in southwestern Montana, I used carbon (13C) and nitrogen (15N) stable isotopes to assess how red blood cells (RBC), which is a proxy for stored body reserves (endogenous), change over time as local foods (exogenous) are consumed, and to estimate the relative contributions of endogenous reserves and exogenous foods for egg formation. From arrival through the egg laying period, 15N values of RBC decreased while 13C values became more stable, a pattern consistent with expectations of endogenous tissues equilibrating with local dietary sources. In 2006 and 2008, isotopic values for egg albumen and yolk protein were similar to those expected from local dietary sources, which indicated that most protein used for producing egg albumen and yolk protein was obtained on the breeding grounds (exogenous sources). In 2007, endogenous reserves contributed on average 26% and 10% more for producing albumen and yolk protein, respectively, than in 2006 and 2008 combined. Due to small differences in 13C values between female endogenous lipids upon arrival to the breeding grounds and those of local invertebrate lipids, it was not possible to separately estimate the contributions of endogenous and invertebrate lipids to egg lipid formation. My results suggest that local invertebrates and endogenous lipid reserves contributed on average 51% (SE = 7%) to egg lipid production. The remaining contributions to eggs were derived from local seed sources. Despite recent findings of reduced endogenous reserves during spring migration, results from the females in this study suggest that the amount of time that females spend on the breeding grounds prior to nest initiation may be adequate in some years to allow them to attain adequate exogenous foods for reproduction. Future isotopic research is now needed across latitudinal gradients, allowing length of prebreeding season to vary, while separating out the contribution from endogenous versus exogenous sources.