NUTRIENT-RESERVE DYNAMICS OF BREEDING LESSER SCAUP - A TEST OF COMPETING HYPOTHESES

We analyzed nutrient reserves (lipid, protein, mineral) and organs of Lesser Scaup (Aythya affinis) breeding in southwestern Manitoba. Our analysis provided weak support for the hypothesis that paired males are physically superior to those pairing later or not at all. Protein reserves of paired males were larger than those of unpaired males, whereas fat and mineral reserves were similar in size between pair-status groups. Except for a marked decline in lipid reserves, body components and organs of paired males changed little while on breeding areas before laying by their mates. Nutrient reserves of males also did not change relative to the nutrient commitment to reproduction by their mates. Females accumulated protein and mineral reserves and at least maintained lipid reserves while on breeding areas before laying. Lipid reserves of females declined, on average, 0.5 g for every 1 g of lipid deposited in eggs. Mineral reserves declined, on average, 0.1 g for every 1 g of egg shell produced. Declines in nutrient reserves of incubating females could account for 12% of their energy requirements during that period. Lipid-reserve dynamics of females were consistent with predictions of the lipid-limitation hypothesis, but contradicted those of the protein-limitation and migrational-uncertainty hypotheses. Food availability on breeding areas may limit lipid storage by females, and serve as an important proximate factor affecting clutch size of Lesser Scaup.