Patterns of variation in size and composition of Greater Scaup eggs: Are they related?

We studied egg size variation of Greater Scaup (Aythya marila) nesting on the Yukon-Kuskokwim Delta, Alaska from 1991-1996. Mean egg size was 64.36±0.03 (SE) ml. Egg size did not vary with clutch size or serve as an index of body size. There was less than 2% overlap in total clutch volumes for clutches of different sizes indicating that phenotypic clutch size-egg size trade-offs are not occurring among individuals. At the population level, Greater Scaup have less variation in egg size than other species of waterfowl. The proportion of variation in egg size caused by differences among females was 0.20, caused by differences within females among years was 0.25, and caused by differences within females and years (i.e., clutches) was 0.56. The proportion of egg lipid decreased with increasing egg size while the proportion of egg protein increased with egg size. Thus, Greater Scaup appear to trade-off lipid for protein as egg size increases. The proportion of variation that was due to differences among females in total egg protein was 0.79 and in total egg lipid was 0.49. We conclude that in the absence of a fitness trade-off between clutch size and egg size, selection has reduced among-individual variation in egg size.