DOES SEX-BIASED HATCHING ORDER IN BALD EAGLES VARY WITH FOOD RESOURCES

In birds that exhibit sexual size dimorphism, hatching asynchrony, and siblicide, the probability of nestling mortality and the cost of rear ing young may vary with different combinations of sex and hatching seq uence. When food abundance varies spatially and temporally, parents ma y maximize their fitness by manipulating their primary sex ratio. We e xamined the sex and hatching order of nestling bald eagles (Haliaeetus leucocephalus) over 17 years on one lake and 5 years on another. The lakes are approximately the same size, but differ dramatically in eagl e density and prey abundance. In years when the number of eaglets fled ged was high (suggesting higher food abundance), there was a significa nt bias in the first-hatched chick toward the larger sex (females), an d broods with a first-hatched male and a second-hatched female (MF bro ods) were under-represented. In years with lower productivity, and on the lake with lower food abundance, there was a male-bias in the popul ation sex ratio of nestlings and the first-hatched egg. Variation in s ex ratio and hatching order may relate to the probability of siblicide associated with MF broods, or the differential cost of raising the se xes.