Nestling Survival and Growth in the Yellow‐Headed Blackbird, Xanthocephalus Xanthocephalus

I conducted a study on survival and growth of nestling Yellow—headed Blackbirds (Xanthocephalus xanthocephalus). I examined the following hypotheses on the nature of variation in clutch size among the individuals of a breeding population: (1) clutch size variation is maintained by reversing selection favoring different clutch sizes in different years, (2) clutch size variation reflects a trade—off between offspring number and quality, and (3) the clutch size laid by an individual is determined by its ability to rear offspring under prevailing conditions of physiology or territory. I found no influence of clutch size on either survival or growth in a poor year in which 41% of those birds hatching ultimately starved. These results argue against the first hypothesis, rule out of the second, and favor the third. Individual nests of this species show a high level of hatching asynchrony, with over one—third of the nests hatching no more than one offspring per day, and few ever having more than two hatchlings in a single day. By causing older birds to get a head start in growth, hatching asynchrony promotes efficient brood reduction of younger nestlings under conditions of food stress. At the same time, hatching order has no effect on the growth of those birds fledging, indicating that those birds not eliminated suffer no impairment due to their position in the brood. The extreme dependence of survival on hatching position, coupled to processes producing adequate growth of all fledging birds, is an adaptive response to breeding under conditions where weather may suddenly charge adversely. Yellow—headed Blackbirds show an ecological trade—off between those breeding habitats having a high likelihood of nest predation and favorable feeding conditions, and those that are relatively safe from predation but poor for provisioning the young.