The Breeding Season of a Parasitic Bird, the Brown-Headed Cowbird, in Central California

Abstract

The breeding seasons of birds are usually timed so that the young are reared during a period of abundant food. At high latitudes nearly all small birds may nest within a week or two of each other. In warmer temperate climates where plants grow and insects are active for several months, the breeding seasons are longer, and different species may have different breeding seasons. Birds that feed insects to their young often breed earlier in the year than those reared mainly on seeds. Even the insectivorous species may stagger their breeding seasons when their prey appear during different weeks or months (Davis 1933; Lack 1950). Birds that do not feed their young but instead lay their eggs in nests of other species must generally lay when their foster species do. Synchronizing the breeding season with that of the hosts is no great problem for parasitic birds at high latitudes where all the hosts breed in the same few weeks. Like the brood parasites which lay their eggs in the nests of only one species of fosterer, they simply use the same environmental cues in the same manner to time their breeding as their foster species do. Parasitic species are faced with a more difficult problem when they use many foster species which breed at different times. To remain in breeding condition for a prolonged period may place considerable demands on the resources and on the chances of survival in parasitic birds. Even brood parasites that lay in the nests of several species may reap higher reproductive returns for their laying efforts in the nests of some foster species than in others.