Brood parasitism in a host generalist, the shiny cowbird: I. The quality of different species as hosts.

Abstract

Molothrus bonariensis is an obligate brood parasite known to use 176 species of birds as hosts. Eggs of the parasite are either spotted or immaculate in E Argentina and neighbouring parts of Uruguay and Brazil. Most species accept both morphs of cowbird eggs, 2 reject both morphs, and one (chalk-browed mockingbird Mimus saturninus) rejects immaculate eggs but accepts spotted ones. No species, via its rejection behaviour, protects the shiny cowbird from competition with a potential competitor, the sympatric screaming cowbird M. rufoaxillaris. Nestling cowbirds require a diet composed of animal protein. Because most passerines provide their nestings with such food, host selection is little restricted by diet. Species-specific nest survivorship, adjusted to appropriate values of shiny cowbird life-history variables, varied by over an order of magnitude. Shiny cowbirds peck host eggs. This density-dependent source of mortality lowers the survivorship of nests of preferred hosts and creates natural selection for greater generalization.-from Author