Summary
The Yellow Warbler (Dendroica petechia) is frequently parasitized by the Brown-headed Cowbird (Molothrus ater) resulting in reduction of nest success at parasitized nests (Schrantz 1943, McGeen 1972). There are several avenues open to a Yellow Warbler once a cowbird egg has been deposited in its nest. It could accept the egg, thereby running the risk of the egg hatching and the cowbird nestlings competing with the Yellow Warbler nestlings. Alternatively, it could reject the egg. This could occur by ejection, where the cowbird egg is removed from the nest (cf. Rothstein 1975), by nest desertion, or by burial, in which the cowbird egg, along with any Yellow Warbler eggs present at the time, are covered by the addition of nesting material. The response favored by natural selection depends upon the potential for a successful nest attempt. The possibility of success varies with the amount of time and energy already invested in the nesting attempt, and the possibility of the cowbird egg hatching. The objectives of this study were to determine the frequency of occurrence of these various responses by Yellow Warblers to naturally deposited cowbird eggs and to investigate the factors eliciting each response.
Methodology
Yellow Warbler nests were located in several study areas near the Queen's University Biological Station, Chaffey's Locks, Ontario, from 1975-1977. Most nests were found during nest building. Nests for which the date of clutch initiation was unknown have not been included in this analysis unless noted. In 1975 and 1976, nests were checked every second day. In 1977, nests were checked daily during egg-laying and early incubation and then every third day until the young fledged. All references to number of Yellow Warbler eggs indicate the number present when the nest was checked. In some parasitized nests, 1 or more Yellow Warbler eggs may have been removed by cowbirds. Our measure of nest success was the number of young leaving the nest as a proportion of the number of eggs laid. All references to nest success are only to those nests not preyed upon. For nests which received more than 1 cowbird egg, only the response to the first egg is included in tables and text unless specified otherwise.