Unsuitability of the House Finch as a host of the Brown- headed Cowbird

Abstract

Brown-headed Cowbirds (Molothrus ater) parasitized 99 (24.4%) of 406 House Finch (Carpodacus mexicanus) nests observed at Barrie, Guelph, Orillia, and St. Catharines. Ontario, Canada, during the periods 1983-1985 and 1990- 1993. Hatching success of cowbird eggs was 84.8%, but no cowbird was reared. Cowbird growth was severely retarded; nestlings required about twice as much time to accomplish the same amount of growth observed in nests of other hosts. Estimated final body mass of nestling cowbirds was about 22% lower than normal. Cowbird nestlings survived on average only 3.2 days. Only one cowbird fledged but died within one day. Lack of cowbird survival in nests of the House Finch appears to be the result of an inappropriate diet. We conclude that nestling diet may be important in determining cowbird choice of host.