UNSUITABILITY OF THE HOUSE FINCH AS A HOST OF THE BROWN-HEADED COWBIRD

Brown-headed Cowbirds (Molothrus ater) parasitized 99 (24.4%) of 406 H ouse Finch (Carpodacus mexicanus) nests observed at Barrie, Guelph, Or illia, and St. Catharines, Ontario, Canada, during the periods 1983-19 85 and 1990-1993, Hatching success of cowbird eggs was 84.8%, but no c owbird was reared. Cowbird growth was severely retarded; nestlings req uired about twice as much time to accomplish the same amount of growth observed in nests of other hosts. Estimated final body mass of nestli ng cowbirds was about 22% lower than normal. Cowbird nestlings survive d on average only 3.2 days. Only one cowbird fledged but died within o ne day. Lack of cowbird survival in nests of the House Finch appears t o be the result of an inappropriate diet. We conclude that nestling di et may be important in determining cowbird choice of host.