Cowbird Parasitism, Host Fitness, and Age of the Host Female in an Island Song Sparrow Population

Abstract

Mandarte Island, British Columbia, was colonized by Brown-headed Cowbirds (Molothrus ater) after an intensive study of the resident Song Sparrow (Melospiza melodia) population. I studied the Song Sparrow population from 1975 to 1979, when it was parasitized by one to three female cowbirds a year. Breeding success was lower than during the earlier study, but comparisons of breeding success before and after the arrival of cowbirds each year and after cowbird removal in 1977 suggested that cowbirds were not a major cause of the reduced breeding success. Parasitism depressed the breeding success of adult female Song Sparrows by 0.6 independent young per nest and that of yearling females by 0.5 independent young per nest. The costs of parasitism were almost entirely due to egg removal by female cowbirds. Parasitized nests of adult females contained 0.8 fewer eggs than unparasitized nests. The corresponding figure for yearling females was 0.5 eggs. Nests of adult females were parasitized twice as often as nests of yearling females. A behavioral explanation is proposed for this last difference. Song Sparrows with cowbird nest mates weighed less than control young, but survival of young from six days after hatching to independence did not decrease if there was a cowbird in the brood. Female Song Sparrows that were parasitized one or more times during a breeding season raised as many young to independence per year as females that were not parasitized. This was because parasitized females made more breeding attempts per year.