BROWN-HEADED COWBIRDS AND AN ISLAND POPULATION OF SONG SPARROWS - A 16-YEAR STUDY

We studied brood parasitism by Brown-headed Cowbirds (Molothrus ater) in a strongly fluctuating island population of Song Sparrows (Melospiza melodia). One to three cowbird females visited the island daily to search for host nests in 13 of 16 study years. Individual cowbirds visited on more days and laid more eggs at high sparrow densities, but some factor, perhaps aggression by territorial female cowbirds, limited cowbird numbers at high sparrow densities. Female cowbirds exhibited natal and breeding philopatry. Individual cowbirds laid about every other day on average, began to lay well after sparrows in most years, and avoided laying two eggs in the same nest. Cowbirds rarely laid after hosts had completed laying, and their eggs hatched as often as host eggs. Parasitism rates increased with host age, probably because of age-related differences in host behavior. Cowbirds removed a host egg from two-thirds of parasitized nests, and damaged two per cent of remaining host eggs. Parasitism reduced production of fledgling sparrows per nest by 0.79, but only by 0.27 fledglings in years when food was added experimentally to some territories. Nests failed more often during incubation in years with than without parasitism, supporting our earlier suggestion that cowbirds destroy eggs in host nests to enhance future laying opportunities. Colonization of Mandarte Island by cowbirds had little effect on sparrow numbers because of: (1) incomplete overlap in laying seasons; (2) multiple broods in sparrows; (3) functional and numerical responses of cowbirds to host density; (4) the sparrows' ability to rear their young with young cowbirds; and (5) density-dependent host reproductive success.