Brood parasitism by the Shiny Cowbird, Molothrus bonariensis, on the Rufous-collared sparrow, Zonotrichia capensis: New Observations and Conclusions
Abstract
Data on the brood parasitism of the Shiny Cowbird, Molothrus bonariensis, on the Rufous-collared Sparrow, Zonotrichia capensis, was detected during 10 breeding seasons (1976–1979; 70 nests) in a study done at Buenos Aires Province, Argentina. Although the cowbird parasitized 14 species of local passerines, (40% of the total) most parasitic fledglings (46%) were reared by the sparrow. During the breeding season a sparrow considerably outnumbered the cowbird. Home ranges of banded female cowbirds ranged from 21 to 48 ha, and other females were not excluded; the females were rather gregarious. After 1975 the incidence of parasitism in nests of the sparrow declined significantly; the overall incidence was 61.9%. Two morphs of cowbird eggs were found in similar proportions in sparrow nests; measurements, mean and shape of the egg morph, are included. The distribution of all cowbird eggs in the nests of this accepted host departed significantly from Poisson and Preston (1948) distributions, but those of separate egg morphs agreed closely with expected distributions. Cowbird females laid 1 or 2 eggs per sparrow nest; with 1 exception, sets of 3 or more cowbird eggs represented parasitism by 2 or more females. The nesting success of host and parasite was low, being only 5.56% of cowbird eggs. Cowbirds destroyed relatively few (6.58%) cowbird eggs. In order to have 1 fledgling cowbird reared by sparrows each breeding season, female cowbirds should lay 18 eggs in the nests of this host. The results are compared with those of other studies.
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