Reproductive Interactions of the Shiny Cowbird and the Yellow-Shouldered Blackbird

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Abstract
Studies were made during 4 breeding seasons the reproductive relationships of a brood parasite, the shiny cowbird (Molothrus bonariensis) and the yellow-shouldered blackbird (Agelaius xanthomus), its main host in coastal Puerto Rico. At one study site, in eastern Puerto Rico, all 26 yellow-shouldered blackbird nests were parasitized, while in southwest Pureto Rico, 38 of 54 blackbird nests had cowbird eggs. Lack of parasitism was due to habitat preferences of cowbirds, and there was no seasonal variation in the incidence of parasitism of the blackbirds. Cowbirds parasitized only 5 of 110 nest of other passerines nesting in the same habitats at the same time as the blackbirds. In southwest Puerto Rico, cowbird brood parasitism reduced the reproductive output of yellow-shouldered blackbirds by 0.39 fledglings per nest. In eastern Puerto Rico the effect of brood parasitism was more severe than in the southwest, this difference being related to the greater intensity of cowbird parasitism in the former region. In the southwest, cowbird and blackbird production was nearly balanced: 35 nests produced 27 blackbirds and 24 cowbirds; in the east, 18 nests produced only 3 blackbirds, but 17 cowbirds. In the east, most blackbird nest losses occurred during the egg stage, due to egg-puncturing by cowbirds. This behavior was rare in southwest Puerto Rico. Compared to the effect of the shiny cowbird on its main South American host, the rufous-collared sparrow (Zonotrichia capensis), cowbird parasitism in southwest Puerto Rico caused a lesser reduction in yellow-shouldered blackbird reproductive output. Yellow-shouldered blackbird nestlings are larger than shiny cowbird nestlings, and thus are at less competitive disadvantage than is the rufous-collared sparrow toward the shiny cowbird. The preference shown by shiny cowbirds for yellow-shouldered blackbird nests is thought to be a behavioral result of an expanding parasite population, in combination with features of the blackbird''s biology which predispose it to brood parasitism. Reproductive data from 1 breeding season and survival data collected during 3 yr indicate that the yellow-shouldered blackbird population may not be maintaining itself, in part due to cowbird brood parasitism. Other populations of brood parasites have had similar effects on new hosts.

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