Shiny Cowbird Parasitism in Two Avian Communities in Puerto Rico

The Shiny Cowbird (Molothrus bonariensis), a brood parasite, has recently expanded its range from South America to Puerto Rico via the Lesser Antilles. This species is a host generalist and, on reaching Puerto Rico, encountered avian species with no history of social parasitism. In mangrove habitat study areas, 42% of the resident non-raptorial land bird species were parasitized. Some species were heavily parasitized; e.g., Yellow Warbler (Dendroica petechia)-76% of nests parasitized, Black-whiskered Vireo (Vireo altiloquus)-82%, Puerto Rican Flycatcher (Myiarchus antillarum)-85%, Yellow-shouldered Blackbird (Agelaius xanthomus)-95%, Troupial (Icterus icterus)-100%, Black-cowled Oriole (I. dominicensis)-100%. Others suffered low rates of parasitism (2 to 17% of nests examined); e.g., Gray Kingbird (Tyrannus dominicensis), Red-legged Thrush (Turdus plumbeus), Bronze Mannikin (Lonchura cucullata), Northern Mockingbird (Mimus polyglottos), Greater Antillean Grackle (Quiscalus niger). Cowbird parasitism affected hosts by (1) depressing nest success an average of 41% below non-parasitized nests, and (2) reducing host productivity. Parasitized host nests hatched 12% fewer eggs and fledged 67% fewer of their own chicks than non-parasitized pairs.