Egg laying in inappropriate nests by the Brown-headed Cowbird (Molothrus ater): acts of parasitism or emergency egg dumping?

Abstract

The generalist, brood-parasitic Brown-headed Cowbird (Molothrus ater) has been found to parasitize the nests of about 220 species, mostly passerine birds. Among the thousands of documented cases of parasitism are rare records of egg laying in nests in which the cowbird stands no chance of success, because its diet or developmental strategy are incompatible with those of the “host” species. Forty-four nests of 16 such inappropriate host species are reviewed: 23 nests of nine precocial species (waterbirds and shorebirds) plus 21 nests of seven altricial species (a raptor, doves, cuckoos, a hummingbird, and a woodpecker). Two hypotheses explain inappropriate egg laying. In the “normal laying” hypothesis, Brown-headed Cowbirds may lay dozens of eggs in nests they encounter, including the occasional inappropriate nest. In the “emergency laying” hypothesis, females, on discovering that a selected nest has failed, must lay or “dump” her eggs elsewhere, in nests of inappropriate hosts or already-parasitized nests of regular hosts. Support for either hypothesis will require electronic surveillance of movements of nest-searching and laying Brown-headed Cowbirds to generate fine-scale spatial data that confirm whether parasitism on inappropriate nests occurs at the usual laying time for pre-selected nests (around sunrise) or later in the day if the chosen nest has failed and emergency laying is required.