Use of Artificial Brown-Headed Cowbird Eggs as a Potential Management Tool in Deterring Parasitism

Abstract

We added artificial and real brown-headed cowbird (Molothrus ater) eggs to red-winged blackbird (Agelaius phoeniceus) nests in 1985 and 1991 to determine the effect of egg addition on subsequent nest parasitism. We added large artificial eggs measuring 26.1 × 17.2 mm, similar in size to red-winged blackbird eggs, and small artificial eggs measuring 20.1 × 16.1 mm, similar in size to brown-headed cowbird eggs. Brown-headed cowbirds avoided parasitizing nests to which we added real or artificial cowbird eggs. In 1985, only 5.3% (3 of 57) of experimental nests were parasitized by cowbirds, compared with 31.5% parasitism of control nests (P = 0.001, n = 54)