Intermediate Frequencies of Egg Ejection by Northern Mockingbirds (Mimus polyglottos) Sympatric with Two Cowbird Species

Abstract

Abstract We experimentally parasitized Northern Mockingbird (Mimus polyglottos) nests to determine whether that species is more likely to eject immaculate eggs of the Bronzed Cowbird (Molothrus aeneus) or the spotted eggs of the Brown-headed Cowbird (M. ater); the latter species lays eggs that more closely resemble mockingbird eggs. Mockingbirds ejected 69% of model Bronzed Cowbird eggs, indicating that contrary to previous evidence Northern Mockingbirds eject Bronzed Cowbirds eggs at a high frequency. Mockingbirds also ejected 60% of model and real Brown-headed Cowbird eggs. Bronzed Cowbird eggs were ejected faster than Brown-headed Cowbird eggs (1.3 and 2.1 days, respectively). Ejection by mockingbirds may account for the lack of observed parasitism at our study sites in southern Texas, but it is more likely that cowbirds did not parasitize mockingbirds.