Conspecific Nest Parasitism in Three Species of New World Blackbirds

Abstract

We determined the rate of conspecific nest parasitism (CNP) that occurred in populations of Red-winged Blackbirds (Agelaius phoeniceus), Yellow-headed Blackbirds (Xanthocephalus xanthocephalus), and Brewer's Blackbirds (Euphagus cyanocephalus), and tested the hypothesis that females whose nests are destroyed during laying may deposit physiologically committed, subsequent eggs in nearby conspecific nests. CNP occurred in only 0.5% of Red-winged Blackbird nests, in 1.1% of Yellow-headed Blackbird nests, and in about 3% of Brewer's Blackbird nests. Nearby nests were destroyed during the laying period within three days of detected CNPs in less than 50% of the cases for both Red-winged and Yellow-headed Blackbirds. We discuss possible options of female songbirds for resorbing, aborting or laying physiologically committed eggs after their nests are destroyed during laying, and the potential costs and benefits to these females of CNP.