Nest parasitism by cowbirds reduces nest success by one to two young per nest (Davis and Sealy 1998).

In any case, cowbird parasitism now affects 4-77% of nests (Wiggins 2006; Davis and Sealy 2000; Jones et al. 2010).

Nest predation and brood parasitism by Brown-headed Cowbird may be higher in open habitat that is adjacent to areas with woody vegetation.

Brown-headed Cowbirds (Molothrus ater) lay their eggs in the nests of other songbird species, including Baird’s Sparrows, leaving the host parents to raise the cowbird young.

Distribution, Abundance, and Habitat In British Columbia, the Brown-headed Cowbird is widespread in southern coastal, southern interior, central and northeastern regions.

Some factors negatively affect this species such as the loss of wintering habitat, parasitism of nests by the Brown-headed cowbird and competition for some resources with Blue-winged Warbler.

During displays, a male picks a “singing tree” from where he will sing the typical cowbird song that begins with two liquid whistles, ending in a rapid glissando: “bub-blow-com-seee”.

Forest fragmentation in North American forests has resulted in both increased nest predation and increased cowbird parasitism, significantly reducing their reproductive success.