Nest records for North America suggest that brood parasitism by Brown-headed Cowbirds (Molothrus ater) on cavity-nesting species is relatively rare. Indeed, a summary by Friedmann and Kiff (1985) indicated that cavity nesters constitute only 8% of known hosts of the cowbird. Of these, only the Prothonotary Warbler (Protonotaria citrea) was considered to be a major host.
In bluebirds (Sialia spp.), all cavity nesters, documented incidences of cowbird parasitism are extremely rare (Gowaty and Plissner 1998). Most involve the Eastern Bluebird (S. sialis). Power and Lombardo (1996) reported only four cases of cowbird parasitism of the Mountain Bluebird (S. currucoides), with no accounts of cowbirds raised to fledging.