Warbling vireos (lighter eggs) are parasitized by brown-headed cowbirds (more heavily speckled egg) significantly less frequently when cattle are removed from the pasture for the brief breeding season. Warbling vireos almost invariably fail to raise their own young in nests with cowbirds because cowbirds hatch 4-5 days earlier than vireos, and they out-compete the vireos for food and parental attention.
