Some cowbirds make warblers an offer they can't refuse: Brood over my eggs, or I'll rough up your nest. Cowbirds are a parasitic species that lay eggs in the nests of other birds, called hosts, which ...
Some consider brown-headed cowbirds to be a threat.
Fleischer, Robert C. and Smith, Neal Griffith. 1992. "Giant cowbird eggs in the nests of two icterid hosts: The use of morphology and electrophoretic variants to identify individuals and species." The ...