The Blue Grosbeak is a large bunting of the southern forest edge, often seen singing from roadside wires and tree tops. Although widespread throughout its breeding range, it is generally scarce and virtually all aspects of its biology are poorly known, perhaps owing in part to its low numbers. While the breeding biology of this species is likely similar to that of its relative, the Indigo Bunting, we still lack detailed information about Blue Grosbeak nesting ecology, courtship behavior, and song structure and learning, among other things. In addition, little is known about populations that breed in Mexico and Central America, and even less about northern populations on their wintering grounds.
The Blue Grosbeak's breeding range overlaps with that of the related Passerina buntings, so ecological competition with these species may help to keep its numbers low. Nesting in shrubs or tangles of vines along forest edge or roadsides, Blue Grosbeaks commonly produce two broods per year. Limited data suggest that it is quite heavily parasitized by the Brown-headed Cowbird (Molothrus ater) and a known host for the Bronzed Cowbird (M. aeneus). It feeds mostly on insects, especially grasshoppers and crickets, and less so on the seeds of wild and cultivated grains; in fall, flocks often gather in ricefields. Most Blue Grosbeaks nesting in the eastern United States appear to migrate across the Caribbean, as individuals are sighted regularly on Caribbean islands during spring and fall migration.
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