BIRDS AND BUTTERFLIES
By Julie Jansen
The Brown-headed Cowbird is a stocky blackbird with a fascinating approach to raising its young. Females forgo building nests and instead put all their energy into producing eggs, sometimes more than three dozen a summer. They are called "brood parasites" which means they lay their eggs in the nests of other birds, abandoning their young to foster parents, usually at the expense of at least some of the host's own chicks. Even though Brown-headed Cowbirds are native to North America, many people consider them a nuisance bird, since they destroy the eggs and young of smaller songbirds and have been implicated in the decline of several endangered species, including Kirtland's Warbler and Black-capped Vireo. Cowbirds often flock with other species of blackbirds, and they may come to your yard if it contains open ground or lawn, or if you scatter grain for ground birds. If your yard is large enough to keep livestock, there's a good chance you will find cowbirds there.
Named for the habit of feeding near cattle and other grazers, cowbirds walk on the ground with their tails held up. The male is glossy black with contrasting brown head. The females are a plain dusty brown with a dark sparrow like bill.
Some interesting facts about cowbirds:
Brown-headed Cowbird lay eggs in the nests of more than 220 species of birds. Recent genetic analyses have shown that most individual females specialize on one particular host species.
Some birds, such as the Yellow Warbler, can recognize cowbird eggs but are too small to get the eggs out of their nests. Instead, they build a new nest over the top of the old one and hope cowbirds don't come back. Some larger species puncture or grab cowbird eggs and throw them out of the nest. But the majority of hosts don't recognize cowbird eggs at all.
Cowbird eggs hatch faster than other species eggs, giving cowbird nestlings a head start in getting food from the parents. Young cowbirds also develop at a faster pace than their nest mates, and they sometimes toss out eggs and young nestlings or smother them in the bottom of the nest.
In winter, Brown-headed Cowbirds may join huge roosts with several blackbird species. One such mixed roost in Kentucky contained more than five million birds.
The oldest recorded Brown-headed Cowbird was a male, and at least 16 years 11 months old when it was recaptured and rereleased during banding operations in Wisconsin.