Cowbirds will sometimes dominate bird feeders and have been proven to have contributed to population decreases in birds like the Kirtland's  and  Golden-cheeked Warbler.  Trapping programs have been used to reduce cowbird populations in some areas.  If cowbirds are dominating your feeder try different food choices, such as safflower or suet.

Males and females are different in appearance.  Males are black with a brown head, naturally.  The females are a dull gray-brown in in color.

 

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The Brown-headed Cowbird is a parasitic nester, in that it lays its eggs in the nest of other species.  One theory is that the cowbirds followed buffalo herds as the buffalo roamed, feeding on the insects they buffalo scared up.  Since the buffalo roamed widely, the cowbirds were forced to either stop following the buffalos or lay their eggs in the nest of other species, to raise their young.  Some song birds seem to happily raise the cowbirds, while others create a new nest when a cowbird egg is discovered,.  These days the lawn mower has replaced the buffalo and the cowbirds have become common in suburban areas.