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Brown-headed Cowbird (r.). Females
The Brown-headed Cowbird is found in 48 states; open country more than urban environments. The female (shown here) chooses the nest of another bird species. The Cowbird hatchling emerges sooner and is larger, than most other species. It will often shove unhatched eggs out of the nest.
The Cowbird is not intentionally being mean; it is in survival mode. For centuries, it followed herds of wild buffalo and cattle, eating the insects stirred up (hence the name "cow"bird). There was no way the female could incubate her eggs in her own nest, while the herd moved on. So it developed the practice of using other birds' nests.
Cowbirds avoid "parasitizing" the nests of House Finches. Why?? Most baby birds, including Cowbirds, are fed easily-digested insects by their parents. Not House Finches. House Finches are one of the few birds that feed seeds to their nestlings. Baby Cowbirds cannot handle the all-seed diet.