The Brown-headed Cowbird is one of three brood parasites in the US. Its range covers the entire US.
The BHCO is slightly smaller than the Abert’s Towhee (1.5 oz vs 1.6). Here is a male:
This guy has an overbite that is not normal, the female that follows has a standard looking beak.
Per Wiki, this bird followed the buffalo back in the day eating bugs kicked up by the herd.
Had the Scots founded the US rather than Columbus, this could have been a sheepbird.
The Brown-headed Cowbird has over 220 hosts of which only 170 are successful (Wiki).
Cowbirds are omnivores and some of their hosts are herbivores (vegetarians). When that combination happens the chick dies of malnutrition. Here is the female:
This little lady can lay up to 36 eggs in one season. They watch the activity in the first couple of days and will lay a second egg if needed.
They will displace the eggs of the host. One study indicated that the Abert’s Towhee’s clutch size reduced from 2.9 to 1.7 due to the cowbird kicking eggs out (Finch, 1983).
I have read that removing a cowbird egg from a nest actually increases the parasitizing activity. In areas where there are active controls to save a particular species the cowbirds are euthanized.
Brown-headed Cowbird juvenile
Brown-headed Cowbird Juvenile