How now brown cow?

Brown-headed Cowbird, female,
Molothrus ater

As, all around,
they build and brood,
she watches,
waiting.

Brown-headed Cowbirds are brood parasites, meaning they lay their eggs in other birds' nests and leave their young to be raised, sometimes displacing the young birds of other species. Cowbird eggs have been discovered in the nests of 220 species, including everything from songbirds, to small ducks and woodpeckers.
More about Brown-headed Cowbirds may be found through this Audubon site.