Brown-headed Cowbird Female

Brown-headed Cowbird Female
(Molothrus ater )

Distinctive bill.

npwrc
Breeding Birds of North Dakota
By Family Icteridae (With blackbirds, orioles, grackles, meadowlarks)

cornell
(Has a pic of a Parasitized Yellow Warbler nest.)
"Females forgo building nests and instead put all their energy into producing eggs, sometimes more than three dozen a summer. These they lay 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."

enature
"Upon choosing a nest, she removes one egg of the host's clutch, and deposits one of her own in its place. Unlike parasitic Old World cuckoos, which lay eggs closely resembling those of a host species, cowbirds lay eggs in the nests of more than 200 other species, most smaller than themselves. Some host species eject the unwanted egg, others lay down a new nest lining over it, but most rear the young cowbird as one of their own. The young cowbird grows quickly at the expense of the young of the host, pushing them out of the nest or taking most of the food."

More info

UPDATE:
Sat, 19 Jun 10, got a pic of a male.