I have seen so many of these Brown-headed Cowbirds around my yard and feeders this spring, many more than usual. They are not at all skittish, and often stay in the feeders or trees while I get very close to them with the camera. According to allaboutbirds.org, the cowbirds do not build their own nests and put all their energy into producing eggs, up to three dozen a season. They lay these eggs in the nests of other birds, including those of vireos, wood thrushes, woodpeckers and other songbirds.
From wikipedia: These birds feed on insects, including the large numbers that may be stirred up by cattle. In order for the birds to remain mobile and stay with the herd, they have adapted by laying their eggs in other birds’ nests. The cowbird will watch for when its host lays eggs, and when the nest is left unattended, the female will come in and lay its own eggs.
The brown headed cowbird has over 220 hosts (Friedmann 1997). The other cowbird species have less hosts that are known, but they are all generalists when it comes to choosing a host. This means that the eggs may look different from the hosts’ eggs. The cowbird chicks grow quickly, and may consume most of the food the host brings. If starvation does not kill the other birds, in some species the cowbird will use its large size to push the other chicks out of the nest.