I was tromping through the woods the other day and came upon this little cup nest (literally the size of a medium-sized tea-cup) with two eggs in it. It turns out to be a Red-eyed Vireo nest, but you'll notice that the eggs are different patterns and sizes. The smaller white egg belongs to the vireo; the larger speckled egg is a Brown-headed Cowbird egg. According to Sibley's Guide to Bird Life and Behavior, vireos are conspicuous nest-builders and often attract the attention of cowbirds while constructing their nest. The cowbird then lays an egg (or two or three) in the nest and the vireos raise them alongside (or instead of) their own children.
This nest is deep in the woods, a good half-mile as the cowbird flies from our neighbor's horse pasture (the nearest cowbird habitat to the vireo nest). Cowbirds are relatively recent migrants to Eastern North America. They're original North American habitat was in the Great Plains, where they followed herds of bison, eating the insects that were drawn to the bison. Apparently the ancestors of cowbirds developed nest parasitism before they came to North America, but it is obviously well suited to the nomadic behavior of bison. Over the past century or so, they have not only adapted to the virtual disappearance of bison, but have also moved Eastward, making themselves at home in the cleared openings and livestock pasture dotting the Eastern forests.
Their continuance of this nesting tradition in the Eastern forests is a fine example of the cowbirds' exceptional adaptability. It's remarkable that they know where to go to find the woodland bird nests that they parasitize. They are prolific egg-layers, often parasitizing multiple nests, laying many times the number of eggs of their host species. This situation has contributed to the threats to survival of some of the rarer woods-warblers. They also represent one if the many ways, subtle and unsubtle, that humans have impacted the Eastern forests.