Carolina wrens built a nest in a plant on my front porch recently. Before the wrens could lay eggs I noticed a large speckled egg just outside the entrance hole to the wren nest, way too large an egg for a wren to produce. I’ve seen this before; it was the egg of a brown-headed cowbird.
Unfortunately this and similar sights are pretty common during the nesting season. I have found blue grosbeak nests with three light blue grosbeak eggs and one brown-speckled cowbird egg. If you have ever found a nest with an odd egg in it, very different from the others, you can bet it is a cowbird egg.
Brown-headed cowbirds are brood parasites. That is, they lay their eggs in the nests of other species and leave the parenting chores to them. Invariably the species chosen to raise the cowbird chick is much smaller and less aggressive than the cowbird. This size advantage for the baby cowbird ensures it will out-compete the foster parent’s chicks. Generally songbirds will feed the largest open mouth inthe nest. The cowbird gets fed every time.
You might also see a small bird such as a song sparrow or an Eastern phoebe feeding a fledged, begging, much larger bird. That will be the brown-headed cowbird chick. I see this several times a year, and I have never seen the foster bird’s chicks anywhere around. The brown-headed cowbird works its sinister system to perfection.
Cowbirds are native to North America, so some species have adapted to the cowbirds and successfully thwart their efforts. Cavity and bird-box-nesting birds are not affected as much. Yellow warblers can recognize the different egg and will wall it off in the cup, depriving it of incubation. Sometimes a new nest is just built over the original.
Unfortunately many of our Eastern birds did not evolve with cowbirds, so they have not learned to cope with the situation. They think the cowbird egg is their own and expend all of their energy raising a single cowbird chick.
Every time you see a brown-headed cowbird, it represents a failed nesting attempt of another species. And cowbirds form flocks of thousands of individuals each year.