This male Brown-headed Cowbird started showing up at my bird feeder last week and is now a regular visitor. I haven’t seen a female yet. I actually heard this bird before I saw it. It has a very unique sound (Brown-headed Cowbird Call).
The interesting thing about these birds are that they lay eggs in the nests of other small birds, particularly those that build cup-like nests. The young cowbird is fed by the host parents at the expense of their own young. Brown-headed cowbird females can lay 36 eggs in a season. More than 140 different species of birds are known to have raised young cowbirds.
Brown-headed Cowbird Facts
Brown-headed cowbirds do not raise their own young, instead laying their eggs in the nests of other bird species. Because of this, cowbirds are not exposed to species-typical visual and auditory information like other birds. Despite this, cowbirds are able to develop species-typical singing, social, and breeding behaviors.
The brown-headed cowbird eggs have been documented in nests of at least 220 host species, including hummingbirds and raptors.
Some host species, such as the house finch, feed their young a vegetarian diet. This is unsuitable for young brown-headed cowbirds, meaning almost none survive to fledge.
The acceptance of a cowbird egg and rearing of a cowbird can be costly to a host species. In the American redstart, nests parasitized by cowbirds were found to have a higher rate of predation, likely due in part to the loud begging calls by the cowbird nestling, but also partly explained by the fact that nests likely to be parasitized are also more likely to be predated.
Host species sometimes notice the cowbird egg, with different hosts reacting to the egg in different ways. Some, like the blue-gray gnatcatcher, abandon their nest, losing their own eggs as well. Some, like the American yellow warbler, bury the foreign egg under nest material, where it perishes. And some, like the brown thrasher, physically eject of the egg from the nest. (Wikipedia)