Cowbirds are very unique in their nesting habits in that they do not build nests of their own and is what is considered to be a “brood parasite.” Female cowbirds will lay her eggs in the nests of other bird species who will then raise the young cowbirds. It’s estimated that brown-headed cowbirds will lady their eggs in more than 220 species of birds!
There are some birds that recognize the cowbird egg in their nest but are too small to get the eggs out of their nests. In these cases, new nests are built over the old one to try to prevent cowbirds from coming back. There are other species of birds that will throw cowbird eggs out of their nests or puncture the shells. The majority of birds do not recognize cowbird eggs at all.