These birds lay their eggs in the nests of other birds, mostly song sparrows, yellow warblers, common yellowthroats, and so on. However, many birds have learned to recognize the eggs of an interloper and will often build the nest over the egg and lay their own again on top, or they will turf it out. However, some species that have only recently had to contend with parasitism don't have the same coping mechanism. And even those who do cope fairly well, often end up with a huge baby cowbird. There is nothing more bizarre than seeing a little bird like a yellow warbler, feeding a very demanding (and huge in comparison) baby cowbird.