Some interesting facts:
Two varieties of cowbird are the only birds that parasitize nests in North America. They can lay up to 40 eggs a year in the other nests of over 220 different species (in about 140 of those species successfully). A lot of birds do recognize the intruder and remove the egg, bury it under more twigs, or abandon the nest for another site. The female is believed to attempt to lay her eggs in the nest of the same species that raised her. The chicks do not kill the host chicks, but rather eat most of the food, leaving the others to starve.