Robin's (Turdus migratorius) nest with a brown headed cow bird (Molothrus aster) egg. Cowbirds (genus Molothrus) are brood parasitic birds that lay their eggs in other bird's nests. When they hatch, the chicks grow quickly and will often consume most of the food brought by the host parent. If starvation does not kill the host's young, the cowbird will often use its large size to push them out of the nest.