Chestnut-sided warbler

Chipmunks sometimes steal the warbler eggs and newly hatched young. Nest parasitism, however, is much more common. The female brown-headed cowbird spies on the nest of the chestnut-sided warbler and as soon as the parents leave the nest unguarded, the cowbird replaces one of the warbler eggs with one of its own, and then leaves. The cowbird egg generally hatches before those of the warbler and the young cowbird pushes the eggs and young of its host out of the nest. Often, the warbler doesn't notice the switch and raises the cowbird youngster as if it were its own. To reduce the threat of parasitism, the female warbler sometimes covers its eggs and that of the cowbird with a layer of plant material and starts a new clutch of eggs.