In this video a female cowbird attempts to lay her egg in a Bell's Vireo nest. Except that her timing isn't great — the mother vireo is still inside the nest, and the father goes on an all-out-attack. But the cowbird appears to proceed unphased—hops onto the nest, leans forward and ditches her egg right on top of the mother bird! If you watch closely, you'll see the mother peek out after the cowbird leaves and the egg rolls right off her back out of the nest.

For more about cowbirds' bizarre behaviors, check out http://www.pbs.org/wgbh/nova/sciencen...

Video credit: Kevin Ellison