
The most hated bird: brown-headed cowbird (Molothrus ater) at San Pedro House.
From the Cornell Lab:
The brown-headed Cowbird is a stocky blackbird with a fascinating approach to raising its young. Females forgo building nests and instead put all their energy into producing eggs, sometimes more than three dozen a summer. These they lay in the nests of other birds, abandoning their young to foster parents, usually at the expense of at least some of the host’s own chicks. … Some birds, such as the yellow warbler, can recognize cowbird eggs but are too small to get the eggs out of their nests. Instead, they build a new nest over the top of the old one and hope cowbirds don’t come back. Some larger species puncture or grab cowbird eggs and throw them out of the nest. But the majority of hosts don’t recognize cowbird eggs at all. …
Cowbird eggs hatch faster than other species eggs, giving cowbird nestlings a head start in getting food from the parents. Young cowbirds also develop at a faster pace than their nest mates, and they sometimes toss out eggs and young nestlings or smother them in the bottom of the nest.