Spotted in Marine Park’s wild west side a week ago: the identity of this bird baffled me for while. And then it hit me. Young Brown-headed Cowbird (Molothrus ater). This bird was raised by another species, for Brown-headed Cowbirds are brood parasites: they lay their eggs in the nests of other bird species. Yellow Warbler, Song Sparrow, Chipping Sparrow, Red-eyed Vireo, Eastern Towhee, and Red-winged Blackbird are typical targets, but there are 220 species possibilities! Talk about adaptation.
Some birds will recognize the alien egg and push it out, or, if not strong enough for that, build a new nest on top to it. Other species, however, can’t recognize that the egg doesn’t belong to them (even though it may be larger). Hatchling cowbirds will then out-compete if not outright kill their step-siblings in the nest.
Brood parasitism is a remarkable adaptation by several bird species around the world. The BHC use other nests because, we think, they followed the bison around the grasslands of the American west. This left them no time to make a nest and brood a clutch of eggs themselves. They’ve expanded their range eastward as we’ve destroyed forests and otherwise paved paradise.
Some people get very moralistic about BHCs–the birds can negatively affect rare bird populations–but as usual the problem was created and/or exacerbated by us humans, definitely the world champion nest-wreckers.
Also, close by was a singing Eastern Towhee male (Pipilo erythrophthalmus) atop a cherry tree. Coincidence?