The birds in my Decatur yard usually give me joy, but the other morning a couple of birds hanging out at the feeders made me wince. They were brown-headed cowbirds, which, during the nesting season, usually are up to no good.
No other bird, perhaps, is loathed by birders as much as the brown-headed cowbird. That’s because it is a “brood parasite, ” which lays its eggs in the nests of other songbird species. (Cowbirds, which breed April through July, are in the same family as blackbirds and orioles.)
In effect, cowbirds, which build no nest of their own, notoriously trick other birds into raising cowbird babies. Let‘s say that a female cardinal (a favorite cowbird target) leaves her nest just for a few minutes — but long enough for a female cowbird to sneak into the nest, quickly lay her egg next to the cardinal’s eggs, and then disappear. When the mother cardinal returns, she regards the cowbird egg as her own.
She then will incubate the cowbird egg along with her own eggs. The cowbird, though, has a shorter incubation period than most other songbirds and likely will hatch first. The baby cowbird will grow large very quickly. Nevertheless, the cardinal parents will regard it as one of their own.
The cardinals then will feed their foster baby along with their own babies. But the cowbird’s bigger size will allow it to monopolize the food, leaving the baby cardinals to starve to death.
Dozens of other Georgia songbird species — including the wood thrush, Eastern phoebe, sparrows, several warbler species and others — suffer similar threats from cowbirds. Only a few species, such as the yellow warbler, can recognize cowbird eggs and reject them or build a new nest on top of them.
Cowbird populations appear to be rising, probably due to an increase in their preferred forest edge habitat. Biologists say that cowbird parasitism is a major factor in the decline of several songbird species.
Brown-headed cowbirds are native to Georgia, and therefore may provide some ecological benefits. Unfortunately, I can’t think of any.
In the sky: From David Dundee, Tellus Science Museum astronomer: The moon will be first-quarter Sunday. Mars and Jupiter are in the southwest and Saturn is high in the east around nightfall.