The grassland-foraging, brown-headed cowbird got its name because of its relationship with livestock, in particular, bison. The hungry birds would tag along after roaming herds, feasting on beetles and grasshoppers that the cows flushed out as they grazed. Neat story—but cowbirds are actually better known for their sneaky trick of stashing their eggs in the nests of other birds. The unwitting mothers raise the alien babies, and female cowbirds, meanwhile, are free to breed continuously until mid-July.
One cowbird lays, on average, 40 eggs per year. That’s four or five times as many eggs as plenty of other birds. (And a huge amount compared to species such as loons. They only lay two eggs per year!) The cowbird’s crafty brood parasitism probably evolved out of necessity. Since cowbirds were always on the move, following their prairie herds, they had no time to build a nest, and couldn’t stay put as they waited for their eggs to hatch.
If cowbirds lay so many eggs, why isn’t North America overrun with cowbirds? Because only a small portion of eggs actually hatch successfully. Certain species, including robins, blue jays, and orioles, can recognize the distinct, dark-spotted eggs, and know to throw them out of the nest. Other species just abandon the entire nest and start again. If the cowbird eggs do hatch, the foster mother will usually raise the nestling as her own.
Cowbird babies grow faster, and beg more persistently, than their fake siblings. This means their fake mother will feed them more frequently, guaranteeing their survival, while their adopted brothers and sisters starve. The brown-headed cowbird has more than 200 known “host” species, including, unfortunately, the endangered Kirkland’s warbler and the black-capped vireo. Oh, Nature. You sure are cruel sometimes. And kind of creepy.
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