The Bad Habits of Cowbirds
Cowbirds have an unusual life strategy: they lay their eggs in other birds species‘ nests. North America’s Brown-headed Cowbird first evolved this strategy in order to follow herds of buffalo. The cowbirds fed on insects flushed up by the buffalo’s feet. As the buffalo migrated around the Great Plains, the cowbirds followed- even during the birds’ breeding season. If the cowbirds took care of nests, they couldn’t follow the buffalo. So they left their eggs behind in other songbirds’ nests.
Buffalo are now scarce in North America, but cowbirds’ numbers have exploded. They now follow grazing cattle, and also benefit from land cleared for farming. Once confined to the Great Plains, their range now covers most of North America.
Many wildlife managers are very concerned about cowbirds- they create a real problem for other songbirds. In one year, a single female cowbird may lay eggs in 40 different nests. At least 144 different species of birds have unknowingly raised a cowbird’s chick.
Some host birds don’t fall for the cowbirds’ trick: they’ll break open the cowbird’s egg, or will abandon the nest entirely. But others do not notice the newcomer. When the cowbird chick hatches, it usually crowds out the true chicks, by taking most of the food brought by the mother songbird.
Cowbirds share this habit of nest parasitism with a few other bird species, such as the cuckoos of North America and Europe.