Cowbird Eggshells Dropped From Above Break Other Birds' Eggs

A red-winged blackbird, one of the species parasitized by brown-headed cowbirds, near Los Angeles. David McNew/Getty Images

If you've heard anything about cowbirds, it's likely been some harsh criticism, since the birds have a bad reputation: Rather than take care of their own chicks, they drop their eggs into the nest of other birds and let them do all the hard work. But whether or not you agree with their tactics, you have to admire their ingenuity—the birds are carefully adapted to the absentee parenting lifestyle.

Scientists have long believed that the oddly thick shells of cowbird eggs are one of these adaptations, but they hadn't been able to figure out why precisely the heavy-duty eggs might help the birds survive. The secret may be in how the birds slip their eggs into other nests, according to a recent paper published in the journal Behavioral Ecology.

That's because cowbirds don't deposit their eggs into strangers' nests particularly gently. Instead, they perch on the rim of the nest and let the egg fall, usually at least two inches. The new study suggests the thicker eggshells protect cowbird chicks while slightly increasing the odds that the nest owner's eggs are damaged when hit by the projectiles.

This isn't the only explanation scientists have come up with for why the eggshells of cowbirds and other nest parasites are relatively thick. Perhaps the shell gives baby cowbirds extra calcium for when they kick their would-be foster siblings out of the nest. Perhaps the shell stops the attacks of angry nest owners trying to destroy the invading egg. Perhaps the risk actually comes from the laying process itself, which usually causes some fuss as the nest's owners try to shoo away the freeloader.

The scientists behind the new paper wanted to test some of these theories. So they gathered up eggs from two species of cowbird, one that lives in South America and one that lives in North America, as well as eggs from the birds that these species typically foist their chicks off on, like red-winged blackbirds in North America.

Then, they set up two fake nesting scenarios. In one cowbird eggs were dropped onto the other eggs; in the second, cowbird eggs were just placed in the mock nests, but then stirred up to bounce eggs off each other. In both cases, the cowbird eggs fared pretty well, but in the egg-dropping test, almost half of large host eggs and about a third of small ones were damaged.

That suggests that it's really the laying process that makes thicker eggshells helpful for cowbirds. The same principle may also hold true of other nest parasites, since they also tend to lay their eggs in a hurry and from a height, the avian equivalent of dining and dashing. It may be a little cut-throat, but for cowbirds, the tactic gets the job done.