Brown-Headed Cowbird Necks Were Built for Begging

In nature, as in horse racing, sometimes you can win by a neck. A new study shows that Brown-headed Cowbirds hatch from their eggs with extra-strong neck muscles, an evolutionary win that helps them hold their heads up longer than their host species while badgering adults for food. 

Brown-headed Cowbirds are one of roughly 100 bird species known as brood parasites, which lay their eggs in the nests of other species. Brown-headed Cowbirds parasitize more than 200 species of birds. When a targeted host is away from its nest, a female cowbird can sneak in, damage or toss out a host egg, and replace it with its own, sticking host parents with the hard work of caring for the chick. 

The paper, published last month in the journal The American Naturalist, shows that baby cowbirds outcompete their nestmates by begging longer and harder. Thanks to neck muscles that don’t easily fatigue, the interloping chicks can hold their heads in a begging posture more than twice as long as Prothonotary Warblers, one of their common host species. 

While some brood parasitic chicks eliminate host offspring by pushing their eggs out of the nest, Brown-headed Cowbirds don’t directly kill their nestmates. Instead, they outcompete them by hatching earlier, growing rapidly, and begging vigorously to bogart food resources. Lead author Nick Antonson, an animal physiologist at Brown University, wondered whether they evolved stronger muscles to support their lifestyle. The results clearly showed that to be true, he says: “I was surprised by just how stark the effect was.”  

In the first phase of the study, Antonson and his team tested how long the two species beg for food. They placed cowbirds in Prothonotary Warbler nest boxes alongside host nestlings, then pulled chicks from both species late in their first week after hatching. They placed young birds in dark boxes and tapped the enclosures three times to mimic the sound of a parent arriving. The scientists then recorded the chicks’ begging behavior with infrared cameras before returning them to their nests. 

Chicks of both species raised their heads about one second after the tapping, but cowbird chicks held their heads high for an average of about five seconds compared to two seconds for the warblers. 

The following day, the team performed another experiment to gauge neck-muscle endurance in both species. They anesthetized the chicks, stimulated the muscle the birds use in their begging posture, and held it in constant contraction until it lost force. The researchers found that Prothonotary Warbler chicks fatigued four times faster than Brown-headed Cowbirds. 

James Kennerley, an ornithologist specializing in brood parasitism at the Cornell Lab of Ornithology who wasn’t involved with the study, says that the work is novel and could forge a whole new field of research into the physiology of begging behavior. 

One strength of the study is how noninvasive the methods are, Antonson says. The entire muscle experiment was completed in 15 minutes, and the scientists returned the chicks to their nests in under an hour. To ensure no birds were harmed, the team continued to monitor the nests to confirm that all the birds tested in their experiment successfully fledged.

Antonson says the ability to beg for longer could allow cowbirds to dominate food resources because parents often can’t decide which chick to feed when they first return from foraging. Persistent brood parasites could benefit if host chicks quit begging before the food gets doled out. 

One might expect warblers to have evolved stronger necks in response to brood parasitism. But that may not have happened because, to some extent, it’s in a chick’s favor for its siblings to get food, Antonson says: Because nestmates share so much DNA, they receive an evolutionary benefit by seeing each other fledge and reproduce—a phenomenon known as kin selection.

Kennerley adds that for warblers, growing strong necks could be a waste of resources if a nest doesn’t get parasitized. The energy and nutrients used to fortify that muscle could have been spent on more essential bodily functions, creating “a trade-off between the best adaptation for a parasitized nest versus the best adaptation for an unparasitized nest,” he says.

Next, Antonson and his team want to know how Brown-headed Cowbirds compare in endurance to their nonparasitic blackbird relatives. They also hope to replicate their experiment in other brood parasites to test whether neck stamina evolved each time nest-sharing parasitism did. If the pattern holds, then he—and the baby birds—will have reason to hold their heads high.