Why Do Male Birds Take on Larger Predators? Maybe Just to Impress the Ladies

Some mobbing behavior may be less about survival, and more about sexual selection

Ecologists tend to think of mobbing behavior as primarily a way that smaller birds protect their nests and chicks from larger predators. Shown here, a Willie wagtail attacking an Australian raven. Ray Wilson / Alamy

Have you ever seen a group of small birds wantonly attack a larger bird? Whether it’s chickadees, mockingbirds or crows, there’s something awe-inspiring about a bunch of tiny floofs taking on a sharp-taloned owl. This behavior is called mobbing, and it’s typically understood as a cooperative strategy that prey animals use to ward off a would-be predator—the weak rising up against the strong, David-and-Goliath style. 

But that understanding may need reworking, according to a new study published in the Springer journal Behavioral Ecology and Sociobiology. It’s not just because nature doesn’t deal in human conventions like underdogs and bullies. By setting up faux owls and seeing how smaller birds reacted, researchers found that these avian gang members may not be exactly the heroes they think. In fact, some of them are probably just trying to get laid.

Remember that evolution isn’t just about survival; it’s about passing down your genes to the next generation. And with that in mind, sometimes mobbing may be less of a survival strategy in the face of a potential attacker and more the behavioral equivalent of the  peacock’s alluring plumage. Because what better way to show the ladies the quality of your DNA than by finding a scary predator and screaming in its face?

“Costly signals may not only manifest as ornaments, like the peacock's tail, but also as behaviors,” explains Filipe Cristovão Ribeiro da Cunha, lead author of the study and a biologist at the University of Zurich. “The take home message here is that risky behaviors may have been sexually selected, as honest signals.”

Scientists have suspected that mobbing behavior might be linked to displays of fitness for a while now, says Cunha. “However, our study is the first empirical test of this hypothesis,” he says.

To induce mobbing behavior, Cunha and his coauthors presented wild birds in south-eastern Brazil with life-size models of two different owl species, while also playing vocalizations from these predators on a loud speaker. Then they dressed up in full camouflage, nestled into the foliage and watched the birds’ responses from a short distance.

The first surprise was that birds of all sizes and species came out of the woodwork to harass the fake owls: 79 different species in all. Researchers saw everything from hummingbirds and flycatchers to antbirds and tanagers, birds that aren’t particularly closely related and who employ a variety of breeding and social strategies. To work out the mysteries of mobbing and sexual selection, they focused on 19 of these species in which males and females are noticeably different colors.  

Out of these species, the vast majority of the mobbers were dudes. The researchers logged 165 individual birds engaging in mobbing behavior, almost exactly two-thirds of which were male. Even more interesting was what happened when ladies were present (either as mobbers themselves or merely spectators): The males mobbed the owls more intensely. That is, they were more likely to fly closer to the predator and even engage in physical attacks as opposed to just puffing up their breast feathers or shrieking from a distance.

But the last insight is the most intriguing. The two different owl species used as models were chosen for a reason: their diet. The ferruginous pygmy owl is a regular predator of small birds, while the burrowing owl only snags birds very rarely and instead focuses its hunting efforts on insects and other arthropods.

Earlier studies have shown both that birds are more likely to mob a predator they perceive as dangerous and that the species that mob are those most often eaten by a predator—reinforcing the image of brave martyrs protecting the group. But what Cunha and his colleagues found ran counter to these earlier studies. Overall, the little birdies of southeast Brazil—most of which were males, mind you—saved their most intense assaults for the burrowing owl. That is: the one less likely to eat them.

You might call this a calculated risk. By attacking a less-dangerous predators, you can show females that you’re a badass without actually putting your neck on the line. Or maybe it’s just a way of advertising acrobatic prowess, skills that translate into the ability to find food or defend your nest. We’re not sure.

Which of these scenarios is more likely is something Cunha is still investigating. For his next experiment, he hopes to evaluate whether females choose males who mob more intensely. That would prove definitively that mobbing is selected for sexually.

Until then, take comfort in the fact that not all mobbing behavior can be traced back to mate choice. There are still plenty of circumstances where tiny birds must band together to protect their young from large and fearsome foes. Cunha’s research, however, shows that birds are at least a little bit like humans.

Some of us are altruistic heroes. Some of us are Romeos. And some of us are just a bunch of macho-bluffing bird bros.

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