Living alongside a group of crows can be murder for a raven.
Across North America, common ravens are regularly harassed by gangs of crows, according to a new study published Wednesday in The Auk: Ornithological Advances. In 97 percent of reported interactions between crows and ravens, scientists report, crows were the aggressors.
This grudge match goes way back. In North America, ravens are both competitors and predators of the crow. Given the chance, brazen, jet-black ravens will happily make a meal out of a crow’s clutch of eggs. And one on one, a crow doesn’t stand a chance against a raven, which can reach up to three times a crow’s size and weight. (Read more about how ravens hold grudges against humans.)
But what crows lack in size, they make up for in numbers. Ganging up “gives crows the upper hand,” says Ben Freeman, postdoctoral fellow at University of British Columbia and lead author of the study.
Many bird species, including crows, flycatchers, and red-winged blackbirds, will conduct coordinated attacks, a behavior known as mobbing, to deter encroaching predators. Mobbing birds will dive-bomb, squawk at, and even poop on an animal they consider a threat.
But the way that crows mob, Freeman says, sets them apart from most other birds. They’ll form small, tightly-knit groups of two to five, whereas other birds usually mob in larger, less cohesive squads. (Related: “Why Ravens and Crows Are Earth’s Smartest Birds.”)
Freeman thinks that crows may be using this effective mobbing technique to drive ravens out of their territory.
To reach this conclusion, Freeman and scientists from the Cornell Lab of Ornithology examined reports on eBird, an online database where bird-watchers can report their sightings. These citizen scientists described more than 2,000 interactions between crows and ravens. (Related: “We Knew Ravens Are Smart. But Not This Smart.”)
Though crows and ravens are both members of the corvid family, they differ greatly in behavior. Crows are vastly more social than ravens, which rarely cooperate with members of their flock. In this case, cooperation and sociality beats brawn.
According to Freeman, crows are increasingly dominating cities and agricultural areas in the southern United States, likely forcing ravens out or preventing them from spreading to new areas. “I suspect that one reason that ravens don't move there is that crows are keeping them out,” says Freeman. (Read: “Think ‘Birdbrain’ Is an Insult? Think Again.”)
“I certainly think that crows could preclude ravens from establishing a territory by engaging in this behavior,” says Kaeli Swift, a Ph.D. candidate at the University of Washington's Avian Conservation Laboratory who wasn’t involved in the study. Swift points to Seattle to illustrate this point. The coastal city is home to a large crow population but very few ravens.
“Ravens could easily live in Seattle; there’s nothing about the habitat that precludes them,” says Swift. “One of our ideas for why we don't see more ravens in this area is because of that interaction. It's just hard for them to get a foothold.”
Both Swift and Freeman suspect crows may be bullying ravens out of their territory, but further research is needed to confirm their suspicions.