Newswise — Ithaca, NY— New research is adding validity to the adage "Birds of a feather flock together." A citizen-science-based report published in The Auk: Ornithological Advances suggests that when crows team up to take on a bigger foe it can be a highly successful strategy. Crows and many other birds resort to "mobbing" to drive off a predator. Even though a Common Raven is two to three times heavier than a crow, when crows band together against a single raven they usually succeed in driving it away. "Our data show that when there are chases between crows and ravens, 97 percent of the time it is crows chasing ravens, not the other way around, a much higher rate than we expected," says study co-author Eliot Miller, a postdoctoral researcher at the Cornell Lab of Ornithology.
In the study, researchers from the Cornell Lab and the University of British Columbia, Vancouver, combed through thousands of voluntary citizen scientist comments describing interactions between crows and ravens submitted to the eBird online database. eBird is the world’s largest biodiversity-related citizen-science project, with more than 100 million bird sightings contributed each year by from around the world. Crow mobbing was reported in 67 percent of the behavioral comments used in the study. The study’s lead author, University of British Columbia, Vancouver, postdoctoral researcher Ben Freeman, pursued this research after witnessing crows mobbing ravens. "Just spending time outdoors, I'd frequently see crows harassing ravens…that was interesting to me because in nature its usually the bigger birds that dominate the smaller birds," says Freeman. "I wondered if this was a real 'thing' and if so, did it matter?" Miller says the crows' social behavior gives them an advantage: "In the descriptions, bird watchers noted that crows usually did not take on a raven one-to-one. Instead, multiple crows gang up, cawing loudly, to drive off a single raven, the typical mobbing pattern." Ravens tend to be be much more solitary.
The study also found another trend: ravens are most frequently mobbed during the crow breeding season from March through May. This is when crows are especially vulnerable to nest predation by ravens. It's possible crow and raven interactions are becoming more frequent because the Common Raven is expanding its range into areas that overlap even more with that of crows. The study authors are hoping to answer other bird behavior questions by tapping into eBird comments for different species. "I'd love to find out whether the presence of one species discourages another from moving in," says Freeman. "It's hard to tease out how much the choice of where to live is based on the habitat itself and how much is based on the presence of the other species and what other factors come into play."
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The Auk: Ornithological Advances