When species come into conflict, as birds so often do, we learn a lot about the way the world works by studying where, when, and how these interactions play out in nature’s arena.
“In nature, when you look at aggressive interactions between species, usually the big guys beat up on the smaller guys,” notes Ben Freeman, a postdoctoral researcher at the University of British Columbia, Vancouver, and a former Cornell graduate student. “But I’ve personally witnessed 17 encounters between crows and ravens and in every case I saw multiple crows harassing a single raven, even though a raven is two to three times heavier than a crow.”
Freeman wondered if the flip-flop he witnessed in the crow-raven dynamic would hold true at a much larger geographical scale and if he could determine what motivated crows to take on the bigger bird. Freeman turned to a surprising source of untapped behavioral information—the voluntary species comments entered on checklists submitted to the Lab’s eBird program. The results from this analysis were published today in The Auk: Ornithological Advances.
First, Freeman downloaded all North American crow and raven reports to eBird from areas where both species occur (American and Northwestern Crows were lumped together for the study). Of those reports, more than 307,000 contained comments. Checklists that did not specifically name both species, did not describe an interaction, and did not clearly state which species was the aggressor and which the target of the aggression were filtered out. In the end, more than 2,000 observations remained for analysis. Statistical methods were used to “normalize” the data so it would not be affected by the fact that birders typically spend more time watching birds at certain times of the year, which would otherwise produce biased monthly sample sizes. The data showed that crows were nearly always the aggressors during encounters with ravens—but only if crows had the edge in numbers.
“In the comment descriptions we used, bird watchers noted that crows usually did not take on a raven one-to-one,” says study coauthor Eliot Miller, a postdoctoral researcher at the Cornell Lab. “Instead, multiple crows would gang up, cawing loudly, to harass a single raven, a familiar behavior called ‘mobbing.’ What’s new here is that our extracted eBird behavioral 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.”
Though previous behavioral studies have shown bigger birds usually have the upper hand during feeder interactions, it’s also clear that having a mob mentality can upend the size dominance hierarchy.
Crows may do the mobbing or be mobbed in turn by other smaller birds they prey upon. In this case, the fact that crows are very social and can join forces in a mob seems to work in their favor. Ravens are much more solitary. Mobbing is a common behavior among many species of birds because it levels the playing field just a bit for the little guys. There is usually no physical contact.
Freeman and Miller also found a seasonal spike in crows harassing ravens. Though the two species carry a grudge year-round, the data showed crows mobbed ravens more often during the crow breeding season from March through May—a bump in aggression that may also be influenced by changing hormone levels. During the 3 months of the crow breeding season, eBirders made note of nearly 1,200 crow mobs going after a raven as compared to just 124 such descriptions during July, August, and September—dropping from an average of 394 mobs per month to a low of 41 per month. Ravens raid crow nests to eat the eggs or young, clearly plenty of motivation for crows to gang up for a raven rout. Freeman and Miller also suggest that higher levels of mobbing during winter months could reflect increased competition for scarcer resources.
The findings raise an obvious question beyond the scope of this study: Do the ravens even care about all this mobbing or is it just a minor annoyance because they’re “too big to fail”? Cornell Lab researcher Kevin McGowan, though not involved with this analysis, has spent 30 years studying American Crows and has seen his fair share of crow-raven encounters.
“Ravens do respond to the mobbing if the crows are diving at them in flight,” McGowan says. “In fact, the raven will actually do a barrel roll to get out of the way because there’s still a potential danger of being hurt by one of these smaller ‘punks’ diving on you. But if the raven is perched and the crows don’t get too close, the raven doesn’t have much to fear from them. Most of the time, a raven might be annoyed by mobbing crows but just keep on doing what it wants.”
It’s possible that the two species may be coming into contact more often. Ravens are no longer being indiscriminately shot by humans as they were in the past and that may be one reason the species is expanding its range. According to The Second Atlas of Breeding Birds in New York State published in 2008, the Common Raven population increased fivefold during the previous 20 years. There have been similar increases in other Eastern states.
Given their continent-wide distributions, crows and ravens have many opportunities to encounter each other:
After testing this new source of behavioral information from eBird, Freeman and Miller would like to extract observations about other species interactions. For example, as Western Scrub-Jays have expanded northward in the Pacific Northwest, there are reports of very aggressive encounters between the scrub-jays and Steller’s Jays. “Citizen scientists can really tell us a lot about bird interactions and behavior,” says Miller. “We think there’s a lot of potential for future behavior studies on a larger scale.”
B. G. Freeman and E. T. Miller. 2018. Why do crows attack ravens? The roles of predation, resource competition, and social behavior. Auk: Ornithological Advances.
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