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I’ll admit that I don’t have much of an ear for the language of crows. Ron says they have a specific call that means “Raven!,” uttered when the crows are harassing one of their larger relatives or escorting it out of the neighborhood. This wouldn’t surprise me at all, since they do have an extensive vocal repertoire.
One researcher described eight variations on the basic caw, used in different contexts: territorial defense against other crows, warnings when a predator is sighted, and so on. Crows also have many non-caw vocalizations, even a kind of song, a mixture of “coos, caws, rattles, clicks, and grating noises” strung together in long sequences with “a rambling, improvised quality.” Crow jazz.
It hasn’t been established whether these birds can recognize each other by voice, but it doesn’t seem too much of a stretch. Voice recognition is how seabirds find their own offspring in a crowded breeding colony, how emperor penguins locate their mates after that long trudge over the ice. It’s what allows territorial birds to distinguish a known neighbor from an unknown intruder, and flocking birds to synchronize their movements.
But it was up to four New York biologists—Jessica Yorzinski, Sandra Vehrencamp, and Kevin McGowan at Cornell and Anne Clark at Binghamton—to demonstrate that the calls of individual crows have distinctive acoustic fingerprints, as reported recently in The Condor.
Yorzinski and her colleagues focused on the inflected alarm caw, a short, sharp burst of sound given in the presence of a soaring bird of prey, or sometimes an unwelcome human. They worked with wild crow flocks in Ithaca, all of whose members had been tagged, banded, and identified as male or female. Each bird’s inflected alarm caw was measured in terms of 25 acoustic variables, including duration, bandwidth, and center frequency.
I’ll spare you the math, but the researchers found that the crows had individually unique alarm caws, differentiated on seven factors. If the biologists could tell them apart by acoustic properties, presumably the birds could as well. They also found a gender difference: in general, female calls had higher frequency, stronger frequency modulation, wider bandwidth, and shorter duration than male calls.
The exception was a male who was low in his flock’s pecking order and whose vocalizations were more like a female’s. Since male crows tend to dominate females, this would make you wonder about the confounding effects of gender and status.
Why would it matter if a crow could identify the source of an alarm? Yorzinski and colleagues suggest that not all crows are equally reliable sentinels. Young, inexperienced birds may issue a disproportionate number of false alarms.
It would be important to know if the warning came from some mistake-prone rookie or one of the flock’s keen-eyed elders. And if a neighbor’s alarm call sounded different from any made by a flock member, the birds could gauge the approximate distance of the threat.
Mind you, this is only one call out of a couple dozen. The crows’ signaling system may also be able to specify the degree of danger. This appears to be the case for another bird, the black-capped chickadee, which had never been suspected of any kind of vocal sophistication. Black-capped chickadees, like our local chestnut-backed species, are known for mobbing predators like hawks and owls, giving a characteristic “chick-a-dee-dee-dee” call. This is often how birders are able to detect the presence of a roosting—and disgruntled—owl.
In an ingenious experiment, Chris Templeton, then a doctoral student at the University of Washington, measured the responses of chickadees in an aviary to the presence of live predators. The hawks and owls came from Raptors of the Rockies, a rehabilitation facility in Florence, Mont.
Templeton also used two mammalian predators, a domestic cat and a ferret (standing in for native weasels), and a bobwhite quail as a control. He found that different predators elicited different numbers of “dees” in the alarm call. In general, the smaller raptors triggered more consternation than the larger ones.
Great horned owls rated only a few “dees”; northern pygmy-owls, up to 23. Since pygmy-owls are specialized predators on small birds and great horned owls take mainly mammals, this makes sense. “A great horned owl going after a chickadee would be like a Hummer trying to outmaneuver and catch a Porsche,” says Templeton.
But the size of the test bird was clearly not the only factor. The chickadees didn’t react at all to the non-predatory quail. And a Cooper’s hawk, a mid-sized raptor that preys on smaller birds, drew more “dees” than would have been predicted from its size alone. These little birds are either equipped with some kind of mental field guide to the local raptors or are remarkably fast learners.
If chickadees can pack that much information into their calls, Lord knows what the bigger-brained crows are capable of. They may well be saying not just “Raven!” but “Raven with a missing right primary feather at ten o’clock!” And the response from the other crows may be “George knows what he’s talking about—let’s scramble!”