Alarm Calls
It’s a beautiful, crisp October day and I am sitting in the porch with my dog, watching the world go by. Right now, the main source of interest for the both of us is a grey squirrel, scurrying across our neighbor’s driveway and into their yard, where he sniffs around, parting the grass and occasionally picking up a small seed or other piece of food to process and stuff into his cheek pouches. Somewhere on the street behind us, a raven prruks and a woodpecker babbles, though my mind is not quick enough to focus and discern what kind.
Suddenly, there is movement in a bush at the edge of our neighbor’s yard, and my dog and I both turn our heads just in time to see the legs and tail of a black cat glide into the cover of its lower branches. We both shift slightly, tracking the speed of the cat and settling our gazes on the next gap in the bush. A moment later, there is movement and the dappled shadows in the gap darken and solidify. Then, stillness. We keep watching the gap as the seconds tick by, but nothing happens, and after a while, I start to wonder if the cat is still there. Maybe he passed through, and the gap was always this dark? I try to think back to what it had looked like before but can’t remember. Nearby, my dog sniffs intently at the air.
Then, suddenly, my doubt is broken in a cerulean flash — a blue jay, gliding into the bush and scolding the air. Blue jays are noisy birds, and this particular call has several purposes, including settling disagreements with other jays and pursuing mates. Here, however, it is clearly being used as an alarm, a warning that there is a predator hiding in this bush. Taking heed, the squirrel feeding on our neighbor’s lawn goes stiff, then rushes across the driveway and up our backyard maple, hanging face down on the trunk and flicking his tail against the bark. This flicking behavior is the squirrel’s own, quieter version of the blue jay’s warning — when his tail hits the bark, a vibration travels up the trunk of the tree, where any other squirrels or birds perched in the canopy will be able to intercept it and know to be on the lookout for danger. In this case, the squirrel’s tapping on the maple elicits a series of high-pitched, sees from a chickadee, which I was not even aware was in its branches.
All three of these responses to the presence of the cat — the blue jay’s scolding, the squirrel’s tapping, and the chickadee’s see — have been fine-tuned by evolution to balance the risks and advantages of giving an alarm call. On the one hand, it is clearly beneficial for all involved when prey animals communicate with each other about the predators in their environment — even if you are able to spot the predator on your own this time around, it is probable that you will benefit from somebody else’s superior senses or vantage point in the future. And yet, communicating the presence of a predator to others still carries a not insignificant amount of risk, given that you could reveal your own position in the process. What is a community minded songbird to do?
In the case of the chickadee and the squirrel, the answer is to make their alarm calls as subtle as possible. When a squirrel taps on the trunk of a tree with their tail, they make no discernable noise and only a slight extra movement, but still manage to transmit the presence of a threat to any other squirrels that may be in the tree, including their own young if the tree contains a nest. The chickadee’s see, meanwhile, may reveal the presence of a chickadee, but its sonic properties make it difficult for a predator to determine where exactly that chickadee is. The call is short, high pitched, and tapers in and out rather than having an abrupt beginning and ending. The high pitch places the call at the edge of most predator’s hearing abilities, while its short duration and tapered beginning and ending make it harder for the predator’s ears to lock in on the exact location that the sound is coming from. A longer call, in addition to being easier to locate the source of, would also sacrifice reaction time — when you are being stalked by a cat or swooped at by a hawk, every second counts and it is important that warnings get to the point quickly.
The blue jay, on the other hand, is taking a very different, much less subtle approach. After a few moments of scolding the cat, she is joined in the bush by two other jays to form what ecologists call a mob. The goal of mobbing is to communicate not only the presence of a predator to other prey animals in the area, but also to make the predator aware that they have been spotted and so lost the element of surprise. In doing this, the blue jays reveal their location to the predator, but this isn’t actually all that risky for any individual bird in the group. Predators want to get their food while expending as little energy as possible and that’s not very likely to happen with a group of birds directing all their focus and vocal energy on you. Thus, when mobbing begins, any possibility of a hunt happening typically ends and the predator leaves.
While blue jays and crows are particularly well known for mobbing predators, the behavior also occurs in many songbirds and squirrels, especially when a predator sticks around in the area after a failed hunt. A few years ago, while cutting through a small patch of woods surrounding a pond at the edge of my parent’s neighborhood, I came across a red-tailed hawk sitting in the middle of the path with what looked like a squirrel tail in her talons. Hanging on a nearby tree, chattering ferociously, was a very angry, conspicuously tailless squirrel, whose warnings had raised the entire forest into a state of emphatic alarm. I’m no Doctor Dolittle, but the message was clear enough: You have been spotted and your hunt has failed. There is nothing for you here. Leave now.
If this is what the jays are saying to the cat in the bush, well, he seems to get the message — after a few minutes of their screeching, the shadow in the gap moves again and the cat rushes off behind the other side my neighbor's house, towards the street. The blue jays, of course, follow, but soon settle down. The chickadees have already stopped chirping and, after another minute or so of glaring suspiciously at the bush, the squirrel climbs the rest of the way up the maple, down a branch, and jumps on top of the porch with a thump. My dog leaps to her feet and follows the patter of the squirrel’s feet before settling in front of the door, sitting and hurumfing expectantly.
I chuckle to myself: “I don’t think he’ll be coming in, Katara!”
Sources:
Haskell, D. G. (2023). Sounds Wild and Broken: Sonic Marvels, Evolution’s Creativity, and the Crisis of Sensory Extinction. Penguin.
Kershenbaum, A. (2022). The Zoologist’s Guide to the Galaxy: What Animals on Earth Reveal about Aliens — and Ourselves. Penguin.
Kricher, J. C. (1998). A field guide to eastern forests, North America. Houghton Mifflin Harcourt.
Image Credits:
(1) "Grey Squirrel (Sciurus carolinensis)" - Peter O'Connor aka anemoneprojectors's photostream (CC BY-SA 2.0)
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