The North American walnut sphinx caterpillar produces a whistle that sounds just like a songbird's alarm call--and the whistle seems to startle birds. Christopher Intagliata reports.
Songbirds produce a rich repertoire of sounds: songs, of course, but also chatter calls, flight calls and alarm calls: like this high-pitched warning from a black-capped chickadee: <<seet call>>. But chickadees aren't the only ones endowed with chirping abilities. Turns out a certain type of caterpillar can, too. <<caterpillar whistle>> Once again, the bird, then the caterpillar. <<seet call, caterpillar whistle>> Sounds pretty darn similar.
"<laughs> Yeah it's pretty crazy." Jessica Lindsay is a biologist at the University of Washington. She says this species, the North American walnut sphinx caterpillar, makes the sound using tiny pairs of breathing holes, called spiracles. "They compress themselves lengthwise like an accordion and so that compression pushes air out of the spiracles, making whistling sounds."
A cool coincidence, for sure. But here's where it gets interesting. Lindsay then played the caterpillar whistle through a speaker near a bird feeder. And the birds—of many different species—dove for cover. "Sometimes we'd see nuthatches flicking their wings, which is a big sign of distress for them. Sometimes it would take a really long time to return to feeding, which is a big indicator that they were taking that whistling noise pretty seriously."
And whereas the birds were unperturbed by the song of a house finch—a control sound—they responded to the caterpillar whistles with almost the same urgency as when they heard that real alarm call <<seet call>> from a chickadee. Suggesting the caterpillars… are onto something. "They're basically using bird speak to say that there's a dangerous predator in the area and that the birds better get into cover."
Lindsay presented the results at the International Symposium on Acoustic Communication by Animals in Omaha. [Jessica Lindsay and Erick Greene, Whistling Caterpillars Mimic the Alarm Calls of their Predators]
Previous work suggests the caterpillars whistle when pecked at by birds. And they sometimes squeal, too: <<caterpillar squeal>> Suggesting they may have more than one scare tactic, to worm their way out of trouble.
[The above text is a transcript of this podcast.]