Why whistling caterpillars scare birds

ANCHORAGE, Alaska — A caterpillar that emits a squeaky whistle may be mimicking bird calls that warn of an immediate, deadly threat.

If so, the walnut sphinx caterpillar (Amorpha juglandis) is the first known invertebrate to make phony alarm calls, researchers reported June 12 at the annual meeting of the Animal Behavior Society.

Broadcasting recordings of some of the whistles given by the moth caterpillars can send some birds at feeders fleeing for cover; others freeze for minutes at a time, reported Jessica Lindsay of the University of Montana in Missoula. The birds react much as if they have heard the high-pitched “seet” that many species worldwide, including robins, titmice and even squirrels, give when a hawk looms near, Lindsay said. The main component of one type of caterpillar whistle is a little swoop at the same frequency as a chickadee seet.  

Like these other species, the caterpillars whistle in a crisis. When birds peck at them, the caterpillars clench their bodies to blow air out of a particular pair of breathing holes along their sides, researchers at Carleton University in Ottawa reported in 2011. The Carleton researchers proposed that simply by making a sound the caterpillars startled the birds into breaking off their attack. But Lindsay’s coauthor Erick Greene, also at Montana, has studied genuine seet calls and suspects the caterpillars are defending themselves with sounds more potent than “boo.”