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"When danger approaches, sing to it." No one takes this bit of wisdom more seriously than songbirds. With a combination of seets, screeches, and hisses, they can create a woodland security system to tip each other off when predators are on the prowl. The calls are ubiquitous and well studied—by both bird-loving humans and other species of birds.

Yet as Christopher Solomon , scientists are just beginning to understand the complexities of these sounds. Erick Greene, a biology professor at the University of Montana, and other researchers have determined that birds can recognize the alarm calls of other species, including squirrels and chipmunks, and that their warning cries can pass from bird to bird through the forest at speeds of 100 miles per hour.

Greene also discovered that Black-capped Chickadees will vary their chick-a-dee-dee-dee alarm call based on the size of the threat. When faced with a large raptor that's unlikely to eat a chickadee, they may only say chick-a-dee-dee. But if it’s a particularly dangerous Northern Pygmy Owl or Sharp-shinned Hawk, they may tack on up to 12 dees at the end of the call.

This doesn't come as a complete surprise; it’s long been assumed that birds use each other’s alarm calls when foraging in mixed-species flocks, says bird expert and ���Ϲ��� field editor Kenn Kaufman. “But to actually quantify this, as Greene and some others are doing, is important and intriguing,” Kaufman adds.

He says that in North American forests, woodpeckers, nuthatches, and Brown Creepers can’t easily see predators because they’re climbing up trees with their faces close to the bark. So they partially depend on chickadees and titmice, which are flitting around and looking in all directions, to let them know when trouble arrives.

Experienced birders likewise listen closely to alarm calls, except in their case it’s to help them view, and not to avoid, hawks and owls. Kaufman describes a recent trip he took to Ohio’s Magee Marsh in which Red-winged Blackbirds continually gave a high, thin teeeer, teeeer call upon spotting a raptor.

“I don't know if other birds react to this note, but the birders do!” Kaufman says. “Thanks to these alarm calls from the red-wingeds, I saw several Sharp-shinned and Cooper's hawks that I might have missed otherwise.”