When song birds detect a threat, they make an alarm call. Not surprisingly, when another bird hears this, they start making their own seet alarm call. Recently, University of Montana, biologist Eric Greene, who studies bird calls, discovered something quite surprising about these bird alarm calls, something no one had imagined — he realized that these bird warnings are adopted, and passed along, by completely different types of animals, animals that aren’t even birds.
Greene explains how this works. When small bird spots a dangerous predator, a hawk or owl for example, flying toward the small bird, it warns other birds by making a soft, high-pitched seet call "It's very high frequency," Greene explains, "and it's hard to locate."
It turns out the maxim: there’s safety in numbers, applies especially to small birds. When they see a perched predator, smaller birds make another call referred to as a mobbing call. It’s an irritating sound like screeching tires. This brings other birds out of the trees to "mob" the predator and chase it away.
Greene has been watching birds do this for years, but recently, while he was watching birds do this, he noticed another creature. He noticed that squirrels seemed to be mimicking the birds’ warning calls, and adding to the alarm as soon as they heard it. Even more surprising, he found that the squirrel's mimicry is nearly perfect, even though squirrels have a very different vocal apparatus than the one birds use.
Once he noticed this, he also realized that Chipmunks also make these calls. Greene found himself astonished by the fact that mammals and birds, species that are not even from the same order could share this early warning system.
"We've got these complex communication networks," says Greene, "and it's not just one species yakking to members of its own kind. It's all these different species — and not just birds, but mammals as well. And they're all sharing information.”
Being a scientist, Greene realized that he needed to observe and record this behavior many, many times, consistently to make sure he was interpreting it correctly. But he also knew that waiting for predators to make their appearances would take tremendous amounts of time. So, to speed up the process and provide a reliable predator to provoke the birds and squirrels in a consistent way, Greene turned to a simulacrum: a mechanical bird of prey.
In his university lab, Greene holds up a dead predator and explains, "This is going to be a robo-pygmy owl.” The terminator-like device has real feathers on the the outside, but it’s stuffed with small motors and a computer board that allow it to move. The head swivels in a very creepy way.
In the woods near Ithaca, N.Y., Greene and his team set up two of these “robo-raptors”, as Greene calls them, on artificial stumps Greene and his team can raise and lower by remote control. After the humans clear the area, and all is quiet, the research team activates the trunk. It slowly slides down, exposing the robo-raptor.
A tufted titmouse spots the “predator" first, and begins its mobbing call. Next comes a white-breasted nuthatch, who joins in. Then come house sparrows, jays and cardinals. And then, just as Greene has observed numerous times before, the squirrels begin their mobbing calls. This wall of sound, created by all of these different animals, in their way, working together, travels through the forest at 100 mph.
"It's almost as if you've got a bow wave preceding the raptor," says Greene. "In many ways, I've come to appreciate that it's hard to be a hawk. As soon as it's spotted, the alarm flies far faster through the woods than the hawk can."