Decoding The Forest: Researchers Track Bird Alarm Calls

In recent years, biologists have learned that birds use a variety of signals to communicate life-saving information about predators. Researchers from the University of Montana are trying to find out how development affects these signals and what this could mean for bird populations.

Biology Graduate Student Cedar Mathers-winn says bird songs are more than just pretty sounds.

"As soon as you realize that they're talking about something or sort of start to figure out what the different things that they might be talking about are, they really open up your ears," says Mathers-winn. "The forest is a much richer place now."

For example, the sound of a mobbing call is an alarm that’s part of an elaborate communication network that can transfer life-saving information to other birds — and even other animals.

"Mobbing calls in nature are used to signal that there's usually a sitting raptor or a threat that birds can handle by mobbing and driving them out by harassing them," says Isaac VanDiest, one of Mathers-winn’s research assistants.

The mobbing call recruits birds and other species to — you guessed it — mob the predator. It’s basically sending the signal that the predator’s cover has been blown, and it’s not going to get any rest in this neck of the woods.

Another type of alarm is called a “seet” call. Seet calls signify a nearby threat like a flying hawk. When song birds and even small rodents hear this, they’ll dive for cover or freeze in place until the predator leaves. These alarm calls are recognized by birds across continents, meaning that a song bird in Australia will react the same way as a chickadee in the U.S.

"There's a lot of information in these calls, and because they're so important, that information ends up spreading pretty far," says Mathers-winn.

In continuous forest with a lot of birds, a signal can move 100 miles per hour. But what Mathers-winn wants to know is: What happens to the signal when the forest is broken up by logging, highways, or housing developments?

"If you have all these communicating birds, they're all talking, listening to each other," Mathers-winn says. "We can consider that to be a communication network because everybody's communicating together and producing this big network of information … I'm interested in looking at how distance affects and how disjunctures in habitat affect the spread of that information."

If birds have no idea a predator is coming because they can’t hear the signal, "they might completely miss it and get nailed," says Mathers-winn.

To find out whether development breaks up bird communication networks, Mathers-winn is conducting experiments with high-tech microphones and recording equipment. Basically it allows the researchers to track how far and how quickly the alarm calls move across continuous forest and then compare this to areas with habitat fragmentation.

At Sapphire Ranch, south of Lolo, Montana, Mathers-winn and a team of biologists set up two bird feeders, 350 meters apart in continuous forest. Biologists are stationed at each of the two bird feeders to observe bird behavior during the experiment. Near one of the bird feeders is a speaker that allows the research team to play pre-recorded bird calls.

"So we'll play those and watch what happens at the site of the feeder, but then we have a microphone array between that feeder and the feeder at the other end where we know there are birds at those two places," says Mathers-winn.

The array is a series of microphones and synchronizers that use satellite and GPS data. This allows the researchers to triangulate the positions of the birds that are giving the calls between the two feeders.

The biologists play a non-threatening, pre-recorded bird song; the birds ignore it and continue feeding. When the biologists play a seet call, however, the birds hide in the tree cover, and the forest gets quiet.

During the third type of call, the birds show a drastically different response.

"So we just did a mobbing call to see how the birds respond at this end of the array, and we had probably three or four as many birds show up once the mobbing call started," says VanDiest.

The birds stopped feeding, flew up into the trees to scan the area and continued the mobbing call until it was clear that there wasn’t a real threat. After the experiment, Mathers-winn says:

"I feel really good about today."

This was the first time the team conducted an experiment in continuous woods. Mathers-winn says it took about a minute for the mobbing call signal to reach the birds at his feeder, 350 meters away.

"It really does keep going," says Mathers-winn. "And I guess I knew that theoretically, but I'd never seen it so I feel really good about today because it's sort of like proof of concept, like yeah, they actually do this."

Mathers-winn says he’s seen this work on a much smaller-scale, but today’s experiment was the first time he has observed a very clear shift in bird behavior from a distance this far. He’s beginning to get an idea just how far the signal can travel.

"It was very gratifying for me to see this network thing actually happen," says Mathers-winn.

He is still in the beginning stages of his research, but he says he’s always wanted to decode the sounds of the forest.

Watch 'Science x Rhymes' to learn more about bird alarm calls. You can also learn about UM's Dr. Erick Greene and his research using 'robo-raptors' — taxidermied hawks and owls with robotic moving parts.