Aerial Acrobats: Pigeons Whoosh Through Tight Space

Liz is a staff writer for Science.

SAN FRANCISCO, CALIFORNIA—Often the benefactors of bread crumbs, pigeons don't need the aerial acrobatics of goshawks and other birds that hunt on the fly through forests. But these common urban birds still know how to get through tight spaces. Arrays of high-speed cameras have captured pigeon maneuvers on film, revealing two ways that these birds can squeeze between branches and other obstacles that constrain their flight paths, researchers reported here on Friday at the annual meeting of the Society for Integrative and Comparative Biology. The work is part of a large project involving biologists, engineers, and mathematicians who are trying to understand how birds fly in order to build robots that can do likewise, without human guidance.

"The [pigeons] are doing a lot of things aerodynamically and flightwise that we haven't realized," says Kristen Crandell, a biomechanist at the University of Montana, Missoula, who was not involved with the work.

Until recently, most studies of bird flight have looked at how birds fly in a straight line through clear airspace. But to survive, birds must also know how to avoid hitting trees, light posts, even other birds that get in their way. Any airborne autonomous robot must do the same. So a team at Harvard University set up a system of high-speed digital cameras to see exactly how birds get around. The researchers chose pigeons because these birds are easy to work with and are good flyers, says Charles Williams, a Harvard postdoctoral fellow and member of the group.

For their experiments, Williams trained eight pigeons to fly from one perch to another at the opposite end of a long, narrow room. Five overhead cameras tracked the birds' progress through the space. Each bird was outfitted with a tiny backpack containing a battery and three very small light-emitting sensors, or LEDs. Williams attached additional LEDs to the head, wingtips, and wrist joints of the birds to make tracking those parts of the body easier. Then he put up bars midway down the room that forced the birds to go through a narrow space. Cautious, he started with gaps between the bars that were just shy of a pigeon's wing span, about 30 centimeters. Eventually he challenged the birds to go through spaces just 11 centimeters wide—the equivalent of a body's width with an extra centimeter or so on either side. "What was surprising to us was how good they were at navigating these gaps," Williams says.

In wider gaps, the pigeons tend to hold their wings up high—pausing briefly at the top of the wing stroke—as they pass through the bars, Williams reported at the meeting. In this way, the birds can quickly do a down stroke once they get through to help stay airborne. But when the gap is too narrow, the birds instead fold their wings, holding them close to the chest, and glide through. Thus, they are less likely to be knocked off course should they strike a bar.

The study "is one of the first in looking at how birds interact with the environment in a more realistic, ecologically relevant way," Crandell says.

Next, Williams wants to get even closer to reality by monitoring what pigeons do in a natural environment. Toward that end, he is designing a new backpack that will hold the same kinds of sensors that smart phones use to tell when to change the orientation of the screen depending on how the user is holding it. He will also put those sensors on the head and wing and will record how each sensor moves through space. In that way, he will be able to tell exactly what the bird is doing as it moves through a forest.

His Harvard colleague, graduate student Ivo Ros, has used a different arrangement of high-speed cameras to study how pigeons initiate turns midair. For his experiments, the cameras recorded the birds from the side and from slightly above. He set up nets to make a flight path with a 90° turn in it. The four birds tested each had bright dots on the head, wing, and body that enabled him to follow the actions of the different parts of each bird's body. The cameras were synchronized to take 250 shots per second and from those images, Ros was able to reconstruct the body positions in three dimensions. The analysis revealed that the birds depend on a stabilizing reflex for turning. During flight, when the head shifts position, the body rotates to match the change so that the bird can keep a steady gaze. When the birds approach a turn, first the head looks to the side, and then the body follows, tilting and banking for the turn. The amount of turning by the body is determined by how much the head—and its gaze—turn to the side, he reported.

That a bird can navigate using "what was thought to be simple reflex patterns is very tantalizing," says Simon Sponberg, a neuromechanist at the University of Washington, Seattle. "The reflexes were co-opted to change direction."

With studies like these two, "We are starting to get at the strategies used to [ensure] robustness, flexibility, and stability" in biological movements, Sponberg says. "Understanding these strategies is really critical for any applied stuff," such as robots that can control themselves in flight. These autonomous flyers would be useful for exploring unknown terrain or doing reconnaissance for search-and-rescue operations after earthquakes or other natural disasters, Ros says. Toward that end, he and Williams' collaborators are incorporating these results to help build pathfinding computer programs that will direct future aerial robots.

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