There's good news for scientists who study animals that are too small to carry a GPS monitor, or that spit ID tags back out through their arms. A setup using an off-the-shelf camera can precisely capture an animal's path in three dimensions—without anyone touching the animal.

Emmanuel de Margerie, who studies animal behavior at the University of Rennes 1 in France, says there are several reasons to seek new animal-tracking technologies. To put a GPS or other kind of tag on an animal, you have to capture and handle it, which can be stressful for your study subject. The tags themselves can be expensive and unreliable, and sometimes get lost. And "small species cannot be tagged at all because the tag is too heavy for walking or flying normally," he says.

So de Margerie thought up a way to track animals purely by sight. He and his coauthors have dubbed it "rotational stereo videography" (RSV). It works like this:

Just like our own eyesight, shooting a scene from two nearby points at once—or, in stereo—lets you measure the distance to an object. (That's why closing one eye ruins your depth perception.) Scientists have tried this kind of stereo imaging in the past, de Margerie says. But their methods usually required two or more cameras on stationary bases.

De Margerie put a camera on a rotating base, so that someone shooting video can keep an animal in view as it flies or walks around. A device in the base precisely tracks the angle of the camera as it moves. The camera is set on a long, T-shaped platform that looks like a crossbow being pointed at the animal. And on either arm of the T is a mirror pointing back toward a camera sensor. This allows stereo vision with only one camera.