Vision and vocal communication guide three-dimensional spatial coordination of zebra finches during wind-tunnel flights

Many bird species fly together with conspecifics in flocks to safely cover flight distances. To avoid collisions with each other and head in the same direction, flock members have to communicate and coordinate their flight positions. Researchers led by Susanne Hoffmann from Manfred Gahr's department at the Max Planck Institute for Biological Intelligence investigated how this communication takes place in zebra finches. In their study published in Nature Ecology & Evolution, they found that zebra finches use both vision and calls to coordinate their flight positions and avoid collisions.

Communication is essential for living together in a functional community. This also applies to flocks of birds: flight maneuvers and the movements of individual animals must be quickly coordinated. This is the only way to avoid collisions and control the movement of the flock as a whole.

"How birds accomplish this in flight is not yet known," says Susanne Hoffmann of the Max Planck Institute for Biological Intelligence, in foundation. "It has been suggested that each bird in the flock observes its neighbors and adapts its own flight maneuvers to their movements. But how many other individuals can a single bird observe simultaneously? How do the birds cope with poor visibility conditions? There are many open questions."

Zebra finches in the wind tunnel

To approach these questions, Hoffmann studied acoustic communication in a small group of zebra finches. In the wild, these talkative songbirds live in colonies and also vocalize during flight. The connection between flight calls and the birds' flight maneuvers hasn't been studied yet.

To investigate the zebra finches' vocalizations in more detail, the researchers used very small and lightweight wireless microphones developed at the institute. These can be easily carried by the birds, which weigh about 15 grams. This allowed Hoffmann and her colleagues to record the chirping of individual birds in the institute's own wind tunnel, even during flight. Video cameras simultaneously tracked the flight movements of the group.

Calls help to avoid collisions

The researchers found that the zebra finches' short vocalizations during flight are almost always followed by an upward flight movement of the calling bird. When calling, this bird was usually located at frontal and bottom positions in the group. From such a position, the animal probably can't see group members above and behind itself.

The scientists also observed that the flight speed of the other zebra finches briefly decreases after an emitted call. This could enable them to observe the flight path of the calling bird and to avoid collisions.

"We didn't expect such a close relationship between the vocalizations of a bird in a group and its flight maneuvers," Hoffmann recalls. "The calls indeed seem to announce a change in the caller's position, and to draw the conspecifics' attention towards the calling bird."

While zebra finches announce vertical position changes by a call, the birds use vision to coordinate horizontal position changes. When turning their head by about 90°, one of the bird's eyes looks to the side while the other eye looks straight ahead—a brief shoulder check, so to speak, before changing direction.

Full moon night and artificial obstacles

To test their hypothesis, the researchers examined how limited visibility and background noise affect the frequency of call emission in flight. They darkened the wind tunnel to limit the visual information available to the birds. Under light conditions equivalent to a clear full moon night, where the birds were just able to fly, call rates increased. However, the frequency of collisions did not—the additional calls obviously help the zebra finches to coordinate their flight paths well despite the limited visibility.

In contrast, when background noise masking the zebra finches' calls was presented during the flight phases, the call rate decreased and collisions occurred more frequently. These observations suggest that in-flight communication via calls is an effective way of avoiding collisions with conspecifics.

Next, Susanne Hoffmann and her colleagues want to take a closer look at communication during a flight maneuver coordinated across all group members. "We are interested in whether the birds also use vocalizations when jointly avoiding a virtual obstacle that we display within the wind tunnel," says Hoffmann. Future studies will also have to show whether the observed behaviors also apply to larger flocks of birds and to zebra finches in the wild.