How birds communicate in flocks

zebra finch

Zebra finches flying in the wind tunnel. © MPI for Biological Intelligence, iG / Susanne Hoffmann

Birds must coordinate when flying together to avoid collisions. How they do this, however, was previously unclear. Researchers have therefore closely observed and overheard a group of zebra finches flying in the wind tunnel. It turned out that the birds orientate themselves by looking, especially when changing their position from the side, but when changing altitude they often use special calls. These calls are also used when visibility is limited by darkness.

Many bird species fly in swarms together with conspecifics in order to safely cover flight distances. However, this requires good coordination between the animals: flight maneuvers must be coordinated and the movements of the individual birds must be coordinated quickly. This is the only way to avoid collisions and control the movement of the swarm as a whole.

Mini swarm in the wind tunnel

“How the birds accomplish all this in flight is not yet known,” says senior author Susanne Hoffmann from the Max Planck Institute for Biological Intelligence. “It was assumed that each bird in the flock observes its neighbors and adapts its own flight maneuvers to their movements. But how many other animals can a single bird focus on at once? And what do the birds do when visibility is poor?” In order to clarify some of these unanswered questions, Hoffmann, lead author Fabian Arnold and their colleagues observed and overheard a small group of zebra finches in flight.

To do this, the scientists equipped their zebra finches with tiny, lightweight wireless microphones that the birds, which weigh only 15 grams, can easily carry. When the birds flew in the institute’s own wind tunnel, they were able to record the communication between the individual animals during flight. High-speed video cameras simultaneously tracked the flight movements of the group, but also the head movements of the individual animals. This enabled Arnold and his colleagues to observe whether the zebra finches also use looks to orient themselves and coordinate when they change position in the swarm.

shoulder look and warning call

The investigations showed that the individual zebra finches in the swarm each take up preferred positions. For example, one bird preferred to fly very low to the ground and formed the bottom of the group, another was usually found at the head of the flock. “Despite these preferences for the rough position in the group, the birds constantly and quickly changed their positions laterally,” the researchers report. The evaluation of the video recordings showed that the zebra finches use their looks to orient themselves: Shortly before they swerve, they turn their heads by around 90 degrees so that one of their eyes looks in the direction of the planned maneuver and the other looks ahead.

In addition to these lateral flight manoeuvres, the zebra finches also carry out vertical position changes in the swarm. These showed that vocalizations in the form of a special call were important for this. In particular, when a bird was flying in the front, lower part of the group, it would emit this call before its flight movement upwards – presumably because the birds cannot see the area behind and above them. These warning calls were heard above all when the area above the warning bird was already relatively densely populated with conspecifics. “We didn’t expect that the vocalizations of a bird in a group would be so closely related to the flight maneuvers of that bird,” says Hoffmann.

Flexible adjustment of communication

The team was also able to observe that after such a call, the flight speed of the other zebra finches briefly decreased. This could make it easier for them to observe the calling bird’s trajectory and avoid collisions. “The calls really did seem to announce a change in position and to warn other members of the species accordingly,” says Hoffmann. When the scientists fed disturbing background noise into the wind tunnel during the flight tests, the birds adapted and called less often because their calls could no longer be heard in the midst of the noise anyway. As a result, however, collisions occurred more frequently. This suggests that the in-flight calls actually contribute to collision avoidance.

Supplementary experiments also showed that the warning calls are used more frequently when the zebra finch’s vision is impaired – for example at night or in a darkened wind tunnel. In such conditions, the frequency of calls emitted in flight increased. This helped keep the birds from colliding despite the poor visibility and consequent limited visual matching. To learn more about how zebra finches communicate in flight, Hoffmann and her team next want to observe how the birds coordinate their movements during a coordinated flight maneuver. “We are interested in whether the birds also use vocalizations when avoiding a virtual obstacle together,” says Hoffmann. Future studies will have to show whether the observed behavior also applies to larger flocks of birds and zebra finches in the wild.

Source: Max Planck Institute for Biological Intelligence (in formation); Specialist article: Nature Ecology & Evolution, doi: 10.1038/s41559-022-01800-4

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