Birds wheel through the air en masse, swooping and diving as a flock but never hitting one another. It’s not because they have magical powers, although that was one suggestion made in the mid-nineteenth century. So how do they do it? And why?
Contrary to popular belief, birds do not simply follow the leader. Each bird in a flock makes its own decisions about how to maneuver in flight, and these decisions are guided by three natural inclinations that all flocking birds share. First, a bird always tries to maintain a minimum distance between itself and every bird immediately surrounding it. By doing this, the bird usually ensures that it does not collide with a fellow avian traveler.
Countering this tendency towards repulsion is the second key inclination – cohesion. An individual bird within a flock is always trying to move toward the average position of its fellow flyers. The net result of these two inclinations is a continuing effort to be as close as possible to the neighboring birds without coming too close.
The third inclination is the flocking instinct itself. This inclination drives each individual bird to keep track of the average heading and general speed of the entire flock. Without this tendency, splinter groups of birds would constantly be breaking away.
In order to remain with the flock, each individual bird constantly responds to these three inclinations. Following the cues of the surrounding birds, an individual moves in order to maintain a balance between distance and cohesiveness without leaving the flock.
A bird is inclined to stay with the flock to take advantage of the many evolutionary benefits that that it provides. First, flocks allow birds to draft behind one another, greatly increasing their efficiency. Drafting allows large groups to travel a greater distance than solo flyers: a flock of geese flies 70 percent further then a lone goose. Riding in the wake also helps birds that are less strong and less competent fliers remain with the flock
A second benefit of flocking is protection in numbers, and that means preservation of the gene pool. If a hawk is circling and swooping into a flock of European starlings, it can be overwhelmed by the sheer number of birds to choose from and be unable to focus effectively on a single bird. In addition, a large group of birds is more likely to see a predator and take evasive action in time.
But the most important benefit of flocking comes during migration. Not all birds in a flock will have migrated before, and others might only have a vague idea of where they need to go. With multiple birds combining their memories and senses of direction, the whole flock is more likely to end up where it needs to be.
Essential to all three reasons for flocking is acute vision. The size of a bird’s eye in relationship to its head is a testament to its importance. Human eyes make up 1 percent of the mass of the human head, but the eyes of European starlings, those fanatic flockers, make up 15 percent of the mass of a starling head. Without keen vision, a bird would not be able to fly, much less maintain its three-dimensional position in a flock of other birds, each constantly adjusting its own three-dimensional position.
Eye position is a key factor in the effectiveness of bird vision. Flocking birds have monocular vision, meaning their eyes are located on the sides of their heads. This allows some birds (depending on how far back their eyes are placed) to see up to 300 degrees without turning their head. Although this compromises depth perception, it allows them to see the bird flying next door much more easily. Since each decision in flight is made based on neighboring birds, this serves a flocking bird much better than eyes located on the front of the head.
Monocular vision does mean, however, that flocking birds are not as effective at being predators. Depth perception – a critical skill for a predatory bird trying to gauge the distance to its prey – requires binocular vision, with eye placement at the front of the head. Though hawks and other predators will fly together during migration in order to draft, they do so more as loose groups than as tightly spaced flocks.
Birds flocking in a cloudless, blue, autumn sky may seem like magic, but it’s really a combination of vision, instinct, and skill. It’s the ease with which birds combine these three things that makes bird flight appear magical.