Our Window on Nature

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Birds of a Feather, Flock…

Filed under: Birds — Lowell Christie -- May 20, 2015 @ 7:17 pm

Sanderlings-1A flock of birds is more than a gathering – it’s a group that remains together over an extended period of time because of common benefits. This definition of a flock excludes the casual association that birds develop when they stop at your feeder, but it includes the groups of chickadees, titmice, and nuthatches that may forage together for hours. And, of course, it includes the array of sandpipers that wheel at the edge of the surf and the V-shaped formation of cormorants or geese flying south in the fall.

A flock of dickey birds may number only a dozen individuals, but flocks of large birds can exceed 100. Compare those numbers to the thousands of blackbirds and starlings that mass together to forage. Is there some kind of organization, some mysterious plan behind flock behavior?

Naturalists have been arguing over this matter ever since Pliny the Elder wrote about geese in Roman times. For example, one of the more popular theories that attempts to explain why waterfowl fly in formation states that flying single file yields an aerodynamic advantage. Proponents suggest that a goose or a gull playing follow-the-leader is actually riding in the slipstream of the bird in front of it (much the same way a car can be pulled behind on the slipstream of an 18-wheeler).They haven’t explained why the tired front-flying bird permits it, however. Birds, unrelated ones especially, aren’t noted for their altruism. And is the lead bird actually the group leader? Perhaps not.

Frank Heppner, zoology professor at the University of Rhode Island, has been pursuing that question for years. He hasn’t been able to prove an aerodynamic advantage, because measurements indicate that the birds aren’t flying in close enough formation to make use of it. He and his trusty flock of graduate students did find other interesting points to ponder, however. For one, flying in V formation permits the geese to stay within sight of each other.

With their eyes placed far back on the sides of their heads, geese have excellent forward and side vision but a blind spot behind. The shape of the V formation coincides nicely with the range of their eyesight, so any bird can see all other birds up and down the line. Heppner noted that this presents the possibility that the leader, if a leader exists, could fly anywhere in the formation, not necessarily at the head. Each possible answer produces more questions.

Another possible reason birds flock together could be to exchange information. Migrating turkey vultures cruise in “kettles” of several dozen birds. These massive-winged creatures must flap mightily to become airborne, but once they are up, they’re masters of soaring flight. How convenient then that when one bird encounters a rising coil of warm air, the others see it spiraling upward. They can all then head for the action. Conversation is unnecessary; before long, a black spiral of birds can be seen gaining altitude and peeling off to soar for miles.

Other instances of information sharing can be seen among smaller birds. When part of a flock of blackbirds lands in a freshly harvested field, it telegraphs the possibility of food to the rest of the group. Similarly, when part of a group of warblers and chickadees (mixed flocks are not uncommon) finds a feeder, the others soon arrive. Behavior communicates more loudly than words.

Another benefit of flocking is predator control. A flock of blackbirds or starlings with thousands of members responds to the sight of a falcon or accipiter (hawk) by drawing into itself as if caught in a vortex. This sudden clumping of birds into a ball is an example of what specialists call “bunching behavior.” There truly is safety in numbers; an individual is less vulnerable when surrounded by a flock.

On the California coast, biologist Barbara Kus studied the flocking behavior of sandpipers. Over the course of three winters she saw flocks attacked by merlins (a type of hawk) more than 600 times. Her studies confirmed others’ theories that the merlins’ success rate was directly related to the size of the sandpiper flock. Groups of fewer than 25 birds were the most vulnerable. As the flocks grew larger, their swooping and swirling increasingly confused the falcons and made dinner an ever dimmer possibility. Again, safety in numbers.

All of these explanations fit both our sense of logic and our understanding of the way nature works. That’s why a more recent report from Frank Heppner is a little disturbing. As he pointed out, much starling and blackbird behavior isn’t related to food or the presence of a predator. When a flock comes together to roost, it may take more than an hour to settle down. Meanwhile, the birds swoop through the sky and drop into the trees, only to take to the air again for another flight. The scene is anything but serene. If they are flocking to avoid predation, why are they calling attention to themselves”?

Heppner entered data concerning the flight of starling masses into a computer, hoping to learn whether the behavior was random or organized. After months of sitting near a computer screen with a group of mathematicians, Heppner now theorizes that the birds may be genetically programmed to be attracted to others of their own species. This dramatic kind of flight may result from individualized behavior, rather than from organized flight. So much for romance.

Heppner admitted that there is more we don’t know about flocking behavior than what we do know. He suggests that flocking serves many purposes, and we have yet to learn them. Luckily for nature lovers, an understanding of the science of it all is not necessary to enjoy these spectacular flights.

FMC0992

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