Let’s say you’re being chased by zombies. Are your survival odds better if you’re in a crowd of 100 people being chased by one zombie, or better if you’re running alone, chased by 100 zombies? OK, so now you understand why birds flock together. Imagine you’re in a flock of 100 sandpipers, being chased by a peregrine falcon. There’s a 99 percent chance the pending victim won’t be you.
There are, of course, other possible reasons why birds would flock together. But we can be pretty sure that defensive flocking is a real thing because shorebird plumage tells us so. All over the world, virtually every species of shorebird shares a similar characteristic. Despite being different species, they all have long, distinctive stripes on the upper wing. It’s a curiosity on individual birds, but it’s downright mesmerizing in a large, cart-wheeling flock. And that’s the point. It makes it more difficult for a predator to pick out an individual target. It’s why zebras are striped. When a herd dashes away from a cheetah, the pattern of undulating stripes is confounding.
Birds that fly in large, tight flocks have always intrigued biologists. How do the birds do that without crashing into each other? Without a leader, who decides when to turn?
You can watch flocking behavior at the Bangor Mall while finishing your holiday shopping. European starlings often fly in small flocks around the mall. In Europe, where starlings originated, flocks can be huge, cartwheeling in amazing patterns. Go to Youtube and search “murmuration of starlings.” Prepare to be mesmerized.
Scientists filmed these undulating flocks about a decade ago and postulated that each bird was paying little attention to the other 20,000 in the flock. Each was really just keeping a set distance from the six or seven around it. Furthermore, when executing a turn, birds reacted predictably.
Balloon-shaped flocks would elongate as they turned, with adjacent birds automatically moving into line behind their neighbors. Slow-motion analysis suggested the birds had specific rules on how to interact with neighbors and react to change.
This suggestion was consistent with observations published in 1984 in the scientific journal Nature. After frame-by-frame analysis of a video featuring European shorebirds, biologists noted that turns and rolls started slowly but rapidly picked up speed. They theorized that birds were aware of changes far from them, and anticipated how they would have to adjust flight when the change reached them. It’s like when humans do the wave at a football game. Usually, the change is triggered by birds on the outside edge moving closer to inside neighbors, a behavior one might expect when those outside birds wish to avoid being eaten by an approaching falcon.
So, it appears that this leaderless choreography in the sky is possible because it happens automatically. Birds don’t think about it. They just react as a group to outside stimuli, following specific rules that keep them from crashing into each other. For humans watching, it’s astonishing. For the birds, it’s just another day at the office.
The phenomenon has even been studied in humans. Some students in Germany assembled a large group of people and bunched them in a field. The crowd would start moving forward in unison, but when someone designated as a “predator” approached the group, the whole blob of people reacted in much the same way a flock would, following mathematical models that were consistent with computer-simulated flocks of birds.
Of course, there are other benefits to flocking such as increased vigilance. The more eyes in a group, the better the chance to spot trouble. Scientists have filmed flocks feeding on the ground. The bigger the flock, the smaller the percentage of birds actually looking up for hawks. In other words, flocked birds are able to forage more efficiently, while maintaining sufficient watchfulness.
You can observe this in the backyard. Chickadees and nuthatches usually dart into the feeder individually, grab a seed and skedaddle to safety. Goldfinches often arrive in bigger numbers. They settle on the feeders and chow down, confident that somebody in the flock will spot trouble in time.
Flocked birds are better able to find food, either because the chance of spotting food increases or because they learn from each other where to find it. Experiments in Britain demonstrated that groups of birds solve problems faster.
Most flocks disperse in summer, as individuals defend territories large enough to feed a growing family. Winter is the time to watch who’s flocking.
Bob Duchesne serves as vice president of Maine Audubon’s Penobscot Valley Chapter. He developed the Maine Birding Trail, with information at mainebirdingtrail.com. He can be reached at duchesne@midmaine.com.