The Flocking Behaviour of Birds

By Lorraine Julien

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We’ve all noticed clouds of birds swooping and diving and marveled at the precision and speed with which they fly in perfect unison, no matter how large the flock. This flocking behaviour is similar to that of a school of fish or a herd of large mammals. I decided to check into this natural phenomenon.

When you first notice these flocks flying, they turn and move as though they are one – at fairly high speeds and with incredibly fast turns and dives, their wings never touching and mere inches or centimeters apart. It’s an awesome sight and obvious the birds have much faster reflexes than humans!

Why do they fly like this? It may be easier to explain the “why” rather than the “how”. Perhaps flocking is a defense mechanism whereby there is security in numbers: it’s much more difficult for a predator to single out one bird from a large flock and a predator isn’t likely to take the chance of flying into a swarming mass of bodies. Some small birds bunch up and fly in an even more compact formation if a hawk or other predator is spotted. Also, many eyes are much more likely to spot predators and the birds can take evasive action. Perhaps this is also why many fish swim in schools and many land animals congregate in herds.

Flying in flocks may be more efficient and safe as far as detecting food sources. Single birds are much more apt to get lost if they stray from the safety of the flock.

Through research and a frame-by-frame analysis of high-speed films of certain birds, it was found that any individual can initiate a flock movement. One individual bird can begin a “maneuver wave” which radiates throughout the flock. Once one of these waves begins, it spreads through the flock much quicker than individual reaction times. This information was derived from research by Wayne Potts, published in the journal “Nature” in 1984 and helped explain how flock movements are initiated and co-ordinated.

Potts proposed that birds farther away from the initiation site were able to see the wave approaching them, and could “get set” to respond before it actually reached them. He called this the “chorus line hypothesis” as it reminded him of the Radio City Rockettes at Radio City Music Hall in New York, where one can see and anticipate a high leg kick when it is still well down the line. It reminds me also of the waves that spectators sometimes do at ball stadiums.

Flocking behaviour is too complex to go into much detail here but flocking is, and has been, a favourite subject of computer modeling. Craig Reynolds, in 1986, created “boids”, a generic flocking of computer organisms. Reynolds’ simulation program relied on three basic rules of flocking: (1) Separation – avoid crowding your neighbours; (2) Alignment – steer towards the average heading of your neighbours; (3) Cohesion – steer towards the average position of your neighbours. Using these three rules, a flock can move in a very realistic way. It sounds fairly simple but is a considerable accomplishment given the lightning speeds at which these decisions take place.

Computer-generated flocks of bats and penguins based on “Boids” were first used in the movie “Batman Returns” in 1992 and in many other films since then.

In addition to information from Wayne Potts’ research, other information was also derived from Boids: http://www.red3d.com/cwr/boids/ In fact, if you check out Craig Reynolds “Boids” website, you can see one of his computer models. It looks very similar to some computer screensavers.

In another type of V-shaped formation flying, such as that of geese and other large birds, the flocks do not swoop and dive generally but are more interested in covering vast distances as efficiently and safely as possible. Usually the stronger, more experienced birds will fly at the forefront. This type of flying breaks up the air and provides an energy advantage for those flying further back in the flock. Once the lead flyer becomes tired, it drops back to one of the outer Vs of the flock and another takes up the lead.

More than any other kind of wildlife, birds have an almost magical hold on our imaginations. If not for birds and their awesome gift of flight, would humans have ventured into the skies in their “flying machines”? 

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Please feel free to report any observations to Lorraine Julien at naturewatching@gmail.com  or Steve Blight at natureobservations@rogers.com