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Think of a flock of birds and you probably imagine that classic V shape, a leader with sets of trailing birds on either side, but not all flocks fly this way.  Starlings, for example, travel in large, three-dimensional clusters that seem to move like a wave.  So why do some species fly in Vs and others in clumps?  Well, it turns out it has a lot to do with the individual birds themselves.

Some, like geese heading South for the winter, are making long treks.  The V formation helps them stay in visual contact with each other, avoid collisions, and conserve energy.  It's the structure of their wings that lets them take advantage of the V.  As the wing flaps, each wingtip creates a vortex that spirals up from the bottom of the wing and over the top.  This vortex trails off behind each bird as it moves forward and is encountered by the next one in the line.  The trailing bird positions itself to catch just the upwash of that vortex, or the upward force, and that requires being behind and just to the side of the leading bird.  Lots of birds behind and to the side of one another creates that V shape.

Studies have estimated that birds flying in this way can save around 15% of their energy.  So, why don't all birds fly this way?  We talked to Professor Erick Greene from the University of Montana Bird Ecology Lab and he explained that this has to do with the size of the bird.   You may have noticed that birds that fly in a V, like geese, pelicans, swans, and ibises are typically larger creatures with a long wing-span.  These species move their wings only a few degrees up and down with each flap.  This motion creates vortices that lie pretty neatly behind the bird.

Small birds, on the other hand, tend to flap their wings all the way up and down.  The vortices created by these motions are all over the place, not consistent enough for their flock-mates to actually use, and the small birds that do flap their wings like larger ones just don't generate a big enough vortex because of their size.  For small birds, flying in groups sometimes uses even more energy, not less, but these species have another need that's even more important: protection.

In 1971, evolutionary biologist William David Hamilton proposed a theory called the selfish herd.  It suggests that the risk to an individual is reduced if that animal places another animal between itself and a possible predator.  Repeat this across enough individuals, and you end up with a herd or in this case, a flock.  

Other theories offer similar explanations, but whether you're talking about schools of fish or swarms of insects, it's clear that this is a pretty common survival strategy.  So the next time you see a group of birds flying by, you'll know it might be to save energy or it could just be to stay alive.

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