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? It turns out to have 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, 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 wing tip 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 line.

The trailing bird positions itself to catch just the upwash of that vortex, or 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 the V shape.

Studies have estimated that birds flying 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 wingspan.

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 even uses 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 between

itself and a possible predator. Repeat this across enough individuals and you end up with

a herd — or in the case, a flock.

Other theories offer similar explanations, but, whether you're talking 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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