First we will look at wing size.

The area of a bird's wing is directly related to the flight forces it can generate.

The larger the wing, the greater the flight forces.

Longer arms indicate larger wings, so we can use the length of the arm bones as an indirect measure of the flight forces that the wing can generate.

This is a helpful measure to use when we can't measure flight forces directly—such as when we're studying fossils.

The large bird pictured here has longer arm bones.

Is it safe to say that this bird flies better than the small bird because its big wings generate more force?

No, not exactly.

Although the large bird does generate greater flight forces, it is also heavier.

It needs to generate more force to overcome its weight than the small bird.

But how can we estimate the bird's weight unless we can measure it directly?

Again, the skeleton gives us clues.

Larger birds have longer legs, so we can estimate the weight of a bird by the length of its leg bones.

The best fliers generate large flight forces and don't weigh very much.

Therefore, the length of a bird's arm bones relative to its leg bones gives us a rough idea of how well a bird can fly.

With this principle in mind, which bird is the better flier?

The large bird.

The small bird.

Neither, they are equally well equipped for flight.

Oops.

Consider the length of the arms relative to the length of the legs.

Try again.

Although the larger bird has longer arms, it has the same relative arm length as the small bird.

Therefore, these birds are equally capable fliers.