This is the standard measure of beak size in Decapodiformes; hood length is preferred for Octopodiformes.

Darwin's finches exhibited adaptive radiation by evolving different beak sizes to exploit the diversity of seeds present on the different islands.

In birds, BMP4 has been shown to influence the beak size of Darwin's finches.

Another example is the beak size in a population of finches.

Five subspecies are recognised, differing most significantly in beak size.

However, they exhibited disruptive variation in beak size.

When either one occurred by itself on a smaller island, however, the beak size was intermediate in size relative to when the two co-occurred.

Sexual dimorphism in beak size may have affected behaviour.

The beak sizes of jays determine how large acorns may get before jays ignore them.

Their studies clearly show changes in physical characteristics (like beak size), in response to environmental forces (like floods and droughts).