Smaller beaks for colder climates

Birds living in colder climates evolved smaller beaks than their fair-weather cousins to help keep them warmer, a new study shows.

Led by Matt Symonds from the University of Melbourne in Australia, the study published in American Naturalist looked at over two hundred bird species from across the globe - including African barbets and tinkerbirds, Australian parrots, Canadian gamebirds, penguins, and terns - to see how their beak sizes varied with the temperature regime of their native habitat.

We already know from thermal imaging studies that birds like toucans and geese can lose a lot of their body heat through their big beaks.

Now Symonds and the team have robust evidence that birds living in colder environments, whether that's towards the poles or up a mountain, tend to have smaller beaks.

It could be that birds in warmer climates have evolved bills to shed heat, but the authors think it's more likely that cold temperature is a constraint on the size of birds' beaks.  A big beak radiating heat would be too much of a liability in a cold climate, no matter what other advantages might come with it, like communication or attracting mates.

This is the first in-depth study that backs up a 133 year old theory, called Allen's rule, which predicts that warm-blooded animals from colder parts of the world will evolve smaller appendages including ears, tails, and limbs.

The team didn't find such a strong relationship between bird leg length and temperature, suggesting that beaks are more important as a source of heat loss, and consequently the need to regulate body temperature has been an important factor in shaping the evolution of bird beaks.