MRE Symonds, GJ Tattersall (2010) Geographical variation in bill size across bird species provides evidence for Allen’s rule, American Naturalist 176: 188-197. PDF
What’s it about?
We show in a comparative analysis across 214 species of birds representing eight taxa including penguins, parrots and toucans, that species that live at higher latitudes and altitudes tend to have smaller beaks. This provides extensive comparative support for an old ecogeographical rule called Allen’s rule, which states that appendages should be smaller in colder climates, in order to reduce heat loss.
What’s the story behind it?
In 2009 I got an email out of the blue from an old friend from PhD days, Glenn Tattersall, who is now based at Brock University in Canada. He was out visiting Australia and wanted to catch up. During the course of his stay, he excitedly told me of his work on toucans (subsequently published in Science) which had shown that they use their prodigious beaks as enormous thermal radiators, to shed heat loads. Glenn has always been a thermal physiologist par excellence, but had felt uncomfortable in the land of phylogenetic comparative methods, and yet he was intrigued by a possibility that bird beaks might demonstrate Allen’s rule and wanted to test it with toucans (there are over 40 species of toucan, something I didn’t appreciate until I talked to Glenn). We collared together some data for the group on beak sizes, latitudinal and altitudinal range and found, sure enough, a really nice relationship with the latter. My reaction was “cool!”, but we surely toucans were just weird – so then we looked at other species we could think of – in all but one group we found strong relationships (sometimes R-squared values of 66% relating latitude to beak size, which is incredible). It was one of those cases where I literally almost couldn’t believe we were getting such clear patterns. It really helped ram home to me that at least one of the reason why bird beaks are the way they are is because of their role as thermoregulatory structures, something I had never even considered before. Thanks Glenn!