I’m going to hopefully excuse my rather poor photographs with a bit of a science story to go with them.
Two and a half weeks ago I returned from four weeks in Panama teaching a tropical biology field course. Many of you probably associate the tropics with brightly colored birds: parrots, toucans, and so on. A common question is: why are tropical birds so colorful. What is it about the tropics that results in the evolution of bright colors?
Well first, are there really more brightly colored birds in the tropics? This is not a straightforward question to answer. One complicating factor is that there are a lot more species of birds in tropical forests than there are in any other environment in earth. So any comparison must take differences in the total number of species into account.
Secondly you have to consider carefully what being ‘colorful’ means. A vividly green parrot will seem bright flying over a desert but will blend in the forest canopy. Also bird vision is not exactly the same as our own vision so a bird’s idea of what is bright may not be the same as the impression of a birder.
Finally bright color is the result of evolution. If you really want to know if the tropics in some ways drives the evolution of brightness then you need to look at where groups of bright birds originated.
So when studies have examined this issue it turns out that there is no evidence that bright coloration is more likely to evolve in the tropics than anywhere else. However there are particular groups of tropical birds that have many spectacularly colorful species: tanagers, macaws, toucans, trogons, and the large group of unusual birds known as cotingas.
One thing many (not all but many) of these birds have in common is that they eat fruit. Frugivory (fruit eating) is a very common mode of existence for many tropical birds and mammals. Many birds and mammals in North America will eat fruit but very few are fruit specialists. Cedar waxwings and phainopeplas are examples that come to mind.
Frugivory in tropical birds seems to be associated with strong sexual selection and thus bright colors. The hypothesis is that eating fruit is a fairly easy way to make a living and that individuals can devote a lot of time to competing for mates/choosing mates.
Another interesting pattern is that within these groups of colorful birds, ones that nest in cavities (holes in trees) tend to be species in which both sexes are colorful. In species with nests in the open females are typically fairly dull. This is thought to be associated with the high level of predation on bird nests in the tropics. There is strong natural selection on females to have dull coloration if they will be sitting on a nest in a location where they can be seen.
Thanks for reading this week’s Dawn Chorus. Please discuss all things birdy in the comments. Sorry the pictures aren’t better.