Island living may call to mind vivid flowering vines and colorful plumage. But in reality, birds on islands around the world have evolved less-colorful feathers than their mainland relatives. Their drab, simple patterns are only the latest evidence that island evolution is kind of weird.
Claire Doutrelant, an ecologist at France’s Centre d’Ecologie Fonctionnelle et Evolutive, and her coauthors studied 116 pairs of bird species, or 232 species in all. Each pair included an island bird and its closest relative that lives on the mainland and breeds at a similar latitude. For example, one pair was made up of the Hawaiian nene and the Canada goose.
You can see another pair at the top of this post. On the left is Ptilinopus regina, the rose-crowned fruit dove. It lives on mainland Australia. On the right is its cousin Ptilinopus regina, the gray-green fruit dove, from the island of Tahiti. In this case, the Australian bird—with its rainbow breast and hot-pink hat—is clearly the showier member of the pair.
The researchers wondered whether this pattern would hold up across island birds in general. Species that live on islands tend to evolve in interesting ways. Large species may shrink compared to their mainland relatives, like the pygmy elephants of Borneo. But small species may become enormous. Several giant rat species (don’t worry, they’re mostly extinct) fit this description.
Colorful birds might evolve to be less colorful on an island, thanks to reduced selective pressures. Or they might become more colorful than ever—think of birds-of-paradise, says Doutrelant, or “the amazing Hawaiian honeycreepers.”
To find out, the researchers looked at samples of all 232 species. The samples were skins from American and British museums. Doutrelant used a spectrometer to measure the colors of each bird’s plumage—three males and three females for each species. Then the researchers ran the data through a computer program that simulates bird vision to find out how those colors appear to other birds. They also counted the number of individual “color patches” on each animal. That is, if the bird were a page in a coloring book, how many separate regions would you have to fill in?
Mainland birds had more colorful plumage, on average. Their colors were brighter and more intense than the feathers of their island relatives. Mainland birds also had more color patches, meaning they had more complex patterns. This was true of both male and female birds, even though females are generally more drab to begin with.
Doutrelant points out that not all birds are duller on islands. Most are, though, and there are likely multiple reasons why.
On an island, birds tend to have fewer related species sharing their space. So it’s not as important for a bird to announce its identity to other birds using very specific patterns. That may be why island birds have fewer color patches. And thanks to smaller populations on islands, there may be less genetic diversity. This could mean that a female bird has less to gain or lose by being picky about her mate. So males no longer need to display bold, flashy feathers to prove they have good genes.
In a way, then, island birds do lead a more carefree lifestyle. Facing fewer evolutionary pressures, the birds can put less energy into building bold plumage. It’s a nice existence—even if it leads to less picturesque postcards.
Images: Left, the rose-crowned fruit dove Ptilinopus regina from mainland Australia, by Geoff Whalan (via Flickr). Right, the gray-green fruit dove Ptilinopus regina from the island of Tahiti, by RyanStudiesBirds (via Wikimedia Commons).
Doutrelant, C., Paquet, M., Renoult, J., Grégoire, A., Crochet, P., & Covas, R. (2016). Worldwide patterns of bird colouration on islands Ecology Letters DOI: 10.1111/ele.12588