It’s gratifying to see stereotypes shattered by hard data, even if the stereotype is merely about bird coloration. We think of tropical birds as lushly colorful. Two scientists, Nicholas R. Friedman and Vladimír Remeš, tested this out on Australian birds. They found it was not true, or at least not confirmed.
Tropical regions support a greater diversity of species, including more colorful birds, but, species for species, arid areas actually have the “fancier” colored birds. Now notice where the hard data starts and ends.
From Science Daily:
Friedman began his study at the Australian National Wildlife Collection, where he examined bird specimens from different regions of Australia. A total of 137 different species from two major songbird families were examined. Songbirds originated in Australia nearly 30 million years ago. Research suggests that these birds began diversifying there before colonizing other parts of the world. The familial relationships of the birds that Friedman examined were compared using an evolutionary tree based on the birds’ DNA. Friedman then used a special instrument to measure the color of the feathers in particular places on the birds….Next, Friedman used data from satellites to describe the geographical region each species lives in. He looked at vegetation, precipitation, and humidity of each region, then combined this data with the evolutionary relationships and color measurements of the birds.
The results of this study, published in Global Ecology and Biogeography, show that bird species do not evolve more colorful feathers in the tropics compared to their cousins in temperate climates. “If you look at birds in the tropics, there are a lot of colorful birds that stand out. But there are really more species in general there, and there are just as many more of the little brown ones” describes Friedman. “Instead, birds living in the harsh arid climates of inland Australia tended to have fancier colors than those in the lush tropical islands.”
In more technical language, from the Abstract:
Here, we compared plumage coloration across two families of Australian birds (Meliphagidae, n = 97 species; Acanthizidae, n = 40 species) in a combined spatial and phylogenetic framework. We assessed the extent to which environmental variables extracted from species ranges explain variation in colour traits, while correcting for the autocorrelation inherent in spatially structured data using extensive simulations.
We found several strong effects of environment on plumage coloration. Inland species with ranges marked by high aridity and temperature seasonality showed greater colour span among acanthizids, and greater saturation among meliphagids. Gloger’s rule was supported in both clades, but more strongly for dorsal plumage. The most consistent correlate in this relationship was vegetation: birds in regions with more vegetation had markedly darker plumage. Ornament hue showed no significant associations with vegetation or climate.
Gloger’s rule observes a “negative relationship between brightness and humidity.”
That is interesting. The “conventional wisdom” is either wrong or, as they say more cautiously, still “largely untested.” Another stereotype is that with evolutionary scholarship there’s a bright red light where the data ends and the storytelling begins. That conventional wisdom has been tested here many times and appears to hold true again in this instance.
Friedman again: “Since desert birds have to scramble for mates during the wet season, we think they may be evolving colors that can attract mates quickly.” (Emphasis added.) More:
“The pattern is really clear,” Friedman reports, “birds living in the desert tend to be more grey on their backs, while birds living in the forest have evolved to be more of a dark green — we think they are evolving these colors to match their background.” This would be an example of natural selection, in this case more camouflaged organisms can survive and pass on their genes.
The data are what they know. The narrative gloss of natural selection is what they “think,” what “may be” or “would be” the case. The thinking, not necessarily wrong, is along predictably stereotypical lines. Before hearing what they “think,” you can probably fill in the blanks yourself from experience of reading this kind of thing.
We don’t need research to tell us that evading predators and mating successfully contribute to passing down your genes, and therefore would be favored given an evolutionary framework. Again, this is not to say they’re wrong. But from the science to the speculation, it’s a steep step down.
Source: Evolution News & Views