An Obscure Bird and the Richness of Biological Specimens

By Rafael Marcondes

Linked paper: Testing the simple and complex versions of Gloger’s rule in the Variable Antshrike (Thamnophilus caerulescens, Thamnophilidae) by R.S. Marcondes, K.F. Stryjewski, and R.T. Brumfield, The Auk: Ornithological Advances.

Museum specimens are the gifts that keep on giving. The great beauty of the specimens in biological collections is that they keep serving researchers for an indefinite period of time after they have been collected, often providing the raw material to answer novel research questions that may not even have been on the radar of their collectors. The story of our recent paper in The Auk serves as an illustration.

That story started almost 20 years ago, in the early 2000s, when I was in middle school and then-University of Washington postdoctoral researcher Robb Brumfield became interested in a small, nondescript South American bird species called the Variable Antshrike (Thamnophilus caerulescens). The feature that prompted his interest is hinted at by this species’ apt English name, first bestowed on it by noted ornithologist Rodolphe Meyer de Schauensee in 1966. Meyer de Schauensee and Brumfield both noticed that, although dull, the colors of this species are extraordinarily variable. The particular kind of variation observed in this species is what we evolutionary ornithologists call geographic variation, whereby populations or subspecies occurring in separate geographic areas display different plumage colors. But the Variable Antshrike takes that kind of variation to a rarely seen extreme. Its subspecies vary from almost all-black to nearly white, with various shades of grey and brown in between. Most impressively, some of those birds with very different plumages occur within just a couple hundred kilometers of each other in South-central Bolivia.

It was those Bolivian populations that most intrigued Robb back in the day. He wanted to know if their genetic variation mirrored their color variation. So, in two field seasons in 2000 and 2002, Robb collected almost 200 specimens representing the entire breadth of variation in Bolivia. He then showed, using mitochondrial DNA from those specimens, that some of the most color-variable birds, called Thamnophilus caerulescens connectens, were hybrids between two other subspecies, Thamnophilus caerulescens dinellii and T. c. aspersiventer. We know from evolutionary theory that hybridization can, at least to some extent, explain increased color variability in animal populations.

Now fast-forward about 15 years. Robb is a curator and professor at Louisiana State University, and I am starting my PhD under his mentorship and am interested in a classic ecological principle called Gloger’s rule. That principle, in its most discussed version, predicts that birds inhabiting warm and humid areas tend to be darker than their counterparts inhabiting cool and dry areas. However, a more obscure version of the rule also predicts that birds should be browner when they live in drier and warmer areas. That prediction, called the “complex version” of Gloger’s rule, was almost completely forgotten for decades and was totally under Robb’s radar when he collected those antshrikes in Bolivia. But the specimens were still carefully preserved at the Burke Museum at the University of Washington in Seattle. Robb and I realized that they would be perfect for testing both versions of Gloger’s rule. In addition to the dramatic plumage color variation the specimens displayed, the regions in Bolivia where Robb collected them have great variation in climatic conditions.

Thus I traveled to Seattle carrying a nifty piece of equipment called a reflectance spectrophotometer that allowed me to precisely measure and quantify the colors of Robb’s antshrike specimens. Adding those to specimens collected by other researchers and deposited in other museums, I measured a total of 232 specimens ⁠— the largest sample size ever used in a study of Gloger’s rule in any tropical birds. Next, I developed a simple way to put a number on the amount of geographic variation in the Variable Antshrike, to see whether it is really exceptional compared to other birds, as Robb and Rodolphe Meyer de Schauensee previously suggested. Turns out it indeed is.

Finally, we combined my color measurements with data on the climatic conditions where the birds lived. Our results showed that the Variable Antshrikes do conform to that long-forgotten part of Gloger’s rule that predicts birds should be browner in warmer and drier places. As far as we know, nobody had ever speculated on why that might be the case. We suggest it may be related to camouflage: dry regions tend to have sparser, browner vegetation and more exposed soil, so it only makes sense that birds would also be browner to become less visible.

I think it is incredibly exciting to think about how these specimens helped answer a question that wasn’t on anyone’s mind when they were collected just 20 years ago. Who knows what other secrets of evolution they might hold the key to unraveling in 50 or 100 years?

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