Evolution in the “Chickadee” of the Outback—the Red-backed Fairywren

By Dan Baldassarre
July 15, 2011
Illustrations by Dean Ingwersen (Red-backed form) and Tim Laman (Orange-backed form).
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Exotic name aside, the Red-backed Fairywren is sort of an Australian equivalent of the Black-capped Chickadee, readily found singing its heart out in backyards everywhere. But to a fledgling American graduate student, this denizen of the tropical savannah seemed to be hiding some interesting biology behind its unassuming persona—maybe even some insight into how species evolve.

Australia has two kinds of Red-backed Fairywren: one truly red-backed subspecies in the west, and an orange-backed form in the east. The two color forms overlap in northern Queensland. I was intrigued by the idea that females, by preferring mates of one color over another, might be causing these two subspecies to drift apart. This would be exciting, because other forces, such as habitat differences, are what usually cause new species to form.

As I was planning my fieldwork, the plot thickened. A colleague published a study showing that the genetic break between the two subspecies occurs hundreds of miles west of the transition from red to orange plumage. This break is easy to see on a map—it occurs in a lowland known as the Carpentarian Barrier that probably flooded repeatedly as climate changed during the Pleistocene, 300,000 years ago. That could have been enough to divide the fairywren population in two and set each on the path to becoming a new subspecies.

But then how could red-plumaged birds occur on both sides of this genetic divide? In his paper, my colleague suggested the plumage overlap was erroneous, the result of various observers finding orange-backed birds in the overlap zone, but inconsistently judging their colors as sometimes red, sometimes orange.

I had a hunch that something very different was going on. Maybe some of the red birds in the west had recrossed the Carpentarian Barrier, where they could interbreed with orange-plumaged birds. These pairings could produce red-plumaged offspring, causing the two forms to exist side by side as people had reported. To confirm this, I needed more precise plumage data to compare to the genetic data. This would entail traveling across the species’ range, catching birds to collect feather samples.

Easier said than done! The range of the Red-backed Fairywren spans 2,300 miles of unforgiving outback, where temperatures topped 110°F, swarms of flies seemed intent on sucking the fluid from my eyeballs, and thickets of razor-spined spinifex plants shredded my legs. Early outback explorers sometimes died from infected spinifex wounds. I survived, but was still extracting spines from my legs months after returning from the field.

As I approached the Carpentarian Barrier region, the habitat quickly changed from open tropical savannah packed with fairywrens to an expansive dry floodplain devoid of plants. The barrier that had arisen in the Pleistocene still seemed effective in separating the subspecies today.

After 100 miles of driving across this wasteland, the cracked soil gave way to grassland and I found myself in fairywren country again. As I had suspected, the birds on the western side of the barrier were just as red as the birds on the eastern side. Despite this seemingly uncrossable barrier, a few birds had managed to sneak across at some point, leading to the resurgence of red plumage among the orange-backed birds of eastern Australia.

I’d have to wait for hard evidence until I got back to my lab, where I could analyze my data. But being on the ground, suffering in the same harsh and unpredictable climate that had shaped the fairywrens’ evolution, and traveling across the same barriers, made me feel a true connection to these birds. And sleeping out under the Southern Cross in the empty outback made me feel a true connection to Australia, a country like no other on earth.

Dan Baldassarre is a Ph.D. student in the Cornell 
University Department of Neurobiology and Behavior and the Cornell Lab of Ornithology’s Macaulay Library.

Originally published in the Summer 2011 issue of BirdScope.