I saw my first winter wren in Tilden Park during the Ford administration, but I haven’t encountered many in the East Bay since then. The Contra Costa Breeding Bird Atlas (www.flyingemu.com/ccosta) shows them still nesting in the Berkeley and Oakland hills.
In Europe, this species is simply called “the wren,” or the local equivalent. It’s the only wren in the Old World; the rest of the family is American, with its greatest diversity in the tropics. “Winter” probably comes from its status as a seasonal visitor in the South, as opposed to the year-round-resident Carolina wren and the summer-visiting house wren. That name may no longer apply to western populations, though: recent research strongly suggests that our birds are a distinct species, for which “Pacific wren” has been proposed.
These birds are skulking mouselike creatures, only seen when they pop out of some dark root cavity and scold you for trespassing. They’re much more often heard. British Ornithologist S. Cramp wrote that the male sings “with remarkable vehemence … [as if] trying to burst [his] lungs.” Roger Tory Peterson heard it as “a high rapid tinkling warble.” David Sibley characterizes it as “a remarkable continuous series of very high, tinkling trills and thin buzzes.”
The indefatigable songcatcher Donald Kroodsma, professor emeritus at the University of Massachusetts, Amherst, whose The Singing Life of Birds is probably the best popular introduction to the science of birdsong, was the first to notice that eastern winter wrens and western winter wrens had really different songs. He reported in 1980 that birds recorded near Corvallis, Ore. had much larger repertoires of song types. One of them performed 32 song types, which placed it “near the pinnacle of song complexity” among birds. New York wrens, on the other hand, had only two song types apiece.
The Oregon wrens, in fact, seemed to have their species’ largest song vocabularies. French wrens were later found to have a range of four to seven song types. In 1989, Kroodsma and Hiroshi Momose of UC Davis recorded Japanese wrens with six to seven types. The similarity between Japanese, eastern North American, and European songs led the authors to speculate that eastern wrens had “colonized the Old World via the Bering Strait during one of the interglacial periods.”
Kroodsma suspected the eastern and western winter wrens might be biologically distinct species, despite their near-identical appearance. He had noticed that their call notes were also different: sharp and higher-pitched in the West, rich and lower-pitched in the East. Western birds have more higher frequency notes in their songs and a more staccato delivery. You can hear the two types at www.zoology.ubc.ca/~irwin/wrens.
“I’d love to know where eastern and western wrens meet…and what they think of each other there,” Kroodsma wrote in Singing Life. “Someday I will discover the limits of eastern and western wrens, somewhere in the Northwest, no doubt, unless some intrepid explorer rises to the challenge and beats me to it.”
Those intrepid explorers were David Toews and Darren Irwin of the University of British Columbia, whose winter wren study was published last year in the journal Molecular Ecology. They located a population of western-type singers at Gavin Lake, BC and a population of eastern types at Lesser Slave Lake, Alberta, and worked back and forth between the two, looking for the contact zone.
In the summer of 2005, Irwin, searching for wrens in BC’s Peace River Valley in the eastern Rockies, googled his way to maps created by the South Peace Bird Atlas Society that showed a high concentration of wrens around the town of Tumbler Ridge. Jackpot. Here he and Toews found eastern and western types singing on adjacent territories. Every wren they heard could be assigned to one of the two categories: there were no mixed song types. The biologists recorded the songs of eight westerners and four easterners, and took blood samples for genetic analysis.
Irwin and Toews found striking genetic divergence between western and eastern types, and a perfect correspondence between singing type and genetic profile. No hybrids were identified in the contact zone, although one of the Gavin Lake birds had eastern-type genes. Genetically and behaviorally, the Tumbler Ridge wrens looked like separate species. Time to revise the field guides again.
Based on a standard molecular clock, the two lineages split 4.3 million years ago—long before the Pleistocene glaciation began.
So it’s unlikely that the ancestral eastern and western wrens were separated by ice, as other species pairs may have been. The whole speciation process may have been driven not by geographical isolation or ecological differences, but by western females’ preferences for more complex songs. Score one for sexual selection.