UBC graduate student David Toews with a winter wren in Tumbler Ridge, B.C. - photo courtesy of David Toews

UBC Reports | Vol. 52 | No. 11 | Nov. 2, 2006

Birds of a Feather, Discovered Together

By Brian Lin

After millions of years of separation, two divergent forms of North American winter wrens have finally come together, and UBC zoologists have discovered where.

And for this avian version of West Side Story, Maria and Tony aren’t hitting it off… yet.

Divided by glaciation during the Pleistocene Epoch 1.8 million years ago, winter wrens have evolved into different subspecies with distinct songs and genetic codes. Eastern winter wrens are found across eastern North America and as far west as Alberta, while western wrens inhabit the Pacific coastal belt between Alaska and Oregon.

Scientists have theorized that the eastern and western populations would meet somewhere in the middle, along the Rockies, but no one has been able to pinpoint where or, if they co-exist somewhere, whether they would interbreed.

Determined to find the elusive contact zone, UBC Zoology Asst. Prof. Darren Irwin spent a few weeks traveling in northeastern British Columbia and Alberta during the summer of 2005.

“I had arrived in the Peace River area and was actually quite discouraged after a few days of not seeing a single winter wren,” recalls Irwin.

“One day I was at a motel in Dawson Creek and Googled ‘Peace River’ and ‘winter wren,’ and came across a map of bird sightings created by a local birdwatching group called the South Peace Bird Atlas Society, showing a high concentration of winter wrens around a town nearby called Tumbler Ridge,” says Irwin.

He set out on a hike and before long, the birds started singing.

It was music to his ears – both eastern and western wrens singing their own special songs within 100 metres of one another. Subsequent genetic testing of blood samples and song analysis carried out by his graduate student David Toews have confirmed that the two types of wren living in Tumbler Ridge are as different as those from New York and Vancouver – so much so that the pair are making a case in an upcoming journal article to classify the two as separate species.

Despite looking almost identical to the human eye and living in close quarters, the two groups don’t seem to consider each other mate-worthy. Toews’ mitochondrial DNA analysis of the birds suggests the two groups aren’t interbreeding.

“So far, the dramatic differences in song and DNA in Tumbler Ridge make us think that the eastern females aren’t responding to western songs, and vice versa,” says Toews. Only male songbirds sing, and for two purposes: to musically mark their territory and broadcast their availability to females.

“You might say the serenade is the deal breaker.”

That is not to say the birds aren’t keenly aware of potential competition in their gene pool. In fact, Toews plays recordings of songs to attract and catch the territorial males for measurements and blood samples.

The discovery of a habitat where “east meets west” presents an opportunity for scientists to clarify an ongoing debate about what characteristics – ecological, behavioural, or physical – contribute to a single species diverging into two.

“When we add this to other observations of species that have come into contact after long ages of separation and how they behave around one another, it allows us to narrow down what characteristics lead to reproductive isolation,” says Toews.

The next step is to carefully determine if there are any other traits that might distinguish the two forms. For example, birds might be able to recognize subtle differences in plumage such as patterns in the ultraviolet spectrum. For now, however, Irwin and Toews believe that songs are the most important trait preventing interbreeding.

“The discovery is also exciting because North American birds are among the most studied animals, so it is surprising to find what we thought was one species is really two,” says Irwin. “It leads us to believe that there may be many other such ‘cryptic’ species out there.”

To hear the marked difference between the eastern and western wren songs, visit http://www.zoology.ubc.ca/~irwin/wrens.html.