by Patricia Robertson
Did you know that the Yukon has more inhabitants from the East Coast than the west?
No, there hasn’t been a sudden influx of Ontarians and Maritimers. Those inhabitants are birds, and their ancestors may have begun arriving at the end of the last ice age, over 10,000 years ago.
Darren Irwin is a zoologist at the University of B.C. who studies the migratory behaviour of birds and what those patterns tell us about bird origins in northwestern Canada.
He’s found that 34 Yukon bird species originally came from eastern North America, including flycatchers, vireos, thrushes, warblers, and some finches and sparrows.
“Those species are all forest birds, and they followed the expansion of forests at the end of the last ice age,” explains Irwin.
Those forests expanded from a large, ice-free area of forest in the eastern U.S., known as a glacial refugium. Forests in the western U.S. - and western birds - couldn’t expand their range as quickly because the mountains of B.C. were still ice-bound, creating a formidable barrier.
“So in many ways the Yukon is more eastern origin in terms of forest birds than western, even though it’s up in the northwest part of the continent,” says Irwin.
Irwin and his graduate students are studying related pairs of eastern and western birds that share a common ancestor, yet became different species when separated during past ice ages.
The winter wren, for example, is an easterner originally, while the Pacific wren came from the west. Another pair is the eastern mourning warbler and the western MacGillivray’s warbler.
“These pairs have spent all this time separated by the ice and within the last 10,000 years or so have come into contact. Do they interbreed or not? Do they have different songs? Different appearances? Different migratory behaviour? We’re trying to use all that information to figure out how one species gradually splits into two species.”
These bird pairs each evolved from one original species that existed before ice covered North America. Zoologists look at the DNA of these pairs and use what’s called a molecular clock to try to figure out how long ago they split.
“Often it’s roughly a million years or more,” says Irwin. Over those millennia, multiple cycles of glaciation and interglaciation, or ice-free periods, meant that bird species expanded and retreated accordingly. “But the basic point is that in each case there was one ancestral species that at some point got split into two groups, and those two groups get more and more different, and eventually they get so different that they don’t interbreed, and then we would say they’re definitely separate species.”
Amazingly, these birds continue to follow the same ancient migratory routes their long-ago ancestors did. Western species such as the MacGillivray’s warbler migrate down the west coast of North America and winter in southern Mexico.
Eastern ones, however, such as the mourning warbler, fly southeast from the Yukon through the southern and eastern U.S., across the Gulf of Mexico, and down to Colombia and other parts of South America. Both of these very different migratory routes take the birds around the Rockies and the deserts of the American West.
These migratory instincts are incredibly strong - so strong that birds caught during migration and kept in a cage will tell researchers the direction in which they want to migrate. “It’s called migratory restlessness,” explains Irwin. “Their instinct to migrate is so intense they’ll take little flights in the direction they want to migrate in.”
Yet these ancient instincts are still not fully understood. Since the parents of migrants such as mourning warblers migrate south before the new fledglings do, “the young bird has to fly to Colombia without a map, never having been there before,” says Irwin. “But they know how to do it. Their genes encode for this really amazing behaviour.”
A number of studies show that the birds can somehow sense the magnetic field through vision. When light enters our eyes, the light excites an opsin molecule in the eye, which is how we see. “If there were a little magnetically sensitive molecule connected to that opsin, that could alter the colours we see,” says Irwin. “It may be that when birds look in parallel to the magnetic field, they see a different colour than if they look perpendicular to it.”
A third group of Yukon birds, including the bluethroat, the wheatear, and the eastern yellow wagtail, originally arrived from northeastern Asia and migrate even further afield - all the way to southern Asia or Africa. “They all breed in the Yukon, but we’re at the very eastern edge of their range, which extends all across Alaska and Siberia - and all across Europe in the case of the wheatear,” says Irwin. “The wheatear migrates across Alaska, Siberia, and the Middle East into sub-Saharan Africa, and then back again in the spring.”
But the wheatear hasn’t moved further eastward into the N.W.T. or southeastward into B.C., even though it could likely survive there. That’s probably because it has to fly back to Africa every fall, “and it reaches a point where it just can’t migrate any further.”
Highly migratory species haven’t moved between North America and Asia very much, and when they have they’re like the wheatear, which has just barely gotten into Alaska and the Yukon. Oddly enough, non-migratory species like chickadees, ravens, crows, and jays move between the two continents quite easily. “They show there’s not a problem for birds to move between continents - the forests are similar enough,” says Irwin.
The Pacific wren, for example, which is non-migratory along coastal Alaska and the Aleutian Islands, has figured out how to survive well in the winter. It feeds on spiders and other insects that live in woodpiles. But the birds that catch flying insects, such as flycatchers and warblers (which eat insects that live on leaves), must head off on their age-old migratory journeys to find the right kind of food. “Each species has its little ecological niche,” says Irwin.
Understanding these migratory routes is also important for bird conservation, says Irwin. “We need to know where these different species spend their winters so that, if we notice they’re in decline, we can look at whether the habitat in their wintering areas is in trouble. We’re trying to better understand patterns of migratory connectivity so we can contribute to conservation planning.”
For more information on overwintering bird species, see http://taiga.net/yourYukon/col411.html
This column is co-ordinated by the Yukon Research Centre at Yukon College with major financial support from Environment Yukon and Yukon College. The articles are archived at www.taiga.net/yourYukon.