In November, 2000, a Dusky Thrush became a local celebrity when it visited Whitehorse for seven days, spending most of its time in a mountain ash tree outside of Hellaby Hall. Usually found in northeast Asia, this songbird is a sensational rarity in North America.

Enthusiastic birders from other communities drove for hours to catch a glimpse of the thrush. Even people who do not keep life lists stopped by the church to see the exotic visitor.

Birds that are seen far from their normal range are called vagrants, or rarities, and they normally receive a lot of attention. Birders spread the word among their friends and also post messages on birders' websites.

"They are a lightning rod for birdwatchers' interest and enthusiasm," says Cameron Eckert, a Whitehorse biologist active with the Yukon Bird Club.

But Eckert points out that such sightings can be important for other reasons as well. Sometimes it turns out that unexpected species are not vagrants at all. They might be species that people had just not noticed before, or they might be forerunners showing that a species is expanding its range.

When the first Sharp-tailed Sandpiper was spotted in the Yukon, birders assumed that it must be a vagrant. While small numbers of this species are seen along the west Coast during fall migration, these birds breed across the Pacific Ocean in Eurasia.

But once Yukon birders started to look actively for this species during fall migration, they found that it passes through the territory every year. Last fall nine Sharp-tailed Sandpipers were spotted in wetlands in the southern Yukon.

"We learned that a species that we thought was rare in this area actually moves through regularly," says Eckert. "It shows the importance of awareness and appreciation of the birds that are here among us as part of our regular avifauna."

And then there is the case of the Caspian tern. A visiting birder from Mississaugua snagged the first Yukon sighting of this species in 1996, but this time the news was not totally unexpected.

"We knew this species was expanding its breeding range in western Canada and that it would show up eventually here, and it did," says Eckert.

Since then terns have been spotted every year, and for the last three years pairs have been seen in the Marsh Lake area. Now the hunt is on for signs that this species is breeding in the territory.

"The Yukon is at or near the edge of the breeding range for many species. As a result, it can be a frontier for detecting changes in species," he says.

Eckert says that the Yukon is also turning out to be a good place for rare sightings, such as that of the Dusky Thrush. "For whatever reason, we seem well-positioned for seeing some fairly extraordinary rarities. A disproportionate number of accidentals from the Bering Sea region are showing up. In addition to the thrush, we've recently seen Bar-tailed Godwit, Slaty-backed Gull, Red-throated Pipit, and Bean Goose."

Local birders speculate that these birds might be blown inland from the coast, or they might fly up the Yukon River system from the Bering Sea.

In some places the arrival of vagrants is somewhat predictable, and birders start to wonder when these visitors do not show up. For example, every year a small number of Red-eyed Vireos are blown across the Atlantic from North America to the British Isles.

In the 1990s British birders noted a dramatic drop in the numbers of this species showing up on their shores. Observers think that major air currents may have shifted, and the vireos might now be dropping out of the skies in Iceland.

Some climate change models predict that higher global temperatures could result in changes to the upper atmosphere. If major air currents start to shift, the migration routes of some birds could be affected as well. For example, research in British Columbia has shown that wind changes in the upper atmosphere during migration could decrease the reproduction of Western Sandpipers.

Eckert encourages birdwatchers to report unusual sightings as eventually patterns can start to emerge in the pooled information, helping to explain what is happening.

Eckert does not downplay the importance of individual sightings of vagrants as well. Between 60 and 100 people showed up to see the Dusky Thrush in Whitehorse, which would be equivalent to 9,000 people in Toronto turning out to see a rare bird.

"That one bird did a lot to boost people's interest in birds and birdwatching which leads to an awareness of habitat issues," he says.

The peak seasons for spotting vagrants is during the spring and fall migrations when birds are on the move. Eckert urges people to contact him or other Yukon Bird Club members if they think that they have spotted an unusual bird in the Yukon. He can be contacted at 667-4630 or by e-mail at ceckert@yknet.yk.ca.