In September, one of the massive wildlife spectacles that distinguishes this part of the world will once again pulse across the Yukon's skies. Headed south to their wintering grounds in the southern United States, huge flocks of Lesser Sandhill Cranes will pass over the territory in the space of a few days.

There are more than 500,000 of these cranes in the world, and about half of them migrate across the territory every spring and fall. A few of the cranes breed on the Yukon's North Slope, but most of the birds continue on to western Alaska, while some even cross the Bering Sea to breed in northeastern Siberia.

Lesser Sandhill Cranes migrate two to three times further than other Sandhill Cranes; some of them travel as much as 22,000 kilometres in a year. They are also the most abundant of the three migratory subspecies of Sandhill Cranes in North America.

The cranes, as well as many other bird species, migrate along the Tintina Trench. This long rift valley cuts a 725 kilometre-long line across the territory in a northwest to southeast direction, providing the birds with a low elevation route through the Yukon's rugged landscape.

Records on the Sandhill Crane migration in the Yukon date back to 1904, but not many observers have documented this phenomenon. In one notable account from 1950, an observer at Pelly Farms reported between 1,000 and 5,000 cranes flying overhead daily over a two-week period during the fall migration. The cranes usually fly only during the day, roosting at night in open areas.

Partridge Farm, located off of the Klondike Highway, is not directly on the Tintina Trench, but owner Del Buerge says that the birds will often spend a night there in spring and fall. Last fall he counted about 2,500 cranes on one of his fields.

One spring, bad weather slowed down the migration, and several hundred cranes spent three weeks at his farm -- after the fields had already been planted. Buerge says he continues to have a soft spot in his heart for the birds, even though he had to replant his field that year.

Sandhill Cranes can be spectacular birds to watch, often spiraling high into the skies on the air currents that they seek out. Because of their distinctive bugling calls, the cranes can often be heard even before they are spotted.

With their heavy bodies and long necks, Sandhill Cranes weigh about 3.5 kilograms and stand about 1.25 metres high. The discovery of a six million-year-old crane fossil in Nebraska proves that they are also one of the oldest species of birds still living.

But despite the appeal of these birds, and the huge numbers that migrate through the territory, the crane migration is not nearly as well known as the Yukon's premier waterfowl event -- the spring arrival of Tundra and Trumpeter Swans in the southern Yukon.

For one, the crane migration does not occur within easy range of Whitehorse. Faro is often recommended as the best place to observe the cranes as the Tintina Trench is relatively narrow and hemmed in by mountains there, so the birds must funnel through a more confined area.

Jim Hawkings, a Canadian Wildlife Service (CWS) biologist who has studied swans extensively, notes another difficulty with observing the crane migration; "You can go out to Swan Haven on Marsh Lake almost any time at the end of April and see swans, but the cranes are just passing through. It is easy to miss them."

But Hawkings and other bird-watchers agree that it is worth making an effort to watch these birds as they pass overhead. The spring migration tends to be more predictable than the fall one; in four different years the cranes were observed migrating past Little Salmon Lake in south-central Yukon between April 27 and April 29.

In fall the birds are more spread out, but they usually pass through the territory around the third week in September. After winging their way over the Yukon, the cranes continue on to their wintering grounds in western Texas, eastern New Mexico, and northern Mexico.

Anyone wanting guaranteed sightings of huge numbers of Sandhill Cranes can visit the Platte River in Nebraska in the spring. Most of the approximately 600,000 cranes that use this staging area every year are Lesser Sandhills, but other migratory cranes also descend upon the lowlands there, with some of them spending as much as six weeks in the area, mainly feeding on waste corn in the fields bordering the river.

Most migratory populations of Sandhill Cranes are considered to be healthy, though habitat loss along the Platte River poses one of the main threats to the species. Here in the Yukon, some biologists are interested in protecting habitat of a different sort.

Efforts to save the endangered Whooping Crane have brought forward the concept of recognizing a particular migration flyway as "habitat." While these birds are migrating, air traffic along their route is controlled. Wendy Nixon, a CWS biologist, comments that "Whooping Cranes are an air traffic stopper."

Given the large number of Sandhill Cranes and other birds traveling along the Tintina Trench, some biologists think that recognition of this aerial "habitat" is a concept worth considering.

In contrast to Whooping Cranes, which are closely monitored along the length of their migratory route, the huge flocks of Sandhill Cranes will soon fly over the territory with little fanfare. Yukoners interested in watching this spectacle should consider checking the web page of the Northern Prairie Wildlife Research Center.

This U.S. group put radio-collars on Sandhill Cranes in the spring, and regularly updates their locations as part of its Operation Crane Watch. People interested in the Sandhill Crane migration in the Yukon can also contact the Canadian Wildlife Service at 667-3929.