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Weather JournalSoaring Migrations On this glorious autumn morning, I have ventured away from my office to personally view some of southern Vancouver Island's most uplifting weather. My destination is the shoreline of the Strait of Juan de Fuca at Beechey Head in East Sooke Park. During this time of year, this is a popular gathering spot...for turkey vultures.
From mid-September to early October, as many as 2000 turkey vultures leave the Island and head to their wintering grounds in the United States. The region around Beechey Head is ideal for the birds to cross the 18-20 km (11 to 12.5 mile) Strait of Juan de Fuca as it is a region where strong ascending air currents are common, optimum conditions for the soaring vultures.
There are several ways in which air can rapidly ascend from the surface to great heights. One is along frontal zones, but fronts are ever-moving across a region and are often associated with poor weather conditions for avian flying. Strong solar heating of the land is by far the most prevalent cause of rapidly rising air currents, know as thermals, from the surface. Each cumulus cloud you see during fair weather reveals the upper portion of one of these thermals. A third cause is the influence of topographical features — mountains, hills, ridges — that force horizontally moving winds to become vertical as they flow over the obstacle. Combine sloped surfaces heated by the sun with a properly directed surface wind, and you have the perfect conditions for a soaring bird to exploit. The Beechey Head area is almost ideal for the development of rising air currents because it sticks out perpendicular to the axis of the Strait. Since winds blow predominantly parallel to this axis over the waters, the Head intercepts portions of the flow and forces it upward.
The weeks just after the equinox on southern Vancouver Island are often characterized by clear skies and thus strong daytime sun. This weather provides excellent conditions for thermal heating of the coastal hills. Combine these conditions — strong surface heating and topographical uplift — to produce strong thermals, add the shortest hop across the Strait's waters, and you have an ideal crossing point for migrating birds, particularly the soaring turkey vultures. On this visit to the Beechey Head, I saw one, perhaps two, of the great birds soaring several hundred metres above the tree tops. Whether it was preparing to cross the Strait or only in search of a morning meal, I do not know for I lost sight of the bird moments later. Since the hour was early, perhaps it was waiting for the stronger afternoon thermals before attempting a crossing. When I first spotted this turkey vulture, it had already reached great heights. I did not see its spiral ascent within an updraft. But a few years ago, I had the unique opportunity to watch a bald eagle corkscrew its way upward from a middle point in its ascent. I was standing near the peak of Mt Maxwell, the 589m (1932 ft) summit of Saltspring Island (36 km / 22 miles north of Victoria). As I looked southward toward Fulford Harbour, my eye caught an eagle flying below. As it approached the mountain, the eagle located a rising current and began a spiral ascent. Circling within the current's stream, it rose to my eye level in a manner of minutes. Passing my elevation, the bird continued ever upward until it was but a speck in the sky above. Finally satisfied with its altitude, the eagle headed northward, its white head and tail feathers catching the sunlight as it sailed out of view.
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The turkey vulture (Cathartes aura) is a large, rather homely member of the raptor family and is purported to have the most highly developed sense of smell of any raptor. These birds are masters of the art of thermal soaring, cruising high in the air with their nearly two-metre (~6 foot) wingspan held in a shallow V. When conditions are right, the wings remain nearly motionless as the great birds catch rising air currents and ascend to great heights.
