Posted on Birdchat by Ron Pittaway and Jean Iron. I found this interesting:

Subject: [BIRDCHAT] Snowy Owl Flight Year - Cause(s)

Snowy Owls are being seen south of the Arctic in high numbers this fall. Most of the early arriving owls have been first year males hatched last summer. Reports last summer indicated that lemming numbers were high across the Eastern Canadian Arctic from Churchill, Manitoba, to Bylot Island, Nunavut.

We previously reported that the cause of the Snowy Owl flight was a lemming crash in the Eastern Arctic. However, we've had recent correspondence indicating the cause of the flight was a very good breeding season, which produced high numbers of young Snowy Owls. Gilles Gauthier and his PhD student Jean-Francois Therrien of Laval University in Quebec City report that based on "the high abundance of lemmings we observed on Bylot Island and at all sites we visited on Baffin Island last summer, we predicted that the abundance of Snowy Owls should be very high this winter (in the south). Indeed, some analyses made by Jean-Francois using the Christmas Bird Count data showed a good correlation between the abundance of lemmings on Bylot Island and the number of owls observed the following winter in Quebec and Ontario for the period 1993 - 2007. So far, our prediction is nicely upheld."

So was there also a lemming crash that is contributing to the Snowy flight?

Bruce Di Labio did environmental surveys on southern Baffin Island in August and in central/southern Baffin in September and October. He reports that very few lemmings were caught in live traps. This might be an indication of a lemming decline in September and October when most researchers were not in the Arctic. Lemmings normally crash in fall and winter after a period of high abundance and cycles are usually synchronous across the Eastern Arctic.

The Snowy Owl flight this fall and winter could be caused solely by high numbers of young being fledged this summer due to high lemming populations. Or is the flight the result of a good breeding year and a subsequent decline in lemming numbers this fall? If a large number of adult Snowy Owls come south this winter then we'll be more confident in saying that a lemming crash has occurred. We'll post updates as we get new information.

Baffin Island lies west of Greenland and is the largest island in the Canadian Arctic. Bylot Island (lat 73 deg, long 78 deg) is about 3000 km (1865 mi) north of Toronto. Bylot is much smaller than Baffin. It is at the northeastern tip of Baffin on Lancaster Sound "Northwest Passage"..

Acknowledgements: We thank Ken Abraham, Bruce Di Labio, Bruce Falls, Gilles Gauthier, Jean-Francois Therrien, and Michel Gosselin for information and discussions about lemmings and Snowy Owls.

Ron Pittaway and Jean Iron
Toronto and Minden, Ontario