November 20, 2005
Snowy Owl invasion
I went out this morning to look for a Snowy Owl reported yesterday out at the South Jetty of the Columbia River and found two. Both are heavily barred indicating that they are probably young of the year and both seem active and healthy. Snowy Owls have also been reported at Ocean Shores and Seaview in Washington and at Bullard's Beach in Coos Co. Oregon. A bird turned up last week near Portland, where it was eventually struck and killed by a car. Taken together, the season is shaping up to be a substantial irruptive year.
We get to see Snowies here on the North Coast of Oregon every couple years, but the last big irruption was back in 1996-97. During that invasion, as many as 5 could be seen at the South Jetty from one spot and there may have been as many as 8 in the area. The first well recorded Pacific Northwest irruption was in 1897. A larger irruption occurred in the winter of 1916-17, during which many specimens were taken. CBC data document irruptions in 1947, 1955, 1966, 1973, 1984, and 1996.
Snowy Owl irruptions in the Pacific Northwest are irregular and unpredictable; the reasons for their occurrance are controversial. Shelford (1945) made a case, which has persisted in the literature, that ties periodic irruptions with crashes in lemming populations in the Arctic. However, Snowy Owls that appear in the N. Great Plains were found in generally good condition. Accidents, mainly collisions with human-made objects, were the main mortality cause there. Food habits information summarized in Parmelee (1992) also show Snowy Owls utilize a wide variety of food sources, including seabirds, in addition to lemmings.
CBC data analysis does not support periodic irruptions. Kerlinger et al. found that numbers fluctuate significantly from winter to winter and from region to region. They were found to be regular rather than irruptive winter migrants to the Great Plains, and time series analysis did not support the contention that irruptions in the east and west were regular and periodic. Kerlinger questions the lemming-owl connection and suggests that irruptive events are more probably associated with large scale weather events.
Posted by mbalame at November 20, 2005 9:00 PM