Bob (and Tweeters),



You may have confounded the issue by including Snowy Owl counts from east of the Cascades. Snowy Owl numbers in eastern Washington, unlike those west of the Cascades, tend not to show strong year-to-year fluctuations (although there have been higher numbers in E WA this year). I believe there is some evidence that Snowy Owls wintering east of the Cascades come from a different breeding area (northern Alaska) than those wintering west of the Cascades (western Alaska, especially the Yukon-Kuskokwim Delta). Even if the Snowy Owl irruptions are related to lemming cycles, the lemming numbers in these two breeding areas may peak in different years.



If one looks strictly at owl numbers west of the Cascades?especially those in the Georgia Strait-Puget Sound area?there is strong evidence for an approximately 4-year cycle in numbers, with a larger (about 12-year) cycle superimposed on the shorter one. However, the pattern is far from perfect, especially in recent years; two ?flights? never materialized at all (1988 and 2000), and there was a 6-year gap, instead of a 4-year gap, between this year?s flight and the last major one in 2005.



For years, I have tracked Snowy Owl numbers on CBCs for 5 counts in SW BC and NW WA which tend to get the highest numbers: Vancouver, Ladner, White Rock-Surrey, Bellingham, and Padilla Bay. If one looks at numbers on these counts, significant flights occurred in 1966, 1970, 1973, 1977, 1980, 1984, 1992, 1996, 2005, and 2011. There were also notable ?echo? flights in some years following the main flights (e.g. 1971, 1974, 1978, 1993, and 2006). Up to 1984, the peaks were very regular, with only a couple of 3-year gaps instead of 4-year gaps between flights. The regularity has been broken by the failure of flights in 1988 and 2000, and the longer gaps between flights in the last 15 years, but it still looks like a pretty regular pattern to me.



There is a higher-magnitude cycle of about 12 years; in other words, every third flight tends to be bigger than the intervening ones. This is best illustrated by the Ladner, BC CBC, which recorded 107 Snowy Owls in 1973, 40 in 1984, 62 in 1996, and will probably record about 40 this year. Once again, there is some variation in the gap between the peaks, with a 15-year gap between 1996 and 2011.



I am not sure that there has ever been a long-term study in any breeding area which clearly demonstrates that Snowy Owl numbers ON THE BREEDING GROUNDS are strongly correlated with numbers of lemmings or other prey. The correlation seems to be mainly an anecdotal one. Even if there is a strong correlation, numbers ON THE WINTERING GROUNDS should be less strongly correlated with lemming numbers, if only because Snowy Owls seen to travel farther in some flight years than in others. When all is said and done, however, there is still strong evidence for an approximately 4-year cycle in numbers of Snowy Owls wintering on the northern Pacific Coast (i.e., BC, WA, and OR), and the theory that this cycle is related to numbers of lemmings sounds more likely to me than any other theory.



Wayne C. Weber

Delta, BC

contopus at telus.net







From: tweeters-bounces at mailman1.u.washington.edu [mailto:tweeters-bounces at mailman1.u.washington.edu] On Behalf Of Bobvanden at aol.com
Sent: December-14-11 10:35 AM
To: tweeters at u.washington.edu
Subject: [Tweeters] snowy owl irruptions



Like Dennis, I have been curious about possible correlations of Snowy Owl migration irruptions with climate and prey fluctuations.



I first downloaded the Audubon CBC data for Washington and for British Columbia for the last 40 years. Comparable numbers are reported each year for these two areas, and ups and downs are strongly correlated between WA and BC. I then looked at correlations with climate indices such as the ElNino Southern Oscillation Index (ENSO), Pacific Decadal Oscillation, and an Arctic Oscillation index, and found no correlation. (For example, some seabirds and Whooping Crane populations exhibit correlations with various of these indices).



As has been mentioned, lemmings exhibit a fairly regular 4-year cycle, although the peaks in the cycle are not geographically correlated as one looks longitudinally across the arctic. One can look for a regular 4-year cycle for snowy owls by making a spectral density plot (plot of frequency of a particular time interval versus time interval) as had been done by Kerlinger et al (1985) using earlier data. This approach should show a peak at 4 years for either of Dennis?s two suggested ways, as it does not depend on whether due to a minimum or a maximum in the lemming population. Neither the BC + WA or WA snowy owl count data exhibited a peak at 4 years. I tried this for several thresholds of how many owls constituted an irruption. Thus lemming populations don?t seem determinative in generating irruptions, either directly or a lag effect.



I am therefore left with no explanation for the timing of owl irruptions.



Bob Vandenbosch