First of I will start by saying I have talked to several owl biologists over the years. Most of them agree that we do not know exactly what causes irruptions in pwls or most other birds for that matter. Finches are known to be correlated with seed supplies but for most predatory birds like owls or Gyrfalcons the situation is far more complicated. Now as some of you know I studied Great Gray Owls (hence my email address) in Idaho for a few years back during my academic days. My own research on Great Grays may be of some interest with respect to the current conversation on Snowys in WA. Great Gray Owls generally stay on the breeding territories year round. Young birds generally do not disperse great distances. However there are some factors that produce more movements off of breeding grounds than others. I suspect than when we see these conditions over large areas is when we will see large movements. Adult Great Gray Owls (like Snowys) are well equipped to hunt in the snow and are among the best owls equipped to deal with cold. However the snow can get too deep even for them. When that happens they can not get enough food and even adults wander from nest territories. Keep in mind this is has to do with prey accessibility not prey availability.
The other major factor has to do with reproduction rates. I have seen a couple of physiological studies that have shown in Great Gray Owls that females that have more fat in the breeding season will produce larger clutches. I have seen clutch sizes double in springs with high gopher numbers. With good prey through the summer all the young may survive. Once winter comes even the best territories can not support 6 or more now adult birds and we see a mass exodus of yearlings. So with Great Grays we see a tremendous numbers of birds abandoning the breeding grounds in the following situation of a great spring followed by a good summer then a harsh winter. That is when nearly every bird leaves the breeding territories. When these conditions are over large areas I would expect to see large irruptions events. I would expect to have similar mechanics driving a Snowy irruption. The tricky part about this is that as others have pointed out lemmings do not cycle at the same time throughout North America. Weather patterns on the other hand do tend to occur over broad areas of the North. So if I was looking for an irruption trigger for Snowys, I think I would be looking at a combination of reproductive success and prey accessibility rather than prey abundance. By the way I checked the website for the National Climate Data Centre of Canada and Whitehorse, Yukon has had well above average temperatures and snowfall so far in December. So it is likely that prey accessibility is down this year, at least in the Yukon.



Mike Munts

Colville, WA