archmcc at qwest.net Sun Nov 21 12:25:11 PST 2004 Hello Tweeters
and Obol, It has been interesting seeing all the reports of
mountain chickadees and other species outside their breeding
range. It looks like a major irruption is on, for a number of
species. Someone on Tweeters asked what the cause of this might
be. The usual answer is food. Irruptive species tend to have a
cyclical food supply. The predators build up a large population
while the food supply is abundant, then when the food supply
crashes, the predators emigrate/erupt from their breeding areas.
That makes for an "irruption" into the receiving areas. Voles
and conifer cone crops are notoriously cyclical and their avian
predators, e.g, boreal (small b) owls and finches, montane
corvids, parids, and sittids, are some of our best known
irruptives. Interestingly, this year both boreal and montane
irruptions seem to be under way. I studied this phenomenon with
Mountain Chickadees in New Mexico in the 1980s. If you'd like
more information, read on. The