>From BIRDCHAT:

>Does anyone have an explanation? Is it because three years of good rains
>have produced bumper crops of birds in the mountains and we're seeing the
>spillover? Or is it perhaps, as one person suggested, the mountains may be
>in for a bad winter and the birds know it and are clearing out while the
>going is good? Or is because of a drought in, say, Idaho, driving the birds
>to the coast to find food?
>
>Does this phenomenon qualify as "irruption"? The Birders Handbook says
>"irruption" is caused by a dimunition in food sources after several abundant
>years and that boreal species are especially susceptible to periodic
>irruption. 1996 seems to have been a very good year for seeds and cones in
> Sierra Nevada's. Abundant snowpacks the last couple of years can only mean
>abundant food for montane species.

"Irruptions" can also be caused, I think, when the prey items of an animal
become locally superabundant. When there is a large crop of pinyon pine
seeds, for example, jays and nutcrackers invade such areas in large
numbers. When spruce budworms are abundant, evening grosbeaks and other
species may swarm into the area to take advantage of this temporary
resource. Crossbills are of course another example. These aren't
irruptions by the definition of the Birders Handbook, but I've heard them
called that.

It seems unlikely that either of these scenarios explains what's happening
in California, as the list of species is so varied. I don't think
Red-breasted Nuthatches and Clark's Nutcrackers share a common resource
base.

Dennis Paulson, Director phone 206-756-3798
Slater Museum of Natural History fax 206-756-3352
University of Puget Sound e-mail dpaulson at ups.edu
Tacoma, WA 98416
web site: http://www.ups.edu/biology/museum/museum.html