Subject: Cascades' species on Calif. coast
Date: Nov 16 16:32:56 1996
From: Darlene Sybert - c557506 at showme.missouri.edu
This question appeared on BIRDCHAT and I wondered if the
same thing is happening on the Wash-OR coasts.
Darlene Sybert
---------- Forwarded message ----------
Why are mountain birds, presumably from the Sierras or Cascades,
suddenly wending their way to the coast in such numbers?
Here in San Francisco Bay area and the neighboring coast we are witnessing a
heavy influx of downslope or montane species in last month or two.
Red Breasted Nuthatches, normally uncommon, are now ubiquitous (I have been
seeing and hearing R-B Nuthatches in the oddest places for the last six
weeks). As well, there are several reports of Clarks Nutcracker (a rarity on
the coast); Red Crossbill; Townsend's Warbler; and Varied Thrush in much
larger numbers than previous years.
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.