Re: [SBB] Thrushes

["Pat Bitton" <[[email protected]]>]

It seems to be a bumper year for American Robins and Varied Thrushes up here
on the Humboldt County coast, too - apparently driven by colder than normal
weather inland.

Pat Bitton
Eureka

-----Original Message-----
From: [[email protected]]
[mailto:[[email protected]]] On Behalf Of Bill Bousman
Sent: Saturday, January 27, 2007 5:48 PM
To: [[email protected]]
Subject: [SBB] Thrushes

Folks:

A number of folks have been asking about Am. Robins and Varied Thrushes
recently.  For what it is worth, here is my take on the conventional wisdom.

Robins are variable in winter.  This can be observed by just looking at
local CBC data for the last 50 years.  Year-to-year variation in enormous
(see Fig. 16, http://www.scvas.org/pdf/primer_3.pdf.)  A century ago, robins
were only seen in the winter.  Local breeding started in the early twentieth
century with urbanization and now birds are found nesting throughout the
county.  As a long ago transplant from the east, I can understand why people
who have noticed the large flocks of robins in the last few weeks might
think spring is coming, but I think the truth is probably just the opposite.
These are not spring arrivals, but winter birds chasing food resources or
escaping the cold of the last month (or both).  If you are a careful
observer, you will note that some of the birds in these large flocks are
singing and it is likely that this is hormone-driven, but most of these
birds are from northern lands or mountains and their spring will not be here
but elsewhere.  Our resident birds start to sing in the next month.  In
March and April, they will be nesting, but you can still encounter large
flocks of migrating birds in some years.  To the best of my knowledge, there
are no "spring arrival" robins in central California.

Varied Thrush are variable also, but they tend to follow a two-year cycle
with an occasional slip of a year.  Even within that two-year cycle, there
are highs and lows, just like our twice a day tides.  This is certainly a
great Varied Thrush year, although to keep us humble, they can apparently be
absent from some likely areas, but abundant in others.  Although we don't
think of them as urban birds, in a year like this they are very widespread
in urban areas and on the valley floor.  At my place in Menlo Park, I'm not
absolutely sure I've ever seen a Varied Thrush in the last 35 years, but in
good winters, such as this one, I hear them.  No surprise there.  We can
call them no-see-ums (special joke for transplanted easterners).

Bill Bousman
Santa Clara County records compiler 



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