[SBB] Thrushes

[Bill Bousman <[[email protected]]>]

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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