[SBB] Poorwill Status

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

Folks:

I'm not sure that David Forthoffer intended to start the current thread on 
Common Poorwills, but it has certainly been interesting.  I think that 
there are really two issues that relate to this topic.  First, do Common 
Poorwills overwinter in Santa Clara County and, second, what are the 
physiological aspects of their overwintering.

The Birds of North America account believes that more northerly birds, 
which extend as far as the interior of southern British Columbia, are 
migratory, while more southerly birds are resident, but the exact status is 
poorly understood.  The Coyote Creek Field Station, which is located in an 
area where there are no breeding records, had captured at least eight fall 
birds, with dates extending from 4 to 24 October.  This suggests that there 
is at least a weak fall movement through the Bay area of migrating 
birds.  No equivalent evidence has been obtained for a spring 
migration.  Thus, it would seem, that if the birds are present in the 
winter, we would see them from perhaps November to February.  However, the 
county notebooks indicate that there have been only seven observations of 
birds in the winter period in the past 25 years, which is a small number, 
even for a cryptic species such as this.  There seems to be general 
agreement that birds are resident in San Luis Obispo and Monterey counties 
and David Suddjian has said that he believes that there is a "resident 
component" in Santa Cruz County.

My own speculation is that there is likely a transition region from purely 
resident birds to purely migratory birds that is fairly wide and diffuse 
(as it is for so many of our visible passerines).  Perhaps all of the birds 
in Monterey or San Luis Obispo counties are resident, while all of the 
birds in Oregon or Washington are migratory.  In between, there may well be 
a mix of birds, with some migrating and some staying over.  Moreover, this 
mixture may very well vary from year to year as these inbetween populations 
have to guess as to whether there will be sufficient food resources for 
them to stay over.

Concerning sufficient food resources, some very interesting experiments 
were performed a number of years ago on a three different species that vary 
their metabolic processes to overcome periods of reduced food (Bartholomew, 
G. A., T. R. Howell, and T. J. Cade.  1957.  Torpidity in the 
White-throated Swift, Anna Hummingbird, and Poor-Will.  Condor 59:145-155) 
and this paper is accessible through SORA on the web.  I've not read the 
paper recently, but my recollection is that the authors concluded that the 
Common Poorwill really has two potential strategies.  First, it may use its 
fall fat reserves to support migration or, second, it may use these 
reserves to support torpor/hibernation (I don't think we should be too 
concerned with semantic definitions of these metabolic approaches to 
handling periods of cold and poor food resources).  Based on laboratory 
measurements, they compute equivalent metabolic costs for migration vs. 
torpor/hibernation.

What is really most interesting to me is (1) the variable strategies that 
are available to some species, and (2) how little we know about their choices.


Bill Bousman
Santa Clara County records compiler 

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