The thread on the occurrence of Brown Thrashers in northern VA and nearby
areas is an interesting one.
For the record, you can always quickly check the standard data sources for
recent observations (which are great for procrastinating or otherwise
killing time anyway):
CBC and BBS data maps at the Patuxent Infocenter -- e.g.,
http://www.mbr-pwrc.usgs.gov/id/framlst/i7050id.html (maps are interpolated
snapshots of the raw data)
The raw count data from the CBC --
http://www.audubon.org/bird/cbc/hr/index.html (the Find a Map method being
the most entertaining, IMO)
and of course eBird.
The end result of all that appears to me to be that finding a Brown
Thrasher in Fredericksburg in early Dec is a bit unusual, and cool, but not
shocking.
Added to that is Jay's good point that our ability to detect some
overwintering residents drops off dramatically because they become less
"detectable". This all relates to the common problem with bird counts of
all types -- detectability -- which the bean-counting scientists have been
struggling with for 25+ years. A terrific summary from an article in The
Auk from 2002 is below. Bird studies often find that the detection
probability for a species in a point-count survey is much less than 50%. In
other words, there's a LOT out there that you won't see. It's more reason
to be very careful in our counting efforts (and then, in an ironic twist,
not to take the results too seriously).
Ted Simons and his crew at NC State are doing some neat work simulating
point counts with "All Bird Radio" -- very interesting stuff, but humbling.
The test subject counters don't often like the
results! http://www4.ncsu.edu/~simons/Bird%20Radio.htm
The Auk 119(1):1825, 2002
OVERVIEW: TOWARDS RELIABLE BIRD SURVEYS: ACCOUNTING FOR INDIVIDUALS PRESENT
BUT NOT DETECTED
WILLIAM L. THOMPSON
U.S. Geological Survey Arkansas Cooperative Fish and Wildlife Research
Unit, Department of Biological Sciences,
University of Arkansas, Fayetteville, Arkansas 72701, USA
"Using just the count of birds detected (per unit effort) as an index of
abundance is neither scientifically sound nor reliable. It is necessary to
adjust study counts by the detection probability." (Burnham 1981:325)
COUNTS OF BIRDS seen, heard, or captured are commonly used to elucidate
avian-habitat relationships, investigate responses of avian populations to
management treatments or to environmental disturbances, estimate spatial
distribution of species, and monitor population trends. {SNIP} Point counts
and other methods that are based on observed counts to estimate abundance,
rely on the assumption that numbers of individuals detected (e.g. seen,
heard, or captured) represent a constant proportion of actual numbers
present across space and time. That is, if the true number of birds within
a surveyed area increases by 20% during successive samples, observed counts
are assumed to increase by the same percentage. Similarly, counts in
different areas during the same time period are assumed to represent the
same proportion of birds present within each of those areas. The validity
of this proportionality assumption has been questioned for decades (e.g.
Burnham 1981) because the many factors affecting detection probabilities of
individuals (i.e. probability of correctly identifying the presence of an
individual) are neither constant within and among species and habitats nor
constant across time. ...
Dave Hewitt
Gloucester, VA
