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Re: Comparing Bird Numbers





Since traffic has been light lately I'll bite on this one too...

Good suggestion from Marilyn.  I figured I would also go ahead and throw my
opinion out as well.  Christmas Bird Counts serve several very important
functions including providing data for LONG TERM trends, making the non-birding
public more aware of the hobby, and increasing the knowledge of participants (by
pairing novice birders with more experienced ones).  However, as indicated by
the bold type above, I do not think that the CBC results should be examined with
the intent of determining short term trends.  The data is not, and probably can
not, be collected under conditions controlled enough to produce these types of
results.

Some of the pre-Christmas counts provide a good example.  The poor weather
during these counts resulted in lower than base-line numbers of many passerines.
Were the birds not there?  No, they were just not seen.  Not seen does not equal
not present.

The weather factor is just one of the externalities that make CBC data useful
for some types of conclusions but not others.  To create a micro-example let's
say my count circle was the NCSU dairy pond and there are only 2 species of
birds, Canada Goose and N. Shoveler.  My count this year would show huge numbers
of Canadas and no shovelers.  Last years count would show the opposite.  Should
I assume that the shoveler population is in great trouble and Canada's are on
the increase?  If I only used my CBC data I may come to this conclusion, but
since the original data was not collected under controlled scientific
conditions, my hypothesis is incorrect.  In actuality last year's weather was
nice, this year's was not.  The shovelers left when the pond froze, the Canada's
actually appeared to congregate.

For this, and other reasons, I believe CBC data can be used to detect and
interpret long term declines and gains, but is not terribly useful as a short
term population fluctuation indicator.  Long term numbers should tend to reduce
the influence caused by weather, experience level of the counters, etc.

Opinions & criticisms welcome!!

Best,
Steve Shultz
Cary, NC
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