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September 2010

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From:
Doug Overacker <[log in to unmask]>
Reply To:
Doug Overacker <[log in to unmask]>
Date:
Sun, 5 Sep 2010 21:59:19 -0400
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Brian Menker sent me the following link about radar images and bird roosts.
I agree with him that I don't think all of these rings could be due to
Purple Martins. I have never heard anyone reports such large roosts of
Purple Martins in Ohio. The only large roost in the area that I have heard
of is one at Presque Isle, Pennsylvania. Brian's comments are below.

Doug Overacker
Springfield, Ohio

http://www.erh.noaa.gov/iln/research/birds/

They make some interesting observations about radar "rings" that appeared on
several mornings in early August, that they claim are due to roosting birds.
Specifically, they speculate that they are due to huge roosts of Purple
Martins that return to the same places at night and create radar rings in
the
morning as they leave to feed for the day. 

I find this conclusion to be highly doubtful.  The number, size, and scale
of
the "roost rings" on their example loops from early August would indicate
many massive roosts of martins that we just don't see.  If there were that
many roosts of Purple Martins numbering in the tens of thousands, birders
would be reporting them.  I don't have my Peterjohn copy handy, but I don't
remember ever hearing of those types of roosts here in Ohio.

I think these rings are artifacts of specific atmospheric conditions and
ground scatter.  They even say that the rings show up in the morning and not
the evening because the atmospheric conditions are more favorable for the
radar beam to hit lower objects.

I wanted to post something on this to Ohio Birds to let everybody weigh in,
but I'm not set up to post anymore.  I know there are discussions on radar
detection of birds from time to time, and I think feedback from Ohio's
birders could help the Weather Service see that their speculations are a bit
wild. Could you at least post the link and throw it out there for comments?

If any bird roosts in Ohio would create a radar ring, I would think that the
crow roost here in Springfield would show up on winter mornings.  There are
tens of thousands of them, and they are much bigger reflectors than martins.
Maybe I'll keep my eye on the radar when they show back up for the winter.

Brian

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