Starlings have been know for decades to make "ring angles" in the mornings when they depart their roosts. I've watched them on FAA, Air Force, and avian radars. Tree Swallow roosts have been reported to resemble Purple Martin roost departures but I don't know of any that have actually been identified. The best strategy, of course, is to locate a roost with radar, then go out in the morning to see who is flying away.

Bob Beason
Huron, OH

On Sep 5, 2010, at 9:59 PM, Doug Overacker wrote:

> 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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