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(C) 1999 weatherTAP.com | ||||||||||||||||||||||||||||||
| Base Reflectivity 124nm Elev=0.5 deg 1 km²/pixel FFC: Atlanta GA 33.36N 84.57W 07/25/99 10:37-11:16 UTC Clear Mode VCP 32 Max: 59 dBZ |
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Roost Rings
When colonial nesters such as Purple Martins (Progne subis) finish breeding, they stage for
their southbound migration in dramatic roost sites sometimes numbering in the tens of thousands of birds.
During this staging period, beginning as early as late June or early July, birds disperse in the early morning
to feed. As they disperse from the roost site, a striking 'roost ring'
can be seen as the birds leave the roost and climb into the radar beam. This roost ring expands until the
birds begin foraging after dispersing to much lower densities some distance from the roost.
In the following animation from the early morning of 25 July, 1999 at FFC Atlanta, GA, Purple
Martins are leaving roost sites all around the radar. At least nine roosts (rings or partial rings) are visible
in this series of images (several roosts are actually in Alabama) - 3 to the south, 3 to the north, and 3 to the
east. Notice the pattern of expansion over the course of approximately 40 minutes as the martins leave
the roost sites to forage.
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