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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.

 
dBZBirds
/km3
Precip
(mm/hr)
ND
-28
-24
-20
-16
-12
-8
-4
0
458
865
1281
16123
202270
244891
2811482
NEXRAD bird roost rings
(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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