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Migrating Birds : Exodus

   Birds migrating at night frequently depart 30 to 45 minutes after local sunset, and this departure is visible on radar. As birds leave their diurnal stopover sites and climb to typical altitudes of migration, they enter the radar beam and begin reflecting energy back to the antenna. Such departures appear as rapidly expanding circular (or nearly circular) patterns in a base reflectivity image as more birds climb into the radar beam.
   By examining radar imagery in this window of departure times, CUROL can identify specific areas from which large numbers of birds are departing. These areas, often called "hotspots," are the diurnal stopover sites in which migrant birds have spent the day feeding and resting in suitable habitat. Because all habitats are not suitable for migrant birds and because many landscapes are increasingly fragmented and unequally distributed across the landscape, hotspot phenomena appear even more striking. These areas consistently appear on radar images as areas of high bird activity. Delineating areas of high bird activity is a priority for bird conservation, and studying hotspots on radar is an essential tool for this endeavor on a large scale.
   The following reflectivity image from the evening of March 31, 1999 at HGX Houston, TX illustrates the premise of "hotspot" phenomena. Notice the areas of higher reflectivity (the reds and purples analagous to densities of 60-500 birds per cubic kilometer) that appear prominently (against the yellow and orange background) around the radar stations. See that the time, given in UTC at 0122Z (6 hours ahead of local CST), is approximately 30-45 minutes after local sunset. This image is a characteristic "hotspot" frame, in which we can see specific areas from which large numbers of birds are departing.

 
dBZBirds
/km3
ND
-28
-24
-20
-16
-12
-8
-4
0
458
865
1281
16123
20227
24489
281148
pict of hotspot frame from houston
Base Ref 124nm
Elev=0.5 deg
0.5 km²/pixel
HGX: Houston TX
29.47N 95.08W
04/01/99 01:22 UTC
Clear Mode
VCP 32
Max: 35 dBZ

   The refelctivity image below shows a striking amount of departure detail east of the station.

 
dBZBirds
/km3
ND
-28
-24
-20
-16
-12
-8
-4
0
458
865
1281
16123
20227
24489
281148
pict of hotspot frame from lake charles
Base Ref 124nm
Elev=0.5 deg
0.5 km²/pixel
LCH: Lake Charles LA
30.13N 93.22W
05/17/99 01:41 UTC
Clear Mode
VCP 32
Max: 49 dBZ

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