There is a free internet site hosted by the University of North Carolina called "RAP- Real Time Weather " http://www.rap.ucar.edu/weather/. It is enlightening and entertaining to view bird (and other) migration in a different way. Open the site (or click on the link below) and click on "Radar" on the toolbar. You will see a map of the US with the codes for all the NEXRAD stations displayed. Under "Programs" click on "Regional Reflectivity". In the far right hand column choose 0400 on the "End Time" pull-down menu. The times are displayed in UTM hours also known as Greenwich Mean Time or "Zulu". In the CDT zone the day begins at 5:00 p.m. (0000 hrs. UTM), so 0400 is 9:00 pm. Set the "Loop Duration" menu to "5 hour loop". Now click on the region you want to look at. For southern Texas click near the bottom of the Texas map or the bottom most part of the map will be outside the screen area. After a minute or so the images are loaded and you get a 5 hour loop of NEXRAD coverage roughly centered on sunset from a composite of all the stations in the region. For Texans and other batophiles it is neat to watch the emergences of Mexican Freetail Bats from the huge colonies in the limestone caves of the Hill Country. The first one to appear today is the Devil's Sinkhole colony NE of Del Rio, followed by the several immense colonies in the San Antonio area (Bracken Cave near New Braunfels and the Frio Cave in northern Uvalde County both in the double digit millions of bats plus several smaller colonies.

It is also interesting to watch the exodus of migrant birds that had spent yesterday on the ground. The composite view filters the image much like Precipitation Mode does so most of the non-biological reflectivity from dust, pollen, etc., is filtered out. Of course if precipitation is being observed it will show as very strong reflectance. Last night there was none on the Texas regional view so what you see blossoming forth just after sunset (indicated by an intense, but brief spike that points at the radar site from the NW) is birds lifting off for the nocturnal flight and the emergence of bats and insects. At this season birds predominate except near the above mentioned bat colonies in central Texas.

http://www.rap.ucar.edu/weather/radar/displayRad.php?icao=KBRO&prod=n0r&bkgr=black&endDate=20080422&endTime=4&duration=5