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Bird Migration and Radar

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Published on Mar 16, 2012

The network of 142 WSR-88D stations in the continental US is a wonderful tool for studying bird migration as it occurs. This animation is a mosaic of the network as operating on 7-8 May 2009. The blocky, uneven, and brightly colored patterns apparent at the start of the animation in the New Jersey and southern New England, the eastern Great Lakes, scattered across the Gulf Coast states, and scattered across the Pacific Northwest and northern Rockies represent precipitation. However, in coastal Texas, the pattern is more uniform in color and stippled in pattern. This is characteristic of biological targets, and in this case, many of the targets that Texas radars are detecting are probably trans-Gulf migrant birds with some insects and other aerial plankton. As the afternoon advances, the trans-Gulf arrival continues making it easy to compare these patterns with precipitation.

After the sun sets, beginning just after 8 PM EDT, a new pattern emerges. The uniformity and evenness of the pattern, in addition to its timing approximately 30-45 minutes after local sunset is indicative of nocturnal migrants taking to the sky . As local sunset occurs in each time zone, a new blossom of migrants enters the radars' scanning zones. Based on the reflectivity scale at the bottom of the animation, many of radars show reflectivities of 10-20 dBZ, approximating 70-250 birds km-3. Isolated radar stations also show values in the green colors up to 25-30 dBZ, representing substantially higher migration densities between 600-1800 birds km-3. Note migration is apparent in several portions of the West. Also note the odd spiky qualities of some stations in the southwest, characteristic of radars with mountains or obstructions blocking their scans. By local midnight in each time zone, the doughnut shaped reflectivity patterns begin contracting, likely representing the beginnings of birds decreasing in altitude, dropping out of the radars' beams and gradually ending their night's movement. Although precipitation apparent over some parts of the country has dissipated, intense storms are apparent in the central Plains and Ohio River valley. Close observation shows migration interacting with these storms, presumably producing ideal birding conditions for the following morning. Note also the patterns occurring in southern Florida and the Florida keys, where birds that have left Cuba begin arriving in the early morning hours. Toward the end of the loop, as daylight proceeds across the US, the nocturnal pattern we observed is gone, with the most evident features of the animation being the intense storms gradually working east across the Mississippi River valley. Note, however, that the same diurnal pattern we observed over Texas and portions of the Gulf Coast at the beginning of the animation is already apparent in the daylight hours -- with trans-Gulf and even diurnal circum-Gulf migrants (and aerial plankton presumably) moving north and making landfall.

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