Thanks to clear skies with calm or light southerly winds, there is a massive northerly movement of birds tonight in the Adirondacks, and indeed throughout much of the northeastern quarter of the country. The image at left shows the Northeast U.S. Radar Mosaic Thursday evening, April 1. Below left is the the radar loop on Thursday evening, April 1 out of Ft. Drum, NY. The “starburst” of blue and green colors surrounding the radar site is typical of birds, as opposed to weather.
To confirm that these are birds, one can click on the radar image and view a velocity image as shown on the right below. Birds are moving from the S and SW (represented by the blues and greens – objects approaching the radar) to the N and NE (represented by yellows, oranges, and reds – objects moving away from the radar). This is the expected direction of migrants on a night with calm or southerly winds in the spring. (Also, these color patterns on the radar images only appear after dark, when most birds are migrating.) We can expect to see similar radar images throughout the next two months when weather conditions are favorable (clear skies and southerly or calm winds, especially after a cool snap).
Therefore, we can conclude that the colors appearing on the radar are mostly if not entirely birds. Step outside in an otherwise quiet area on a night such as tonight and, if your ears are attuned to high frequency bird calls, you are likely to hear birds passing overhead.
The Cornell Laboratory of Ornithology has pioneered the study and identification of the flight calls of nocturnal migrants. Last summer they started an email list with the primary purpose to provide a forum for experienced birders to discuss the identification of night flight calls of migratory birds, exchange ideas about recording equipment, disseminate information about active and predicted flights, and to understand weather and radar data as they relate to patterns of bird migration.