At this time of year migrating thrushes and warblers spend their days eating and resting.  Then at sunset they don’t sleep, they fly!

Most birds that flap to migrate choose to travel at night because the calmer air makes flying easier and they can see the stars by which they navigate.

From sunset until 2:00am — sometimes until sunrise — they are in the air above us flying in loose flocks kept together by contact calls.  The number of travelers peaks between 11:00pm and 1:00am on nights with a north wind.  We know this because they’re seen on radar.

Back when radar was first widely used during World War II operators noticed that many things showed up as blips on the screen including rain, snow, birds and insects.  After the war, radar came into its own as a weather forecasting tool.  Nowadays it’s easy for birders to monitor nighttime migration because weather radar is available on the Internet.

To demonstrate how birds show up on radar, Cornell University created a time-lapse video showing migration over the U.S. on the night of October 1, 2008.  Read the explanation below, then watch the video above:

“This animation created by Cornell University researchers illustrates the use of a network of surveillance weather radar to record nocturnal migrating birds, bats, and insects in the continental U.S. from sunset to sunrise Oct. 1, 2008. The blocky green, yellow, and red patterns, especially visible on the east coast, represent precipitation; but within an hour after sunset, radar picks up biological activity, as seen in the widening blue and green circles spreading from the east across the country. The birds, bats, and insects take off, fly past, and get sampled by the radar beam. Note, the black areas on the map do not represent places without birds, necessarily, but rather places where radar does not sample.”  — from futurity.org

You can watch migration, too.  Tonight Pittsburgh’s wind will be from the north so you’ll see birds on the move if you tune in to the National Weather Service radar loops after sunset.  Pittsburgh appears on two maps:  Central Great Lakes and Northeast.  Click on the links and watch bird activity appear after sunset and subside at sunrise. Remember that the best time is 11:00pm to 1:00am.

For more in-depth observations and hard core science, this 10 minute tutorial by David La Puma explains how to use Nexrad images to monitor migration. La Puma used to post daily radar migration updates for New Jersey on his blog at woodcreeper.com but has taken a break from it this fall.

(video from October 1, 2008 by Cornell University via YouTube)

p.s.  Click here for Drew Weber’s analysis of last night’s activity posted on Nemesis Bird.