Whirlwinds of feathered bodies, iridescent beetle-blue on top and snowy below, are touching down all along the eastern seaboard. Flocks move in a loose collection of tumbles and dives, sweeping across fields and swamps. They pepper the sky, often collecting over bodies of water to skim for insects and catch a drink. As the sun sets, the scattered birds pull together, gathering like a slow-building storm.
At the peak of migration, flocks of tree swallows can contain hundreds of thousands of birds. Doppler weather radar – yes, weather radar – has revealed that staging points are relatively evenly spaced, almost always 62 to 93 miles apart. Migration flows down the eastern seaboard in a multi-month game of hopscotch as the birds make (comparatively) leisurely stopovers one roost after another. » Continue Reading.