This is the time of year when many birds are moving north for the summer. The flocks show up beautifully on Doppler radar. Cliff Mass shows radar images of bird movement in the Pacific Northwest in his blog at https://cliffmass.blogspot.com/2016/03/bird-migration-on-radar-what-weather.html. Smithsonian magazine also has an article describing how the birds are detected at https://www.smithsonianmag.com/smart-news/how-doppler-radar-can-track-bird-migrations-180952834/?no-ist.
