If you check doppler radar every morning around sunrise you might see a couple of bursts of radar reflectivity across the Mid Atlantic, typically across South Jersey and also across the Chesapeake Bay region in Maryland.
These "bursts" are flocks of birds that are taking off from nesting sites for the day.
Yep, radar doesn't pick up precipitation alone. It can spot and "see" other things.
The South Jersey burst typically comes from near Millville and just south of there, along the Maurice River and across the Egg Island Fish and Wildlife Management Area along the shores of Delaware Bay. These areas are low lying, marshy and flat, and contain nesting sites for large numbers of waterfowl and other birds that tend to take off each morning around sunrise.
Doppler radar tends to pick up the birds' activity a bit after sunrise, mostly due to the location of the radar site about 50 miles northeast of the refuge. Radar picks up the birds as they reach a height that the radar beam can "see" them. Since the birds are in a large enough quantity and in close enough proximity, the beam of the radar detects the birds' as they are in flight every morning.
After the initial burst shows up on radar, the burst fades as the birds disperse off in varying directions. Their movement varies morning -- some mornings on radar will show a doughnut shape, others shows the birds going more strongly in one direction than another. Today shows them tracking south and southeast.
Because radar beams travel out in a straight line over the curved surface of the earth, our local radar is slower to pick up on the flock of birds taking off over the Cheaspeake -- this is about 15 minutes after the initial burst shows up in South Jersey...you can just see the radar pick up on the flock of birds in Maryland. Because this nesting area is farther away from Fort Dix than the one in South Jersey, the birds have to reach a greater height in the atmosphere to be "noticed" on radar, which results in a bit of a delay in radar "seeing" the birds.
Birds are not the only items that show up on radar -- bats have often been seen on radar in various parts of the country at night. Jesse Ferrell has a great write-up on one "batty" night in Central Texas a few years ago.
Bugs can also get noticed on radar as well, especially when doppler radar is in clear air mode or when the radar detects the seabreeze front across South Jersey on many an afternoon in the Spring and Summer. The radar detects "backscattering" from birds and insects that are nudged inland thanks to the breezes off of the ocean. There's a slightly greater concentration of birds and bugs along the front...and sometimes a greater concentration of clouds (it can be clearer southeast of the seabreeze front, with a band of cumulus clouds along the front) will also be detected by radar.
Radar just isn't for thunderstorms anymore and it can show some interesting features from time to time...features that can give meteorologists and others in the weather community a reason to keep the radar on even on a slow weather day!