According to Martin and Shapiro (2007), and Wilson et al. (1994), "insects [emphasis mine]are the most common cause of clear air echoes."

    Martin and Shapiro state that birds can affect doppler velocity estimates, while the presence of insects usually does NOT cause bias in the estimates.  Research is ongoing.


The following are examples of "roost rings" caused by birds.  There may be some insect signatures as well.

  Roost Rings--NWS Greenbay

 Roost Rings, Yuma AZ

 Roost Ring near Lake Okeechobee

 Roost Rings near Melbourne FL

 Roost Rings with approaching squall line

On February 6, 2013, a roost ring on the NWS Melbourne radar was observed lifting off from a point near Lake Poinsett, west of Merrit Island, Florida,  near I-95 and State Road 520, and flying north towards Daytona.  Visual observations from the roof of  the College of Aviation Building viewing south with binoculars revealed the presence of flocks of birds, probably numbering in the hundreds; identification was not possible. Subsequent days showed repeated examples in this area, so Applied Meteorology Program Coordinator, Dr. Fred Mosher, made an in-person visit to the apparent origin of the rings, near Lake Poinsett.  He captured several photographs of thousands of tree swallows flying west at sunset to roost in the area northeast of the lake.

Photos by Fred Mosher.



Image of a tree swallow up close (from Wikipedia).

References:

Martin, W. J., and A. Shapiro, 2007: Discrimination of Bird and Insect Radar Echoes in Clear Air Using
High-Resolution Radars. J. Atmos. Oceanic Technol., 24, 1215-1230.

Wilson, J. W., T. M. Weckwerth, J. Vivekanandan, R. M.Wakimoto, and R. W. Russell, 1994: Boundary layer clear-air
radar echoes: Origin of echoes and accuracy of derived winds.
J. Atmos. Oceanic Technol., 11, 1184–1206.