The hydrometeor classification is one way to check the contribution of birds to a radar image. Although the word hydrometeor refers to atmospheric objects produced by the condensation of water (rain, snow, hail etc.), one radar classification is biological and includes birds, bats and insects. At the present time (2013) the National Weather Service is upgrading its radar stations to enable them to produce hydrometeor classifications. Some stations, including Albany, New York, are already able to do so.
Note: Hydrometeor classifications are interpretations of the radar signal produced by a computer algorithm. It is possible that the classification is wrong sometimes, and I know of one such apparent error. On one cold, cloudless fall night, the Albany, New York reflectivity signal looked and behaved like migrating birds, but the hydrometeor classifications were mainly snow and ice crystals.
You can view the classifications at the College of DuPage weather site. To use this page, click on Radar Site Map at the top right and chose your radar site, and then go to the Product Menu at the top left and choose Hydrometeor Classification --> Current at the very bottom of the list. Hydrometeor classifications can also be viewed with RadarScope, an app for Android and Apple mobile devices.