The third base product of WSR-88D (Nexrad) radars, spectrum width, has been called "the largely ignored stepchild of Doppler weather radar data" (Lemon, 1999). If so, this product should be adopted by biologists, because prominent biological targets on weather radars can produce characteristic values of spectral width that aid in identifying kinds of flying animals and discriminating them from other kinds of scatterers. Results from WSR-88D and research radars are presented on insects, migrating small birds, large aggregations of roosting birds, large flocks of flying waterfowl, and some bats. Beginning with the standard summed series of constituent elements for passive scatterers such as water droplets, a spectrum width model is developed of the more complex case of many small point targets occupying a pulse volume and quantitative predictions from this model are compared with high values of measured spectrum width from ground-truthed migrating birds, insects, and mixtures of the two. The contribution of differing movements of individual animals are developed, as well as the rapid accelerations of single animals and their body parts (e.g. wing beats).

In contrast, narrow spectrum width measured from ground-truthed, well-defined flocks of birds is readily distinguishable from singly-migrating birds and from most passive targets such as water and dust. A radar-documented case of known species of birds being killed in a storm over Lake Michigan vividly illustrates the potential of spectrum width to separate juxtaposed weather and biological echoes.