Detecting bird movements with L-band avian radar and S-band dual-polarization Doppler weather radar

Marine surveillance radars with X-band (3-cm) and S-band (10-cm) wavelengths not only detect birds but also record return from insects, rain, ground objects and rough seas that often make discrimination of echoes from birds difficult. We compare the density of bird tracks recorded by an L-band (ca. 23-cm wavelength) avian radar at the Dallas/Fort Worth International Airport that does not detect insects and rain with the amount of mean bird reflectivity in resolution cells of a dual-polarization S-band Doppler weather surveillance radar. Radar data files of 1.5 degrees scans gathered near midnight from January through May for the years 2014, 2015 and 2016 were processed by filtering radar resolution cells to retain only those with backscatter differential phase values characteristic of birds flying more or less perpendicular to the radar beam. In general, increases and decreases in the density of bird movements were correlated significantly between the two radars, but the low R-2 values confirm that results obtained from one radar cannot be used to make quantitative estimates for the other. A decrease in the mean biomass index of migrating birds from January through May, and the difference in coverage volume and resolution between the two radars likely account for the low R-2 values. Each type of radar has advantages and provides important information on bird movements at different spatial scales.