Tim J. Nohara, Robert C. Beason, Peter Weber, Using radar cross-section to enhance situational awareness tools for airport avian radars, Human-Wildlife Interactions, Vol. 5, No. 2 (Fall 2011), pp. 210 ...

A chart showing the reduction in returned signal for various frequencies of UHF bounced off the AFSS. Less signal returned means a smaller radar cross section and a harder-to-find target.