Wind turbine rotor blades with a reduced radar cross sections include a shell having a leading edge opposite a trailing edge, a structural support member that supports the shell and is disposed internal the wind turbine rotor blade between the leading edge and the trailing edge and extends for at least a portion of a rotor blade span length, wherein the structural support member comprises fiberglass, one or more cavities internal the wind turbine rotor blade, and a lightweight broadband radar absorbing filler material disposed in at least one of the one or more cavities to provide the reduced radar cross section.