doppler / pinging
doppler would be proportional to the speed of approach or departure by the craft relative to the bird's position. Given the height of the bird, and the very small angle subtended by the beam relative to that height, doppler will be very small, and probably with very high error extents. Velocity times sine of the angle from perpendicular @ beam center to the craft. Doppler might be unmeasurably small, relative to frequency accuracy at bird and craft.