  Radar echoes from bird flocks contain modulation signals, which we find are
produced by the flapping gaits of birds in the flock, resulting in a group of
spectral peaks with similar amplitudes spaced at a specific interval. We call
this the formation wing-beat modulation (FWM) effect. FWM signals are
micro-Doppler modulated by flapping wings and are related to the bird number,
wing-beat frequency, and flight phasing strategy. Our X-band radar data show
that FWM signals exist in radar signals of a seagull flock, providing tools for
quantifying the bird number and estimating the mean wingbeat rate of birds.
This new finding could aid in research on the quantification of bird migration
numbers and estimation of bird flight behavior in radar ornithology and
aero-ecology.
