## 2 General Characteristics of Micro-Doppler Signatures from Birds and Drones  
As is well known, the reflected signal from a target that is in motion relative to a radar contains a frequency shift relative to the transmit frequency due to the well-known Doppler effect [16]. Quite simply, if \(f\) is the transmit frequency of a monostatic radar (i.e., transmitter and receiver collocated, as is the usual case), for a target moving at a speed of \(v\) at an angle of \(\theta\) towards the radar, the Doppler frequency shift \(f_{d}\) is given by \(f_{d}=2v\cos\theta/\lambda\), where \(\lambda\) is the transmit wavelength. The radial velocity \(v_{r}\) is given by \(v_{r}=v\cos\theta\). For directly approaching targets, \(\theta=0\), the Doppler shift is positive, whereas for directly receding targets, \(\theta=\pi\), the Doppler shift is negative. For a stationary target (\(v=0\)) or for a target moving at right angles to the LOS to the radar (\(\theta=\pi/2\)), the Doppler shift is zero. In many cases, a structural component attached to a target may exhibit micromotions in addition to the main motion of the object (including zero bulk motion). Micromotions can be caused by small movements around the rest position, such as vibration, oscillation, rotation, swinging, flapping, flutter, etc. [17]. The source of micromotion may be a revolving surface, such as the rotor blades of a drone, or a vibrating surface, such as the swinging arms and legs of a walking human, or the flapping wings of a bird, for example. These micromotions induce frequency modulations in the main Doppler frequency shift caused by the bulk motion of the target. Micromotions consisting of pure periodic vibration or rotation generate Doppler frequency shifts around the central Doppler-shifted carrier frequency and appear as sidebands. The modulation contains harmonic frequencies determined by the carrier frequency, the vibration or rotation rate, and the angle between the direction of vibration and the direction of the incident wave. The time-varying frequency modulations provide unique signatures of the object for further classification, recognition, and identification. Micro-Doppler signatures of birds and drone targets are characterized by sinusoidal modulation owing to the periodic motion of the rotating blades of drones and UAVs and the flapping wings of birds [18]. The modulation frequency of drones and UAVs is higher by one order of magnitude compared to that of birds because these targets contain high-speed rotation blades. Since the number of blades on a drone is usually an even number, when a blade approaches the radar, another blade recedes from the radar; thus, its micro-Doppler signature spectrogram is symmetric. In contrast, both wings flap up and down for birds at the same time; thus, their spectrograms are asymmetric. The main source of micro-Doppler signatures from birds arises from the oscillatory motion of their flapping wings. The frequency of the Doppler fluctuations is primarily related to the wing flapping (or wingbeat) frequency and its harmonics [19, 20]. Periodic fluctuations in signal intensity are attributed to changes in body shape due to flapping [19]. In addition, periodic intensity modulations can be caused by the appearance and disappearance of the \"corner reflector effect\" caused by the wings as they flap to and fro [20]. Therefore, the unique micro-Doppler signatures of birds are caused by several mechanisms. The micro-Doppler spread of a bird is quite narrow, and its characteristic signature takes the form of periodic flashes corresponding to the wingbeat frequency [9]. The micro-Doppler signatures of birds depend on their size, wingspan, wing flapping frequency, radar frequency, and viewing angle. In the case of drones and UAVs, the spinning of the rotor blades is the primary cause of their micro-Doppler signatures [21, 22].