Amplitude modulation on echoes from large birds

Although periodic amplitude modulation of echoes from birds were observed in the very first recordings of birds with radar, few publications on the topic are found in the literature. The phenomenon, which tends to be correlated with the wing beat frequency (WBF), has been suggested as a feature for discrimination between bird species. Early studies were reported mainly in the 1960s and 1970s, however, several questions remain unanswered. Is such modulation prevalent, if so what causes it, how does it vary with aspect angle and signal to noise ratio (SNR), how sensitive is it choice of carrier frequency, and what is its potential as a feature for classification in a real scenario? This paper seeks to address several of these question. The discussion and results presented are based on analysis of X-band radar data and video of flying gannets. These measurements are supported with electromagnetic predictions of 3D gannet models.