Detection of bird migration by centimetric radar—a cause of radar ‘angels’

Abstract High-power centimetrie radar at times records random scatterings and occasionally dense displays of small blobs of echo, which have been called radar ‘angels’. As output powers have increased, these displays have come to present an inconvenience, at times approaching a hazard, on airfield-control radars. They have been widely believed to be of meteorological origin, by reflexion or refraction of energy from atmospheric discontinuities, but no meteorological theory so far proposed has managed to explain all their observed properties. It is shown that these properties can be satisfactorily explained on the assumption that the echoes are received from birds on migration. The directions of movement recorded over several years at East Hill are in good agreement with known migration movements in southeast England, mainly eastward and north-eastward in spring, southward and westward in autumn, and the seasonal distribution and flight speeds are shown to fit this explanation. Visual confirmation has also been obtained. With a telescope mounted on the aerial of a target- tracking radar, small flocks of birds have been seen when the radar was tracking angel echoes, and it was clear that the radar was following their flight. It would seem unnecessary to invoke any further mechanism to explain radar angels, and, because movements of birds are substantially world-wide, it would follow that there is little prospect of removing their effects from airfield-control radars. It is clear that radar can provide information of great value to the ornithologist. The records presented in this paper demonstrate the essentially broad-front nature of inland migration movements and the great heights at which these movements sometimes take place. They provide evidence that nocturnal migration can be much heavier than daytime migration. Finally, analysis of the winds associated with these movements suggests that migration rarely takes place with seriously adverse winds, and that weather movements of wintering birds in particular may be largely down-wind movements.