The use of low-level jets by migrating birds

Birds flying at high altitudes have occasionally been observed above mounta in areas and the open sea. For the first time the regular occurrence of mig rating birds flying within a low-level jet at heights of 5000 to almost 900 0 m asl. have now been verified by radar above the Negev desert in southern Israel. Tracks of rather large birds with wing-beat frequencies of 5-6 Hz were measured to have horizontal flight speeds up to 50 m/s. Visual observa tions, seasonal occurrence, and wing-beat frequencies allowed to associate them with small species of the order Ciconiiformes (mainly Ardeidae) and po ssibly with members of the Laro-Limicolae group. These wading birds seem pr one to continuing nocturnal migration into daytime under favorable conditio ns and to make use of high wind speeds at sometimes extreme altitudes.