300 COMMENTS rBIS 130

COMMENTS

Observations of directional thermal soaring prefere nee in

Thermal soaring may be defined as circling Aight within rising parcels of air called thermals. This activity is a primary mode of locomotion of vultures Cathartidae and Aegypinae. In this note I present observations of the horizontal direction of rot.ation utiliz.,.d hy two groups of vultures after they have achieved a circling glide path within a thermal. Differences in the frequency of direction of rotation found in Old World vultures in East and ]\;ew World ~ in the eastern United States are described. Observations ofTurkey Vultures aura and Black Vultures Coragyps atratus were made from Maryland south to Key West, Florida {latitude 39°N-24° N) during March and April 1977. White-backed Vultures africanus and Hooded Vultures Necrosyrtes monachtts were observed during January 1977 in K enya and Tanzania (latitude 0°-S0 S). Observations were made either by eye alone or with 7 x SO binoculars. The were observed over routes totalling more than 10 000 km. A vulture was located and observed until it established circling Aight within a thermal. The direction of rotation (clockwise or anticlockwise as viewed from above) was rt;:corded after the had made at least one complete revolution. The same bird was never recorded more than once within a given thermal. The data (Table 1) indicated that the cathartid vultures in the Northern Hemisphere showed no

Table I. Observations of directional soaring among Neu· and Old World vultures in the eastern US and

Direction of rotation

n Clockwise Anticlockwise

New World Vultures Cathartes aura 53 24 29 Coragyps atratus 22 13 9 T otal(% ) 75 37 (SO) 38 (SO) Old World Vultures Hooded Vulture Necrosyrtes monachus 19 0 19 White-backed Vulture Gyps africanus 53 17 36

Total ('}'0 ) 72 17 (25) 55 (75)

significant preference for either mode of directional soaring(/= 0·01S,P>0·9). Of the accipitrid vultures 1 on the equator or south of the equator, the Hooded Vulture went anticlockwise significantly more than expected

I wish to acknowledge W. L. Long for his helpful suggestions and interest throughout the course of this work. I also thank C. J. Pennycuick for encouragement and helpful information. K. J. Reinecke, G. L. Hensler, and G . W. Pendleton advised appropriate statistical tests, and R. E. Kirby and M. R. Fuller reviewed drafts of this manuscript. HOLLIDAY H. OBRECHT III US Fish and Wildlife Service, Patuxent Wildlife Research Center, Laurel, Maryland 20708, USA