COMMENTS Observations of Directional Thermal Soaring Pref Ere

300 COMMENTS rBIS 130 COMMENTS Observations of directional thermal soaring prefere nee in vultures 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 Africa and ]\;ew World vulture~ in the eastern United States are described. Observations ofTurkey Vultures Cathartes 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 Gyps 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 birds 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 bird 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 East Africa Direction of rotation n Clockwise Anticlockwise New World Vultures Turkey Vulture Cathartes aura 53 24 29 Black Vulture 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 <x: = 19, P< 0·001 ), as did the White-backed Vulture <x:= 6·8, P < 0·01 ). Pennycuick (1971,J. Exp. Biol. 55; 30) suggested that thermals can contain a radial component of Row which results in air Rowing inward toward the centre; as a result, horizonti1l rotation of air about the vertical axis of the thermal is created. Vortex shells (see Cone 1962, Am. Scientist 50) as well as columnar type thermals could rotate. Pennycuick (University of Bristol, UK, pers. comm.) hypothesizes that it would be most efficient for a vulture to circle against the direction ofAow. This would allow a decrease in the angle of bank, a reduction of the circling radius, and hence a decrease in the rate of sink. Thus, vultures would obtain a more efficient rate of climb by soaring against the radial Aow in a thermal. Directional bias in soaring by some African but not eastern U .S. vultures observed might suggest selection by the birds which could be correlated with the actual direction of flow of air in thermals in the two study areas. A true test would be the actual measurement of the d irection of air Row in thermals used by soaring vultures, a difficult but not impossible task (C. J . Pennycuick, pers. comm.). Meanwhile, I suggest that my data show a response by vultures to thermal air movement that is most efficient for maintaining long hours aloft. Further experimental work needs to be performed to elucidate the physical nature of thermals and whether or not vultures exploit any radial component of Aow. COMMENTS 301 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 .

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