Ardea
Official journal of the Netherlands Ornithologists' Union

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Barnard P. (1986) Windhovering patterns of three African raptors in montane conditions. ARDEA 74 (2): 151-158
Flight-hunting methods of Rock Kestrels Falco tinnuncuIus, Jackal Buzzards Buteo rufofuscus and Black-shouldered Kites Elanus caeruleus were analysed over six months in the Drakensberg massif of South Africa. Characteristics of this montane area, primarily the presence of steep slopes and strong updrafts, fostered efficient windhovering tactics by each raptor. Buzzards, with the heaviest wing loading, and kites, with the lightest, were established as strong-wind and light-wind specialists respectively. Kestrels, with intermediate wing loading, used wind speeds more uniformly. 1. Buzzards wind hovered most often using hanging hovers, wherein no flapping was required to keep aloft. Kites mainly used continuous flapping hovers. Kestrels showed a much more uniform use of these and an intermediate, mixed hover type (Fig. 1). 2. Due to a strong dependence on hanging, buzzard hovers were much lengthier than those of kestrels or kites (Table 2). Hovers requiring continuous flapping were predictably shorter than those in which updrafts aided buoyancy. 3. Kites concentrated their hovers in calm conditions, while buzzards used strong, gusty breezes. Kestrels used all wind speeds, but mainly low ones (Fig. 2). All species tended to use hanging hovers in high winds, flapping hovers in calm conditions, and mixed hovers in moderate breezes. (Table 3). Flapping was required to stabilize flight in very strong wind. 4. Hanging hovers were used more often in warm weather by kestrels and buzzards, probably due to updraft formation. 5. Cloud cover and rain inhibited windhovering by kestrels, but not by buzzards or kites. 6. All species made disproportionate use of hill crests in order to exploit updrafts, but only kestrels and buzzards responded by using hanging hovers there. 7. Discriminant analysis of hover type in each species (Table 4) indicated that wind velocity, hover length and updrafts most strongly influenced windhovering mode in kestrels; grass length, hover length, and ambient temperature were important variables for buzzards and kites. The apparent importance of grass length may be either biologically spurious or a valid effect (section 3.4), but my data cannot distinguish between the two. 8. Wing loading appeared to underlie the observed differences in windhovering mode (Table 5), particularly the responses of each species to prevailing wind velocities and topographic features.


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