The colourful White-headed Vulture Aegypius occipitalis: enhancement of threatening signals for interspecific competition?

摘要

A medium-sized vulture among the Afrotropical species, the White-headed Vulture Aegypius occipitalis is the most conspicuous because of its black and white plumage with an especially contrasting effect in the frontal view, orange bill with blue cere, bare pink skin on face and neck, and pinky-orange feet. Its behaviour contributes to the effect, producing 慳 striking hauteur when standing on the ground and walking about in arrogant pose?(Mundy et al. 1992). Its closest similarity in colours as well as general build is with the larger Lappet-faced Vulture Aegypius tracheliotus, the usual dominant bird in vulture gatherings. Although more variable geographically in its colours, which are less vivid in any case, the latter has comparable colour patterns on the head and underparts. Comparable colour patterns also occurs in the Hooded Vulture Necrosyrtes monachus and Marabou Stork Leptoptilos crumeniferus, apparently minor competitors at a carcass, while the griffons梩he White-backed Vulture Gyps africanus, R黳pell抯 Vulture Gyps rueppellii and Cape Vulture Gyps coprotheres梐re all drabber birds. The griffons are also less different in colours between adults and immatures and less often adults win intraspecific encounters over immatures than Lappetfaced, White-headed and Hooded Vultures do (Mundy et al. 1992). To the opposite extreme, the more extensive white in the adult female plumage of White-headed Vulture has been suggested to serve intraspecifically to assert the female抯 dominance at carcasses (Mundy 1985). When excited, however, also the griffons blush in the face, neck and bare patches of skin either side of the crop, and display white areas in the plumage, though with less contrast. These considerations suggest that some colours are rather general signals for vultures. K鰊ig (1976) sees the reddening skin and contrastingly white plumage as threatening colours and suggests that they might work not only intra- but also interspecifically. Negro et al. (2006) propose the same among the possible functions of flushing colours. To investigate the interspecific significance of contrasting colours among the Old World vultures, a starting point might be considering the position of the White-headed Vulture in the encounters for food competition. A further point might be the geographic distribution of the species mainly involved in these encounters, on the assumption that their status in sympatry would depend on their interaction.