Evolutionary Ecology of Afrotropical and Neotropical Vultures in Forests

Abstract

Several species of vultures are commonly found in neotropical forests, but none has occupied the forested regions of the Old World. It is suggested that a greater abundance of food for large scavenging animals may exist in New World forests because of their higher mammalian biomass and the complexity of their invertebrate communities. Cathartid vultures are highly efficient at searching for food and may be the major scavengers of large mammal carcasses in the neotropical forest. RESUMEN. Se encuentran varias especies de buitres en bosques neotropicales, pero ninguno ha ocupado regiones boscosas en el Viejo Mundo. Se sugiere que la mayor abundancia de comida para animales carroneros, existente en los bosques del Nuevo Mundo se deberia a su mayor biomasa de mamiferos y a la complejidad de su comunidad de invertebrados. Los buitres de la familia Cathartidae son muy eficientes buscando comida y pueden ser los mayores carroneros de carcasas de mamiferos grandes en los bosques neotropicales. Vultures differ from other meat eating birds in that they virtually never kill their prey, but obtain almost all food by scavenging from dead animals. This method of feeding has been exploited by two groups of birds, the New World vultures and the Old World vultures, which are quite unrelated. The 1 5 species of vultures that are found in Africa, Asia and southern Europe are classified in the family Accipitridae together with the eagles and hawks to which they are obviously closely related (Brown and Amadon 1968). The seven species of New World vultures, however, appear to show affinities to the storks, family Ciconiidae (Ligon 1967; Feduccia 1980); these Cathartid vultures are usually placed for convenience as a suborder within the Falconiformes (Brown and Amadon 1968), although it is widely recognized that they do not belong there (Voous 1973). Despite their different ancestry, the two groups of modern vultures are extremely similar in superficial appearance. They are textbook examples of convergent evolution. Both groups have a similar diversity of species, and share many aspects of plumage pattern and a common dependence on soaring flight. In contrast, the habitats occupied by these two vulture groups are markedly different in one respect. The Old World vultures are confined to open habitats such as savannas, grasslands, and semideserts, and none of these species is found in any of the forested areas of Africa or Asia. Among the New World vultures, the two species of condors (Gymnogyps californianus and Vultur gryphus) live in mountainous areas. The five smaller Cathartid vultures, however, have centers of distribution in the neotropical forest region and several species live in dense forest conditions. In this review I consider some possible reasons why neotropical forests are able to support several species of vultures, whereas the tropical forests of Africa and Asia support none; this implies that the scavenger food chains in the two regions of forest may differ significantly. I first briefly consider the evolutionary history of the two vulture groups, and then consider the potential size of the food supply for scavenging animals in forested areas, the potential competition among scavengers for this resource, and finally the comparative efficiency of vultures at locating food. Little information is available about the ecology of any of the Cathartid vultures in forested regions; thus many of these considerations are tentative, and much of what follows is speculation. EVOLUTIONARY HISTORY OF VULTURES The fossil record of both vulture groups is comparatively good (Brodkorb 1964; Rich 1983). Thus, we know that although today the New World vultures and the Old World vultures are clearly separated geographically, they did not develop in isolation; fossil New World species have been found in the Old World, and similarly fossil Old World species have been found in the New World. Ten Old World species from four genera have been discovered from the