An analysis of bones and other materials collected by Cape Vultures at the Kransberg and Blouberg colonies, Limpopo Province, South Africa
AbstractWe compared bones and non-faunal items collected by Cape Vultures at the Blouberg and Kransberg colonies. Bones from the base of the nesting cliffs were on average longer than those from the crops and stomachs of birds. Bones from the Blouberg cliff base were on average shorter than those from the Kransberg. A larger proportion of bones from smaller animals was the reason for this. The smaller size of the crop material was due to a greater proportion of fragmented bone. Fragmentation made bones less identifiable to species. The proportion of fragmented material and the particular skeletal elements discovered at the two sites were very similar and did not influence this size difference. Material from these colonies was, for the most part, smaller than bones collected from other Cape and whitebacked Vulture colonies in Zimbabwe, Botswana and South Africa. Higher proportions of bones from smaller animals and smaller skeletal elements collected were the reasons for the smaller average size.
In wildlifeb reserves, Gyps vultures compete with large mammalian carnivores and other scavenging birds for food, including bone. Where these competitors are absent or rare (i.e. farming areas), Gyps vultures eat more bone. Small bones (i.e. carpals, tarsals, phalanges, etc.), quickly eaten by spotted hyenas in game reserves, are collected in large numbers by Gyps vultures in farming areas, where competition is reduced. As a food, bone is almost as good nutritionally and energetically as meat. Where meat is scarce (e.g. farming areas), Gyps vultures collect more bone as an alternative food source.
In areas of high human density, vultures eat more human-made material. Substitution or confusion of one item for another (e.g. human-made items for bone/food) will occur more regularly as the replacement item becomes more prevalent in the environment. Most of the non-faunal pieces did not resemble bone and were probably not confused for that item. Glass was the most common human-made substance found in vulture crops and stomachs, and rocks the most common overall. Grass and sticks were collected from nestling crops and stomachs but rarely from adults. When food is scarce, vulture nestlings feed on non-food items, particularly nesting material. The increase in collection and eating of bone and non-food items is a result of the shift in Gyps vulture\'s diet where meat is scarce and alternative foods are sought.
Ostrich 2004, 75(3): 118–132