The ecology of Serengeti vultures

The relationship between the whiteback vulture Gyps africanus, and Ruppell's griffon Gyps ruppellii and their food supply was studied in the Serengeti National Park in northern Tanzania for sixteen months during 1969 and 1970. Seven species of vulture occur in this region, although over most of their ranges each species is separated from the others ecologically and they do not compete. These two griffon vultures both specialise in taking the soft parts of carcasses and between them account for 85% of all vulture sightings in this area. In the Serengeti region both species are common, an unusual situation, and they are potentially in direct competition for the same food supply.
In the Serengeti about 80% of herbivores are migratory and the large distances covered by these herds during the year prevents their predators from increasing sufficiently to become a major mortality factor. Other causes of mortality are therefore of far greater importance arid may account for perhaps 80% of all mortality in the area. However, the nature of this mortality causes seasonal fluctuations in the number of deaths, as well as the variations in distribution caused by the movements of the herds. These conditions prevent mammalian scavengers from being able to utilise the food effectively, and these may take perhaps only 5% of the total food available to scavengers. There is therefore potentially available a food supply for scavengers which fluctuates considerably in quantity and distribution through the year. This is the food supply that griffon vultures utilise. Predators are usually efficient in their feeding and vultures obtain only small amounts of food from predator kills. A large vulture population can probably only occur in areas where there is a food supply that is not effectively exploited by mammalian carnivores.
In the Serengeti there are probably two peaks of food availability. At the end of the dry season when animals are in poorest condition and scattered widely through the northern woodlands, and a second peak at the time of wildebeest calving when herds are heavily concentrated on the eastern plains. Birds travel widely during food searching and tend to concentrate heavily in areas of large game herds, which is where most of the food is available. The Ruppell's griffon is uncommon in areas away from the migratory herds. They locate food largely by watching the activity of other birds and converging on an area where vultures are seen to be descending. In areas of moderate game density birds search at low altitudes and relatively high density so that birds congregate rapidly at any food source. However, in areas with little game, birds search at higher altitudes and may not always be within sight of other individuals, and fewer birds may be attracted to carcasses. Since birds can cover large distances rapidly, they are able to exploit the food supply despite variations in distribution through the year.
However, food also fluctuates in abundance. Captive birds were used to establish the food requirements for inactive birds. The amount of food actually obtained by wild birds was also recorded from the size of the crop of birds roosting in the afternoon, and this showed that non breeding birds during the first half of 1970 were probably able to obtain sufficient food to satisfy their food requirements. Their fat levels were high throughout this period. Birds can store fat very rapidly after a large food intake and use this to last over periods when the food supply is not adequate to satisfy the food requirements. The body temperature does not fall abnormally at night.
The food requirements of birds could be increased seasonally by both moult and breeding. Moult is continuous throughout the birds life. Primary feather moult starts about ten months after the bird leaves the nest, and the first feather to be shed is the innermost primary. From this position primary moult proceeds in an orderly progression outwards towards the tip of the wing. Subsequent primary moults also occur as similar waves of moult activity moving outwards across the wing. Secondary moult, however, does not show a clear pattern. The innermost secondary is the first to develop, but later feather replacements occur at sites further along the wing, and although there is no fixed position for these additional growing sites, there is a tendency for replacement to occur first in three areas. Further secondary replacement does not occur by the progression of moult waves, but by the irregular growth of feathers. Subsequent secondary moults also appear to be irregular. It probably takes about three years of moult activity to complete the first feather replacement. Adult birds show individual variation in the number of actively growing feathers, but this could not be related to breeding or body condition. Feather growth accounts for perhaps 6% of the total energy intake of the bird throughout the year.
Breeding, however, is confined to a definite season. The Ruppell's griffon lays about two to three months earlier than the whiteback. In the Ruppell's griffon the food requirements for egg laying are considered to be slight, although the provision of calcium for the egg shell might be an important factor. Captive chicks were hand reared to determine their food requirements during growth and these estimates were combined with the figure for the food requirements of adult birds to determine a total estimate of food requirements for breeding birds. The amount of food actually obtained by a group of birds was recorded from the size of the crops of birds returning to the colony in the afternoon. The comparison of food obtained and food required through the season suggested that their may be a period during rearing when there was insufficient food available to satisfy the food requirements of both chick and adult. Chicks were found to have a very high survival rate and they are not able to alter their growth rate according to the amount of food that they are supplied and presumably chick food requirements were being met. The examination of a sample of adult birds for body condition through the year showed a clear decline in the fat deposits through the breeding season, although no bird was found with seriously low protein levels. It was considered that for both species breeding is timed so that the young leave the nest at a period in the year when food conditions are good and young birds can feed with little competition from adults. However, adults have to rear young during a period in the year when food conditions are not always adequate and they have to rely on utilising fat reserves.