Ardea
Official journal of the Netherlands Ornithologists' Union

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Smeenk C. (1974) Comparative-ecological studies of some East African birds of prey. ARDEA 62 (1-2): 1-97
The main problem presented in this paper is how sympatric species of birds of prey manage to live alongside each other avoiding inter-specific competition. The numbers of birds of prey and owls generally are assumed to be regulated by food supply only; consequently, ecological differentiation in these groups, resulting in ecological isolation, is thought to be achieved by avoiding interspecific competition for food. How such competition is avoided is one of the subjects of the comparative ecology, i.e. a comparison of data on habitat preference, food habits, hunting methods and morphological structure of various species. In Africa, the bird of prey fauna of the non-forest or savannah habitats is even richer than that of the lowland rainforests; the savannah, where the ranges of various closely related bird of prey species overlap, therefore was thought to be a promising field of research. This paper is mainly based on field studies in Tsavo National Park East, situated in the belt of dry lowland savannah of eastern Kenya. First, the composition of the bird of prey fauna in the park is analysed. After this the main part of the study follows, dealing with the comparative ecology of five common eagle species in the area. A similar analysis is made of the bird of prey community in a moister type of savannah in the Rift Valley in the central Kenyan highlands, followed by brief notes on the food habits of the dominant species there. Concluding, the differences between the two predator avifaunas are discussed. The structure of the bird of prey fauna of Tsavo East was analysed by means of regular transect counts, carried out over a period of two years (app. 2 and 3).. The most characteristic resident species were Brown Snake Eagle Circaetus cinereus, Pale Chanting Goshawk Melierax poliopterus, Tawny Eagle Aquila rapax belisarius and Secretary Bird Sagittarius serpentarius. The African Hawk Eagle Hieraaetus spilogaster, which was almost restricted to the more luxuriant riverine vegetations, was on the decline in the park, probably as a result of the destruction of trees and bushes by elephants. Common species showing apparent intra-African migration patterns were Bateleur Terathopius ecaudatus, Grasshopper Buzzard Butastur rufipennis and Wahl76 berg's Eagle Hieraaetus wahlbergi. Palaearctic migrants were Pallid and Montagu's Harrier Circus macrourus and C. pygargus, Steppe Eagle Aquila rapax orientalis, Lesser Kestrel Falco naumanni and Kestrel Falco tinnunculus. Special reference was made to the occurrence of these migrants in connection with rainfall patterns in the two years of observation; they were more numerous when rainfall was relatively high. Comparative food studies of Tawny Eagle, Bateleur, Martial Eagle Polemaetus bellicosus and African Hawk Eagle were carried out mainly in the long dry season between May and November, when these species reared their young. The studies were made in two habitat types: (1) heavily wooded bushland (fig. 3 and 4) and (2) more open wooded grassland (fig. 5 and 6). In the dense bushland Kirk's Dikdik Rhynchotragus kirkii, a small antelope weighing up to 5 kg, proved to be the main prey of all four eagle species studied. In addition, Tawny Eagle and Bateleur lived on a great variety of other mammals, birds and reptiles; the Martial Eagle on young antelopes and bustards, and the African Hawk Eagle on small mammals and birds, game birds and bustards (app. 4-7). In table 4 the food composition of each eagle species is expressed both in frequency and weight percentages, the latter based on the estimated maximum food value of each prey item to the respective eagle families. Food overlap between the four eagles is visualized in fig. 9 and 10. The overlap is particularly great when expressed in weight percentages, mainly as a result of sharing of Dikdiks. The total annual predation by these eagles on the Dikdik population was estimated as 40-60% of the annual Dikdik mortality, or 36-56% of the total Dikdik population in the area. This heavy predation on one prey species by a number of different predators is in a way contradictory to the competitive exclusion principle, according to which no species with similar ecological requirements can persist in the same area, especially since the situation here is a permanent one and the Dikdiks are not temporarily superabundant. It is postulated that the numbers of anyone eagle species in this area are not limited by food supply, but by intra-specific territorial behaviour, which prevents a species from increasing to the point where food would begin to operate as limiting factor and serious inter-specific competition would come into effect. Supporting evidence for this is offered by the regular spatial distribution of Martial Eagle pairs along the river courses (fig. 1), the only places where there were suitable nest trees for this species (fig. 7). This resulted in a lower population density of this eagle than if nest trees had been equally distributed over the park. Such a mechanism enables a community of various bird of prey species to divide the available prey efficiently between them; particularly as each species has its own specific hunting methods (see below). In the more open habitats of Tsavo East, where Dikdiks were less abundant, food overlap between Tawny Eagle, Bateleur and Martial Eagle was less (table 7, fig. 11 and 12); the African Hawk Eagle could not be studied here. In addition to Dikdiks, the African Hare Lepus capensis was an important food source to the three eagle species studied in this area. For the rest, the Tawny Eagle showed the same variation in diet here as in the dense bushland, with bustards and snakes as particularly important items. The Bateleur had a predilection for small birds, the Martial Eagle for bustards. Comparison with literature data from elsewhere in Africa (table 8-11) confirmed the trends of ecological differentiation among the four eagle species discussed above, as found in Tsavo East; in many areas, however, game birds appeared to be an important food source to at least three of them, being less important to the Bateleur only. The Tawny Eagle is the ecologically most wide-ranging African eagle. The Bateleur is very catholic in its food habits, with a certain preference for small birds. The Martial Eagle preys mainly on game birds and bustards or on small antelopes. The African Hawk Eagle lives on game birds, bustards and smaller mammals and birds. This ecological differentiation is accentuated by differences in hunting methods. The ecologically most similar species proved to be Tawny Eagle and Bateleur, but these have strikingly different hunting methods, the Tawny Eagle hunting mainly near trees and bushes, the Bateleur higher up in the air over more open vegetation. Consequently, in the mosaic pattern of most savannah types each of the two species tends to catch those prey animals that are less easily available to the other, resulting in what seems to be a remarkably efficient utilization of the prey species they have in common. Similar, though slighter differences exist between Tawny Eagle, Martial Eagle and African Hawk Eagle. The Hawk Eagle prefers relatively dense woodland, the Tawny Eagle more open country, whereas the Martial Eagle apparently is not dependent on any particular vegetation type and is able to hunt over more open country than the Tawny Eagle. Chances on inter-specific competition are thus further reduced. Wahlberg's Eagle (app. 8) differed markedly from the other species studied in that it fed mainly on small birds and reptiles. In other areas (table 12) rodents may form an important food source; where these abound, the population density of Wahlberg's Eagle is reported to be higher than in Tsavo East. Its hunting methods characterize this species as a bird of more heavily wooded savannas. A less profound study was made of the predator avifauna near Nakuru in the central Kenyan highlands, where transect counts were carried out over a period of one year. Counts were made in two habitat types: (1) intensively cultivated farmland and (2) drier grassland with scattered trees, utilized as cattle ranch land (app. 10 and 11). The most characteristic resident birds of prey were Augur Buzzard Buteo rufofuscus, Long-crested Eagle Lophaetus occipitalis, Tawny Eagle, Black-shouldered Kite Elanus caeruleus and Secretary Bird. The most common migrants were Black Kite Milvus migrans, Pallid and Montagu's Harrier, Steppe Eagle, Lesser Kestrel and Kestrel. The Augur Buzzard was found to live mainly on mole rats Tachyoryctes splendens (table 14). Additional food data on this species and the other common residents were derived from the literature. Augur Buzzard, Long-crested Eagle, Black-shouldered Kite and Secretary Bird are all mainly rodent hunters, as well as, at higher altitudes, the Tawny Eagle. Long-crested Eagle, Black-shouldered Kite and Secretary Bird were more abundant in the agricultural land than in the ranch land, which coincided with the much higher rodent density in the moister vegetation types (table 15). Mole rats, however, were also numerous in the grasslands, where the Augur Buzzard is the only bird of prey known to hunt these subterranean rodents. Consequently, the buzzard probably is less dependent on other rodent species and is common in the drier country as well. It is unknown whether the Tawny Eagle too, is able to utilize the same food source. Highly favourable food conditions in combination with geographical factors lead to impressive temporary concentrations of migrant birds of prey in the agricultural land, particularly of Black Kites, Steppe Eagles and Lesser Kestrels. Better food conditions as a consequence of higher rainfall probably are at the base of the larger numbers of migrants in the Nakuru area as compared with Tsavo East. Comparing the resident birds of prey of the two study areas, it appears that the characteristic Tsavo species have either catholic food habits, or else, they are reptile specialists; near Nakuru rodent hunters dominate. This corresponds with the virtual absence of diurnal rodents in Tsavo East. Generally speaking, Black-shouldered Kite and particularly Long-crested Eagle are species of well-watered areas. The Secretary Bird too, apparently is more abundant in areas with a good rodent supply; elsewhere it is largely insectivorous. The Augur Buzzard is a typical highland species; at lower altitudes it is restricted to the direct vicinity of rocky hills, probably in connection with climatic factors. Throughout their ranges Bateleur and Wahlberg's Eagle are mainly dry-country lowland species; the reasons for this are unknown. The Tawny Eagle, able to live on the greatest variety of prey, is equally represented in both study areas. In appendix 1 the systematic position of Wahlberg's Eagle is discussed. This species is regarded as closely related to the Palaearctic Booted Eagle Hieraaetus pennatus, in view of similarities in general appearance, dimorphism in both adult and downy plumage, and voice; consequently, it should be placed in Hieraaetus.


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