Aerial survey of raptors in the lowveld of Swaziland, July 2002

A summary report Ara Monadjem

Summary

An aerial survey, exclusively for the censusing of vulture and eagle nests, was conducted in the lowveld of Swaziland in July 2002. The position of all nests located from the air was stored on a GPS. A total of 272 active nests was recorded, of which 219 nests belonged to the African White-backed Vulture. This species nests in three discrete regions in Swaziland, with the highest population and density located in the Hlane-Mlawula-Mbuluzi network of protected areas. The nesting density in this region is very high, with the Nzotho section of Hlane supporting the highest known density on the continent (108 nests in 40.6 km2). Nesting densities of Lappet-faced and White-headed Vultures were within the range (or higher than that) reported from adjacent areas of South Africa. In addition, nests of seven species of eagles were recorded. Nests of vultures and eagles were almost entirely restricted to protected areas, with only a few nests scattered in suitable, but unprotected habitat. Finally, a Cape Vulture colony, on the Swaziland-Mozambique border, was also visited and nesting activity recorded.

Background

Five species of vulture have been reliably reported from Swaziland, of which three (African White-backed Vulture Gyps africanus; White-headed Vulture Trigonoceps occipitalis; Lappet-faced Vulture Torgos traheliotos) have nested in the country since 1985 (Parker 1994). Small numbers of Cape Vultures Gyps coprotheres are regularly sighted over the eastern lowveld of Swaziland (Parker 1994). These birds probably originate from the Goba colony in Mozambique (Parker 1997). The current status of this colony is uncertain, but has been estimated to support between 10-15 pairs (Parker 1997). The Hooded Vulture Necrosyrtes monachus is an irregular non-breeding vagrant to the country (Parker 1997).

The first population estimates of vultures in Swaziland were made by Parker (1994) who estimated a total of 200 breeding pairs of African White-backed Vulture based on observations of congregations at carcasses. Esterhuizen (1995) recorded 59 nests of this species during ground surveys in the lowveld in 1995. Monadjem (in prep.), also based on ground surveys, estimated that 163 pairs bred in 2001. Since up to a third of pairs may not attempt to breed in any particular year (Mundy et al. 1992), the total breeding population of African White-backed Vultures was estimated to exceed 200 pairs (Monadjem in prep.). Estimates of breeding populations of the other vulture species has mostly been based on educated guesswork, although Monadjem (in prep.) recorded four active White-headed Vulture nests in 2001.

Practically nothing is known about the status of eagles in the country (cf. Parker 1994).

The main objectives of this aerial survey were:

1. To map the distribution of raptor nests in the lowveld of Swaziland, with emphasis on vultures and eagles.
2. To determine the population size of vultures and eagles in the lowveld.
3. To relate the distribution and density of raptor nests to existing land use.

Methodology

The aircraft used in the survey was a two-seater, fixed-wing Kitfox. Generally, cruising speed was maintained at between 60 - 100 km/hour (depending on direction and speed of the wind speed). For most censusing, the aircraft was flown 5-15 m above the tree line.

Parker's (1994) extensive atlas of bird distributions only records vultures in the lowveld (below 500m above sea level) of Swaziland, which lies in the east of the country. The focus of this study, therefore, was the lowveld. The highest density of vulture nests (and possibly eagles as well) is located in the Hlane-Mlawula-Mbuluzi reserve network (Monadjem in prep.). These three protected areas are contiguous and are situated in the north-eastern region of Swaziland. As a starting point, this entire 50 000 ha area (including two dispersal areas to the south and west of Hlane) was surveyed. As most vulture nests in Swaziland are located along rivers or drainage lines (Monadjem 2001), all major riparian zones were surveyed first. These riparian zones were identified from 1/50 000 maps, and were flown in both directions (i.e. upstream and downstream). The pilot maintained the plane over the river causeway and I counted the number of active nests on my side of the river or drainage line. At the end of the drainage line, the pilot turned the plane around and flew back along the same flight path. I then counted nests on the opposite bank. Once the rivers and drainage lines had been surveyed the rest of the area was surveyed by flying parallel strips (straight lines). The Hlane-Mlawula-Mbuluzi area was divided into seven blocs, of which two had large numbers of vultures breeding beyond the riparian zone. These two blocs were flown in strips 100 m apart, and only I counted nests. The remaining five blocs supported very few vulture nests (except along drainage lines) and were flown in strips of 200 m, with both pilot and I counting nests. The entire Hlane-Mlawula-Mbuluzi area, therefore, was completely covered.

Prior surveys (Esterhuizen 1995; Monadjem in prep.) also recorded vultures from Mkhaya Game Reserve and the adjoining Big Bend Conservancy, as well as from IYSIS Cattle Ranch. Hence, major drainage lines in these two areas were censused as described above. In addition, several 200 m strips were conducted at random intervals (i.e. partial coverage as opposed to complete coverage). The habitat in intervening areas between IYSIS, Hlane and Mkhaya is either transformed (to irrigation or subsistence agriculture) or heavily settled. Since vultures and eagles do not nest under such conditions (personal observation), these areas were not surveyed. A number of government and private ranches to the south of the Big Bend Conservancy were also partially surveyed, but agricultural land and heavily populated areas were once again avoided.

The positions of all observed nests were stored, while in the air, onto a Garmin 12 GPS (global positioning system). However, only active nests (defined by the presence of one of the following on/in the nest: incubating adult, egg or chick) were counted. Where necessary, a nest was circled to confirm identification of the species or to check nest contents. Very few birds were disturbed off the nest by the plane, the only species to do so regularly was the Martial Eagle. None of the incubating vultures ever left the nest.

Raptors were identified as any species belonging to the Falconiformes or Strigiformes. A few storks and herons (Ciconiiformes) were also recorded during the surveys, and because of their threatened status in Swaziland, the positions of these nests were also stored on GPS.

Results and Discussion

Vultures were observed to nest in three discrete areas in the lowveld: 1) Hlane-Mlawula-Mbuluzi, 2) Mkhaya and the Big Bend Conservancy, and 3) IYSIS. The same three areas were also identified by Monadjem (in prep.) as supporting breeding vultures. Despite extensive aerial searches in appropriate habitat, no new nesting areas were found. Hence, these three areas are the only areas that currently support breeding vultures. Most of the eagle nests located in this study were also observed here. This aerial survey has, therefore, confirmed the importance of these three areas for raptor conservation in Swaziland.

A total of 272 identifiable raptor nests of 11 species were recorded. A further two owl nests (of one species), one heron nest and 13 stork nests were also located. These data are summarized in Table 1 (below).

Table 1: Species and number of active nests recorded during an aerial survey in July 2002, Swaziland lowveld.

 *Some of these nests may have been misidentified (see text)

Vultures

By far the majority of nests recorded in this survey belonged to the African White-backed Vulture. A total of 219 active nests were recorded during the aerial survey. This is significantly higher than the 163 pairs estimated to be nesting in Swaziland in 2001 (Monadjem in prep.), suggesting that the previous survey had under-estimated this population. Assuming that 20% of the adult population does not attempt to breed each year (Mundy et al. 1992), then the total number of breeding pairs present in Swaziland in 2002 would be 274 pairs.

The density of African White-backed Vulture nests recorded in protected areas of Swaziland was approximately 27 nests/100 km2 (see Table 2). However, the densities varied considerably from one protected area to the next. The highest densities (and the highest absolute numbers) were recorded in the Hlane-Mlawula-Mbuluzi area. Approximately 150 km2 of this 500 km2 area are mountainous and unsuitable for African White-backed Vulture nesting. The density of nests recorded in the remaining (suitable) area was 53 nests/100 km2. Within this, the 40.6 km2 Nzotho section of Hlane supported a density of 266 nests/100 km2. By comparison with other regions of Africa, these density figures are very high. The following "high" figures are available for South Africa: Umfolozi-Hluhluwe 24.4 nests/100km2 (Whateley 1986), Kruger National Park 7.9 nests/100km2 (Kemp & Kemp 1975), Timbavati-Klaserie 9.8 nests/100km2 (Tarboton & Allan 1982), Kimberley 61-110 nests/100km2 (Mundy et al. 1992; Murn et al., in prep.). The density in the Hlane-Mlawula-Mbuluzi area, therefore, compares favourably with the highest densities in South Africa, while the Nzotho area appears to support the highest known density of African White-backed Vultures on the continent.

A total of five White-headed (of which only four were active during this study) and three Lappet-faced Vulture nests was recorded during this survey. All eight nests were recorded in the Hlane-Mlawula-Mbuluzi area. Unlike the African White-backed Vulture, the nests of these two species were spaced well apart and were not situated along drainage lines. Recorded densities at Hlane-Mlawula-Mbuluzi were 1.4 nests/100km2 (White-headed Vulture) and 0.9 nests/100km2 (Lappet-faced Vulture).

Lappet-faced Vulture densities vary considerably across the continent, but "high" figures of 2-7 nests/100km2 have been reported from Kenya and Zimbabwe (Pennycuick 1976; Mundy et al. 1992). The Swaziland density is lower than this, but are similar to that quoted for KwaZulu-Natal (1.5 nests/ 100km2, Mundy et al. 1992), but appreciably higher than densities in the Kruger National Park (Tarboton & Allan 1984).

White-headed vultures nest at low densities throughout Africa, and the density recorded in Swaziland are similar to those quoted for Zimbabwe and Zambia (Mundy et al. 1992), but higher than densities quoted for Kruger National Park and neighbouring private reserves (Kemp & Kemp 1975; Tarboton & Allan 1984) and the Serengeti, Kenya (Pennycuick 1976).

In contrast to the other three species, Cape Vultures breed only on cliff faces. The only known Cape Vulture colony in or near the Swaziland lowveld is situated just over the Mozambique border. This colony was estimated to comprise 10-15 pairs (Parker 1997). During this survey, six active nests were recorded at this colony. However, at least three unoccupied nest sites (judged from the amount and position of white wash) were also observed.

Eagles

Active nests of five species of eagles were located during this survey. Numerous nests of Wahlberg's Eagle were also located, but were not active since this species only commences breeding in August-September (Steyn 1982). Nine African Fish Eagle nests were located along either rivers or water reservoirs. On the Mbuluzi River, four nests were situated along approximately 65 km of river (or 16 km per nesting pair). The Sand River, Van Eck and Nisela dams all supported single pairs, despite vast differences in size of the reservoirs.

A total of four active nests of Martial Eagles were located during this study. In addition, an old nest (which had been active up until 1998, personal observation) was sighted. A further nest, possibly belonging to this species was recorded from Hlane; however, since this nest was not in use the owner could not be positively identified.

Only two active Bateleur nests were located. This species builds a small nest below the canopy and as a result other nests may have been overlooked. Two African Hawk Eagle nests (one active and one inactive) were located. Due to the immense structures built by this species, nests are unlikely to have been overlooked.

Other species

Two Giant Eagle Owls were located incubating in what appeared to be Wahlberg's Eagle nests. This species is rarely recorded in Swaziland (Parker 1994), and only a single nest had been recorded prior to this survey.

Fifteen Marabou stork nests were recorded at Hlane, of which two had already failed by the time of the survey. The other 13 nests contained either eggs or young chicks. This is the only known colony to breed regularly south of the Limpopo River.

A single Goliath Heron nest was recorded on the Mlawula River. This is the only known nesting location of this species in Swaziland.

Distribution of nests

Possibly the most interesting results obtained from this survey are related to the nesting distributions of vultures and, to a lesser degree, eagles. Although a large number of properties were surveyed, vultures (and most eagles) nested only on those where wildlife is actively protected. This was most obvious in situations where two properties were contiguous, with similar habitat but different land use. For example, the Ndukuyamangedla Royal Cattle Ranch lies adjacent to the Nzotho section of Hlane National Park, and is identical in terms of vegetation structure; furthermore, the Mlawula River (together with the associated riparian zone) runs through both properties. The latter area supports one of the highest densities of vultures in Africa, while not a single vulture breeds on the former property. The only difference between these two properties is the fact that wildlife is actively and effectively protected at Nzotho. Similar results were obtained from other areas as well (Table 2), suggesting that this is not a local phenomenon (with a local cause). These results are best visualized as GIS maps of nesting sites overlaid over the various properties. This will be one of the outputs of this study (see below).

Table 2: Number of active vulture and eagle nests recorded from properties (with appropriate habitat) under different land use systems.

 *These figures are estimates; actually figures to be calculated at a later stage

Outputs

• Scientific publications detailing the nesting distribution and conservation status of vultures and eagles in Swaziland.
• GIS distribution maps overlaid on the protection worthy areas map of Swaziland.
• Database of GPS coordinates of all known vulture and eagle nesting sites.

Acknowledgements

Etienne Oosthuizen generously offered his plane and pilot skills to conduct the aerial census. Mickey Reilly (Big Game Parks) arranged free accommodation at Hlane National Park for the pilot and myself for the duration of the study. The Swaziland National Trust Commission managed the funds. My sincere thanks go to Dr. David Garcelon (Institute for Wildlife Studies) for funding this survey.

References

Esterhuizen, J. 1995. Swaziland vulture and raptor census. Unpublished report to the Endangered Wildlife Trust-Vulture Study Group. 

Kemp, A.C. & Kemp, M.I. 1975. Observations on the White-backed Vulture Gyps africanus in the Kruger National Park, with notes on other avian scavengers. Koedoe 18: 51-68. 

Monadjem, A. 2001. Observations on the African White-backed Vulture Gyps africanus nesting at Mlawula Nature Reserve, Swaziland. Vulture News 45: 3-10. 

Mundy, P.J. 1982. The comparative biology of southern African vultures. Vulture Study Group, Johannesburg. 

Mundy, P., Butchart, D., Ledger, J. & Piper, S. 1992. The vultures of Africa. Acorn Books, Randburg. 

Parker, V. 1994. Swaziland bird atlas 1985-1991. Websters, Mbabane. 

Parker, V. 1997. The status of vultures in Swaziland and Mozambique. In Boshoff, A.F., Anderson, M.D. & Borello, W.D. (Eds). Vultures in the 21st century. Proceedings of a workshop on vulture research and conservation in southern Africa. Vulture Study Group, Johannesburg. 

Pennycuick, C.J. 1976. Breeding of the lappet-faced and white-headed vultures (Torgos tracheliotus Forster and Trigonoceps occipitalis Burchell) on the Serengeti Plains, Tanzania. E. Afr. Wildl. J. 14: 67-84. 

Steyn, P. 1982. Birds of prey of southern Africa. David Philip, Cape Town. 

Tarboton, W.R. & Allan, D. 1984. The status and conservation of birds of prey in the Transvaal. Transvaal Museum, Pretoria. 

Whateley, A. 1986. Response of Whitebacked Vultures (Gyps africanus) to environmental change in the Hluhluwe and Umfolozi Game Reserves, Natal. Lammergeyer 37: 5-10.

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