|Range:||Botswana, Namibia, South Africa, Zimbabwe|
|Area of occupancy:||237,369 km2|
|Population estimate in Namibia:||c 380-530 pairs, or (maximum)1400 birds|
|Population trend:||50% decline in last 3 generations|
|Habitat:||Mopane, Kalahari and arid savanna woodlands|
|Threats:||Poisons, declining prey base, drowning farm reservoirs, vehicle collisions|
Distribution and abundance
Click to see distribution map
This species is widely distributed across West and East African woodland savannas, in nw Africa and as far east as India (del Hoyo et al. 1994) . It is absent from Africa's tropical rainforests and appears again in central-s Africa, extending into arid Karoo regions of South Africa (Brown et al. 1982, Boshoff et al. 1983).
From historical records pre and post 1970 it is clear that this eagle has decreased in numbers in Namibia (Brown 1991), South Africa (Boshoff et al.1983), and has largely shrunk into protected areas such as Etosha NP, Chobe NP and Kruger NP (Steyn 1982, Tarboton & Allan 1984, Simmons 1997). However it is more widespread than the Bateleur occurring in small numbers in both Namibia and South Africa outside protected areas, and as far s as 32o S in the grassy Karoo (Simmons 1997). In Namibia it is largely absent from the south and west, thinly spread in eastern farmlands, more frequently seen in central farmlands (reporting rate ~ 20%), and in Etosha (reporting rate > 27%) and spottily distributed through n Namibia and the Caprivi (Simmons 1997). The area of occupancy in Namibia is 237,369 km2 (Jarvis et al. 2001).
It has been estimated that about 5,000 prs occur in southern Africa (Simmons 1997), and 800 prs occur in South African populations (Barnes 2000), but Namibian populations are un-estimated (but see below). Population in s Mozambique number c 20 prs (Parker 1999).
Nesting density varies according to soil type from 0.83 prs/100 km2 on Kalahari sands in Zimbabwe to 1.2 prs on mica-schist farmland in Namibia (Hustler & Howells 1989, Brown 1991). In Etosha NP, 21 nests sites are known in 11 400 km2 of suitable habitat, a density of only 0.2 pr/100 km2 (T Osborne unpubl data). Outside conservation areas density is similar at c 0.2 -1.0 pr/100 km2 in South Africa (Tarboton & Allan 1984). From these figures we can estimate both the extent of the decline in population and the likely present population in Namibia.
In commercial farmlands they are heavily persecuted and in one study declined 89% in less than 10 yr (Brown 1991). The area of occupancy on commercial land is 104,411 km2 (Jarvis et al. 2001), and at a previous (1983) density of 1.2 prs/100 km2 (Brown 1991) the population in this area would have totalled about 1250 prs. At present densities of 0.24 prs/100 km2 (Brown 1991), the population will be about 250 prs. In Namibian conservation areas (where Tawny Eagles have an area of occupancy 31,728 km2 : Jarvis et al. 2001), and a density of 0.2 prs/100 km2 (Osborne et al. unpubl data) populations number about 63 prs. The remaining non-conservation areas (area of occupancy of 102,000 km2) holding birds at a density of 0.1 - 0.24 prs/100 km2 (Brown 1991, Osborne et al. unpubl data) holds 102 - 250 prs. Thus Namibia holds a maximum of 416 - 560 prs of Tawny Eagles (maximum ~1510 individuals).
Assuming (probably unrealistically) that no declines have occurred on communal lands or in conservation areas, the total population in 1983 would be 1,532 prs compared with today's population of (maximum) 560 prs - a decline of 63% in the last 2 decades.
Found mainly in open woodland savanna especially Mopane (34% reporting rate) and dry and mesic Kalahari woodlands (reporting rates average 22%), rarely found in Miombo woodland (Tarboton & Allan 1984, Simmons 1997). Its presence in the largely tree-less Karoo, and other grassland areas is explained by the fact that birds have adapted to man-altered environments, breeding on pylons and in tall alien trees (Boshoff et al. 1983, Tarboton & Allan 1984). Where it breeds, large Knob-thorn Acacia nigrescens are often used (92% of 88 nest trees in ne S Africa) in preference to most other trees (Tarboton and Allan 1984). On pylons they also chose the top-most cross bar (Tarboton & Allan 1984, Barnes 2000).
Takes a very wide spectrum of prey including scavenged carcasses, especially bones of plains and woodland ungulates (eg kudu and duiker) live-caught mammals, birds, reptiles, amphibians and fish. In South Africa's lowveld reserves were more large game are available 65% of the prey were scavenged bones of ungulates. The remainder comprised 63% birds, 34% mammals and 3% reptiles (Tarboton & Allan 1984). Tawny Eagles are known to take nocturnal mammals such as Spring Hares and Genets and an observation of a Tawny Eagle drinking in full moonlight suggests they are opportunistically active at night to take such prey (Steyn 1982). There are no published diets for Namibia. Egg-laying records for Namibia show a peak in May (16) and June (12), with a spread from March - August (Brown & Clinning unpubl data). Lays 1-2 eggs (Tarboton 2001) with c/2 (n = 8) being more common than c/1 (n=3) in Namibia (Jarvis et al. 2001). Nestling productivity in Namibia is poorly known but averages about 0.65 - 0.78 y/pr/yr elsewhere in southern Africa (Steyn 1982, Tarboton & Allan 1984,); c/2 nests are more productive (0.81 y/pr/yr) than c/1 nests (0.50 y/pr/yr: Tarboton & Allan 1984).
The scavenging and hunting behaviour of this species makes it particularly vulnerable to the whim of irate farmers who may lose livestock to Tawny Eagles. Consequently, it suffers poisoning at both large and small carcasses (Steyn 1982, Tarboton & Allan 1984, Brown 1988) and from direct persecution (shooting and gin trapping) on central Namibia farmland (Brown 1991). Birds found poisoned were located up to 18 km from their active nest sites (Brown 1991), thus like Bateleurs they are at risk from even a small proportion of farmers who abuse strychnine in Namibia (Brown 1991). This is also apparent for wide-ranging eagles in South Africa (Davies 1988). Farmers frequently use poisons on the edge of conservation areas such as Etosha NP (Komen 2002) regularly killing vultures and some Tawny Eagles which undoubtedly came from within the park (Bridgeford & Simmons unpubl data, T Osborne unpubl data).
Other causes of mortality include drowning in steep-sided farm reservoirs where it is the second most frequently killed eagle in arid areas of South Africa (Anderson et al. 1999), and collisions with motor vehicles when they are apparently feeding on road-killed carrion (Oatley et al. 1998).
Obvious mortality factors such as poisons may not account for all reductions in population density, despite the good data from Namibia. Reduced wildlife populations in areas now farmed for cattle, also sees less dense Tawny Eagle populations, despite poisons rarely being used in this form of farming (Brown 1988). This suggests that the reduction of large ungulates from intensively cattle-farmed areas, reduces food available for Tawny Eagles ultimately reducing their numbers. Less intensive farms however, probably retain large kudu populations.
Pesticides (DDT residues) that reduce breeding success in some eagles (Davies & Randall 1989) but this has not been investigated in this species. Human interference accounted for 45% of 20 known failures in ne South Africa due to collecting of eggs or chicks (Tarboton & Allan 1984).
This species is classified as Endangered because of a suspected decline in the Namibian population of at least 63% in the last 20 years (above). Given that 3 generations in Tawny Eagles is about 15 years (Steyn 1982), the rate of decline is about 50% in 3 generations. The Tawny Eagle also has a population of about 1,400 individuals in Namibia, which given its decline also qualifies it for the Endangered category. As a long-lived species, that produces less than 1 fledgling/yr (Steyn 1982, Tarboton & Allan 1984), this species like other slow-maturing, slow-breeding species will probably decline for years to come under the high frequency of poisoning. It is not common in large protected areas such as Etosha NP (T Osborne unpubl data) so conservation areas alone are insufficient to protect it. It is attracted to vulture restaurants (M Diekmann pers obs) and this may reduce the likelihood that some individuals will pick up poisoned baits, especially within conservancies where other prey types are available.
It is not classified as globally threatened (Stattersfield & Capper 2000), but in South Africa it is classified as Vulnerable because it is thought to have lost 20% of its population in the last 3 generations through poisoning and direct persecution (Barnes 2000).Given the declines in Namibia this may be an under-estimate and require re-appraisal.
Decreasing the frequency of abuse of poisons is one essential ingredient in preventing further population declines in all scavenging species. There are opposing viewpoints on the way forward in Namibia where poison use is high (Komen 2002). One proposal is to ban completely the poisons used for killing small carnivores from the Namibian environment (Brown 2002). This view advocates that (i) it sets a standard that using poisons for killing predators is unacceptable and not in the national interest (ii) it will cause the thinking farmer to choose other, more environmentally friendly methods; and (iii) when poisons were banned in western Europe vulture populations showed a remarkable recovery. The opposite view is that the total ban is (i) impractical (ii) threatens the agro-chemical industry and (iii) is too idealistic to work (Verdoorn & Komen 2002). Concentrating on stopping abuse and working with farmers to apply poisons responsibly, is seen as the only way forward. In Namibia it is apparent that awareness campaigns and farmer education programmes have not stemmed the tide of poisonings, because of the continuing high rate of scavenging birds that are poisoned each year (Bridgeford 2002, Bridgeford & Simmons unpubl data). The critical point is that all farmers need to be convinced of the need for sound farming and poison-free methods, given that it is estimated that 0.1% of the farming community is responsible for the decline of scavengers (Brown 2002).The draft Parks & Wildlife Management Bill (2002) will ban the use of poisons in predator control, and permits will be required from the Ministry of Environment & Tourism in order for farmers to use poisons in exceptional cases. The Tawny Eagle is protected under this Bill.
A research monitoring programme is required to gauge (i) the success of awareness programmes where groups such as NARREC and REST and other NGOs are intensively targeting farmers (ii) the success of the poison ban on scavenging raptor populations in selected parts of Namibia, and (iii) the density and breeding success of prs inside and outside conservation areas in Namibia. Encouraging vulture/eagle restaurants where Tawny Eagles are still present on central farmland and in conservancies where ungulates are re-introduced will assist the repopulation of such areas once other limiting factors such as poisons are removed.
From: Simmons RE & Brown CJ 2006. Birds to watch in Namibia: red, rare and endemic species. National Biodiversity Programme, Windhoek, Namibia
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