African Marsh Harrier
Circus ranivorus
Status: Endangered
Range:n Botswana , n Namibia , South Africa , Zimbabwe
Area of occupancy:15,260 km2 including 2,074 km of border rivers
Population estimate in Namibia:600 birds
Population trend:Declining
Habitat:Reed beds, floodplains, lake margins
Threats:Wetland degradation, fires

Distribution and abundance


Click to see distribution map

This species is endemic to tropical wetlands of eastern and southern Africa, and is inexplicably absent from the West African and Ethiopian wetlands (Brown et al. 1982) where other migrant harriers occur (Simmons & Simmons 2000)

The bird's stronghold in southern Africa is the Okavango Delta where reporting rates are > 13% (Simmons 1997) but population figures are difficult to gauge. Populations in s African are thought to have declined with numbers in South Africa estimated at 3,000-5,000 prs (Barnes 2000). This figure may need revising given that core reed beds recently surveyed in the Okavango Delta yielded a minimum of 300-400 birds in 2002 (S Tyler unpubl data). However, the only really accurate figures are 500-1,000 prs estimated from the wetlands of the former Transvaal (Tarboton & Allan 1984) and high densities of 8 prs/10 km2 in the s Cape Province (Simmons 1997). Where wetlands have been drained or converted to agriculture the bird has disappeared; alarmingly it has also disappeared from many Gauteng wetlands which are still intact (W Tarboton pers obs). Its population may naturally fluctuate in core areas as rodent populations and wetland flooding fluctuate with rainfall.

In Namibia it is found only in the n-e parts including the Kavango River, and the Zambezi-Chobe, and Kwando-Linyanti systems, with an area of occupancy of 15,260 km2 (Simmons 1997, Jarvis et al. 2001). The largest concentrations have been recorded in the Mahango Reserve (8 birds) and the Salambala Reserve, Chobe River floodplain (9 birds). Birds are vagrant to the Orange River wetlands, Sandwich Harbour wetlands (Berry & Berry 1975), and the cuvelai wetlands of n Namibia (Simmons 1997).

Density of birds along various riverine wetlands probably under-estimates true numbers but have been recorded as follows: Chobe River (4 birds/10 km: Robertson et al. 1998), Zambezi River (1 bird/10 km), Kwando River (3 birds/10 km) and Kavango River (8 birds/10 km); data in Jarvis et al. (2001). Given river lengths (respectively) of 185 km, 155 km, 340 km and 470 km (Mendelsohn et al. 2002), the population in Namibia numbers c 570 birds. Populations may number higher than this when wetlands such as the eastern floodplain of the Zambezi and Lake Liambezi are inundated.

Ecology

Prefers tropical wetlands, including floodplains, reed beds and lake margins, where it both forages and breeds. Rarely found in drier habitats although breeding birds are found occasionally in short sedges, fynbos, and even wheat stubble (Steyn 1982, Simmons 1997, Kemp & Kemp 1998).

As the world's smallest marsh harrier this species is a buoyant flier, flap-sailing at 1-3 m over wetland vegetation. It hunts mainly small mammals (74% frequency: Kemp & Dean 1988, Simmons et al. 1991), but other prey include passerines and waterbirds (23% of 374 items) frogs (2%) and fish (1%: Simmons & Simmons 2000). Raids mixed heron and egret colonies and weaver nests to eat eggs and chicks (Steyn 1982, Simmons unpubl data). Foraging success for mammals is significantly higher (41% of 58 attempts) than success chasing birds (13% of 24 attempts).

Egg laying peaks in southern Africa in September (38% of 123 records) extending over 120 d in any one area (Simmons & Simmons 2000). Virtual year round breeding can occur in n South Africa (Tarboton & Allan 1984). It builds nests in reeds or on the ground (Steyn 1982), laying 2-5 eggs (mode 3), but rears few fledglings. In South Africa only 1.1 - 1.6 y/pr/yr were reared in 2 studies, nest failure accounting for 52% of 25 nests in the Cape Province and 30% of 10 nests in the former Transvaal (Tarboton & Allan 1984, Simmons & Simmons 2000). There are no breeding records for this species from Namibia (Jarvis et al. 2001), although there is no doubt that it does breed here.

Threats

Suffers from habitat degradation through high human population pressure along Namibia's northern rivers. For example, human populations average 50 - 100 people/km2 along the Kavango river banks and cattle, which number over 120,000 head in East Caprivi, graze and trample floodplain wetlands (Mendelsohn & Roberts 1997). With people pressure comes the use of fire. Harriers are especially susceptible to fires that ravage Namibia's northern regions throughout its main breeding period from August to December. By the end of 1996 about 3,000 individual fires had burned about 60% of the entire surface area of Caprivi (Mendelsohn & Roberts 1997). Wetland degradation (rather than loss) is the most insidious factor diminishing population size in South Africa since many intact wetlands no longer support harriers (WR Tarboton pers comm). This may arise from a lack of prey or disturbance given that harriers intolerant of human disturbance during breeding and desert nests at an early stage (pers obs).

Pesticides such as DDT and Dieldrin are sprayed commonly in East Caprivi (Schlettwein et al. 1991) and DDT residues are known to occur in harrier eggs (de Kock & Simmons 1988). However, there was no apparent influence on the output or adult survival of the same population in s South Africa (de Kock & Simmons 1988). Spraying of moluscicides in core nesting habitat of the Okavango waterways occurs but has unknown effects on bird populations. This harrier's propensity to scavenge leaves it open to direct poisoning which has been recorded once in South African wetlands killing a brood and the adult female (RE Simmons unpubl data). All are unquantified threats to this species in n Namibia.

Conservation status

This subspecies is classified as Endangered because of its small population of about 600 birds, which is thought to have declined in the last 20 years through loss of breeding habitat through the frequent use of fires in ne Namibia and wetland degradation from cattle farming (Mendelsohn & Roberts 1997). Like the Black Harrier, African Marsh-Harrier have recently been elevated to a species of conservation concern in South Africa following the realisation that its wide distribution hides the very low and decreasing density at which it occurs (Barnes 2000). It is now classified as Vulnerable there due to a suspected 20% decrease and loss of wetlands (Barnes 2000). Population size is almost certainly lower than the 3,000-5,000 prs estimated for South Africa given its low numbers in the core Okavango Delta (S Tyler unpublished data), and the loss of birds from core wetlands in the former Transvaal (W Tarboton pers obs). It is not listed as globally threatened (Stattersfield & Capper 2000). The entire East Caprivi wetlands have been designated one of Namibia's 21 Important Bird Areas, (Simmons et al. 2001) although this does not confer any legal protective status to the wetlands.

Actions

Given that cattle farming and human population pressure in Caprivi is likely to increase in future years (doubling every 19 yrs: Mendelsohn & Roberts 1997) the best action to conserve wetlands in n Namibia is to formally conserve the larger wetlands of Caprivi as national parks such as the new Babwata NP; this should include transfrontier parks which link Namibia with the Chobe NP in Botswana (Barnard et al. 1998). Concurrently the conservancy programme of the Ministry of Environment & Tourism should promote further conservancies such as Salambala which border important wetlands such as the Chobe (Mendelsohn & Roberts 1997). African Fish Eagles and African Marsh Harriers can be used as indicator species to monitor the health and progress of this action. Research programmes should assess the breeding success, population density and pesticide uptake of this species in selected areas of Caprivi. The use of fires for farming purposes should also be curtailed as it ultimately reduces species diversity and forces numerous wetland-dependant birds and mammals out of their habitats.

From: Simmons RE & Brown CJ 2006. Birds to watch in Namibia: red, rare and endemic species. National Biodiversity Programme, Windhoek, Namibia

References

Barnard P, Brown CJ, Jarvis AM, Robertson A, van Rooyen L 1998. Extending the Namibian protected area network to safeguard hotspots of endemism and diversity. Biodiversity and Conservation 7: 531-547.

Barnes KN (ed) 2000. The Eskom Red data book of birds of South Africa, Lesotho and Swaziland. De Kock AC, Simmons R 1988 Chlorinated hydrocarbon residues in African Marsh Harrier eggs and concurrent reproductive trends. Ostrich 59:180-181.

Jarvis A, Robertson AJ, Brown CJ, Simmons RE 2001. Namibian Avifaunal Database. National Biodiversity Programme, Ministry of Environment & Tourism, Windhoek.

Kemp A, Dean R 1988 Diet of African Marsh Harriers from pellets. Gabar 3:54-55

Kemp A, Kemp M 1998 Sasol Birds of prey of Africa and its islands. New Holland London UK

Robertson T, Jarvis A, Simmons R 1998. Game bird population sizes and quotas in the Salambala Conservancy. Unpublished report, Ministry of Environment & Tourism, Windhoek.

Schlettwein CHG, Simmons RE, Macdonald A, Grobler HLW 1991. Flora, fauna and conservation of East Caprivi wetlands. Madoqua 17: 67-76.

Simmons RE 1997 African Marsh Harrier. In: Harrison JA, Allan DG, Underhill LG, Herremans M, Tree AJ, Parker V, Brown CJ (eds). The Atlas of Southern African Birds. Vol. 1:236-237. BirdLife South Africa, Johannesburg

Simmons RE, Avery DM, Avery G 1991 Biases in diets determined from pellets and remains: correction factors for a mammal and bird-eating raptor. J. Raptor Res. 25: 63-67

Simmons RE, Simmons JR 2000 Harriers of the World: Their Behaviour and Ecology. Oxford University Press, Oxford.

Simmons RE, Boix-Hinzen C, Barnes K Jarvis AM Robertson A 2001. Namibia Pp 639-660. In: Fishpool LDC and Evans MI (eds) Important Bird Areas in Africa and associated islands. Birdlife International, Newbury and Cambridge, UK

Simmons RE 2005 African Marsh Harrier Circus ranivorus In: Hockey PAR, Dean WRJ, Ryan P (eds) Roberts Birds of Southern Africa VII. John Voelcker Bird Book Fund, Black Eagle Publishing, Cape Town.

Steyn P 1982 Birds of Prey of Southern Africa. David Philip, Cape Town

Tarboton, Warwick Dr personal communication (wtarbotn@iafrica.com)

Tarboton WR, Allan DG 1984 The status and conservation of birds of prey in the Transvaal. Transvaal Mus. Monogr. 3:1-115

Stattersfield AJ, Capper DR (eds) 2000 Threatened Birds of the World. Birdlife International, Cambridge.

Tyler, Stephanie Dr African Marsh Harrier counts, Okavango Delta, unpublished data (steph_tyler2001@hotmail.com)

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