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The regional population of Denham's Bustard Neotis denhami is suspected to be undergoing a decline of greater than 30% over a three generation period (50.5 years) which qualifies the species as regionally Vulnerable.
Denham's Bustard has a wide but fragmented Afrotropical range, occurring in a band stretching from Mauritania to Ethiopia, and southwards through Kenya, Tanzania, southern Democratic Republic of the Congo and Zambia to northern Botswana; it is a non-breeding visitor to Angola and Congo (see also Taxonomy). Within the region, an isolated population occurs widely but sparsely over much of mesic eastern half of South Africa, from the Overberg in Western Cape through Eastern Cape and KwaZulu-Natal to the high-lying grasslands of Mpumalanga, with an outlying sub-population in Limpopo Province.
In Western Cape, Denham's Bustard can be locally numerous in mosaics of cultivated pastures, agricultural croplands and natural vegetation with clear seasonal differences in the use of each habitat type (Allan 2002). In Eastern Cape, Clancey (1972) described its range as the coastal and sub-coastal belt, extending to the Karoo Midlands, and noted that it occurred seasonally on the upland grassveld of East Griqualand. The species is uncommon in Free State, occurring only in the extreme north-eastern Highveld of that province (Maclean 1957, Herholdt 1988).
Denham's Bustard is widespread in KwaZulu-Natal, where it is found in both upland grasslands and low-lying coastal grasslands of north-eastern Zululand and adjacent Mozambique (Cyrus and Robson 1980). The sour grassveld areas of Mpumalanga, from Wakkerstroom to Dullstroom, and westwards to the border of Gauteng, hold several populations. At the northern extremes of its range, fragmented populations persist in the Graskop area above the Blyde River Canyon, as well as on the grassy plateau of the Waterberg in Limpopo Province (Kemp et al. 1985).
It avoids the dry western and central parts of South Africa, and occasional (erroneous) claims from the Karoo are attributable to the similarity of Ludwig's N. ludwigii and Denham's bustards (Herholdt 1988); in reality the latter occurs there only as a vagrant or along ecotones where the Karoo meets the Grassland and Fynbos biomes (Allan 2002).
The species occurs marginally in Swaziland, where it is restricted to the western Highveld regions, with breeding records from Malolotja National Park. It is considered a non-breeding vagrant to Lesotho (Osborne and Tigar 1990). Birds have been reported moving to lower-lying areas during winter, but this has not been confirmed (Herholdt 1988).
The global population has not been quantified (BirdLife International 2014) and no current regional estimate exists. Tarboton et al. (1987) estimated there to be less than 300 individuals in the former Transvaal Province. Brooke (1984) provided a figure of between 100-200 birds in Eastern Cape although Allan (2002) mentioned that the species is rare or absent from most of the former Transkei with a small population persisting at Luchaba Nature Reserve near Mthatha (Quickelberge 1989). Allan (2002) provided a population estimate of 956 birds in the ‘southern Cape'. Boycott and Parker (2003) estimated that 10-30 breeding pairs occur in Malolotja National Park, Swaziland. Taking this into account, Allan (2002) estimated the regional population at less than 5 000 mature individuals.
Density varies according to habitat. Mendelsohn (1978) reported a density of 0.05-0.08 birds/km2 in the Springbok Flats area of Limpopo Province, while Tarboton (1989) recorded a density of 0.78 birds/km2 in the Steenkampsberg near Dullstroom, Mpumalanga. Barnes (1998) provides a density estimate of 0.5-1.0 birds/km2 while Allan (2002) reported winter density estimates of 0.04 birds/km2 for the Overberg district of Western Cape, one of the remaining strongholds of this species. A comparison of the EoO between the two atlas periods reveals a slight increase of 1.4% (to 899 620 km2) but the AoO has decreased alarmingly by 39.26% to 117 451 km2.
Declines in the species' range, and correspondingly regional populations, have been reported from Kenya, Sudan and Nigeria (del Hoyo et al. 1992). Within the region, several authors make mention that the species has decreased in numbers and range (Herholdt 1988, Tarboton 1989). The exact rate of decline is unknown although the species may be decreasing at a rate of greater than 30% over three generations. The confidence in this regional population trend estimate is low.
Denham's Bustard faces a wide range of threats in the region. Collisions with power-lines have been identified as a major threat to the species (Shaw et al. 2010). Habitat loss through commercial afforestation of the sour grasslands of the former Transvaal and western Highveld regions of Swaziland (Allan et al. 1997, Allan 2002, Monadjem et al. 2003), as well as the midlands of KwaZulu-Natal (Johnson 1992) is a moderate threat. Monoculture and intensive crop farming was considered to constitute a threat by Herholdt (1988) and Tarboton (1989). Overgrazing in Swaziland has also been identified as a cause of habitat loss impacting upon this species (Monadjem et al. 2003). Hunting has been cited as a cause for the decline of some populations (Quickelberge 1989) and Mendelsohn (1978) proposed that snaring is a specific threat to Ludwig's and Denham's bustards.
The species is listed under Appendix II of CITES. It is also listed under Schedule 2: Specially Protected Wild Animal under the Limpopo Environmental Management Act No 7 of 2003, and as Endangered under the Swaziland Game Act. Following a strategic workshop in 2009, the Bustard Working Group was established with the support of Birdlife South Africa, to assess the growing cause for concern over the conservation of South Africa's ten bustard and korhaan species. Valuable population data is generated through biannual censuses conducted via the Coordinated Avifaunal Roadcount (CAR) Project.
The rate of decline of the regional Denham's Bustard population is unknown and at risk of being undocumented. Slow, inadequately monitored declines over long periods tend to go unrecognised with a result of inadequate conservation measures being put in place. Improvements in the protected area network, ostensibly through biodiversity stewardship models, within Mpumalanga and KwaZulu-Natal provinces would assist in ameliorating the impacts of habitat loss within its core range. Improved management of fires and grazing regimes should be considered in conservation areas and on private land, in order to create heterogeneity in the structural and floristic components of preferred habitats. Work being undertaken by stakeholders in the power transmission sector must continue to focus on abating the threat of collisions to this species.
* An investigation into the taxonomic status of the species, with reference to its isolation from populations in East Africa needs to be undertaken.
* An assessment (including scenario modelling and directed research) of the relative importance of the multitude of threats impacting this species is required before prioritising actions to address key threats.
* Following on the above, development and implementation of mitigation measures against such threats is required. Particular attention should be paid to collisions with overhead lines, the future impacts of climate change and the impact of habitat destruction and degradation, especially from crop farming.