Denham’s bustard is one of the largest bustard species, being only slightly smaller than the kori and Arabian bustards. It is larger than the Nubian bustard, recognized by a sleeker, appearance, much darker upper parts and characteristic slender neck, which is pale grey in front, pale orange at the back linking to an orange nape (see photos). Adult males can weigh over 14kg during the breeding season but drop to about 10kg in the non-breeding season; females are about half the weight of an adult non-breeding male. In the adult male, upper parts are dull brown with fine, black vermiculation. The tail has broad uneven black and white barring at the corners. The upper wing displays a large panel of uneven and broadly blotched black and white, visible on the folded wing and prominent in flight, when it distinguishes Denham’s bustard immediately from the finer grey and white barring on Arabian bustard flight feathers. The crown is grey bordered by black with a narrow black line running from the corner of the bill around the ear coverts.
The chin is white, cheeks, neck and breast are grey, neatly demarcated from the white belly. Females are smaller with grey feathers replaced by more buff colored feathers, more patterned upper parts and a finely barred foreneck. In the field, these subtle differences may not be so apparent. The Niger Bird Data Base (NiBDaB, 2012) has Denham’s bustard (80-100cm) much more slender than the “bull-necked” Arabian bustard (80-100cm). Back and wing coverts are darker than the Arabian and the white spots on the closed wing are bigger. Nubian bustard (50-70 cm) is smaller than the other two species, is bright brown on the back, and does not show white in closed wings. In flight, Arabian and Denham’s bustards show large amounts of white in their wings. The upper wings of Denham’s are more contrasting (white and black and dark brown) than those of Arabian Bustard (white and grey and clear brown). The secondaries of Denham’s are almost black, those of Arabian Bustard are white and grey. In contrast, the Nubian only has white at the base of its primaries, accentuated by the black ends to primaries, the greater primary coverts and the secondaries.
The species demonstrates substantial seasonal movement across its northern range, although details are not well understood. In the Sahel, birds move north between April and August in response to rainfall, when pairs and small groups may be seen searching for prey alongside Arabian bustards. In central Chad they are widespread across the southern grasslands of the Ouadi Rime-Ouadi Achim Game Reserve (up to about 16°N) in September, but typically none are found there by December.
Observations in West Africa support the idea of breeding in sub-Sahelian savanna areas during the local wet season (February-May), with a subsequent move northwards with the rains into the true Sahel for part of the population. By October the birds again move southwards but exact movements are poorly known and would greatly benefit from a programme of satellite-based monitoring.
Where populations are strong, the mating system is said to be polygynous, with males occupying individual territories but in a dispersed lek-like system. In low density populations, males and females may form pair-like associations. Male display consists of bending the head over the back and puffing out the white breast feathers while standing or walking accompanied at times by a bouncing motion. A booming call is emitted in this posture. Breeding season reported from Côte d’Ivoire and Sierra Leone in February-March, Nigeria in May, and December-February in northern Congo. Clutch size is 1-2 eggs in a shallow scrape in the ground. Incubation and parental care is by the female only.
Like most bustards, Denhams’ feed on a variety of insects, small vertebrates and plant material. They have been recorded wading in shallow pools for aquatic animals and/or plants.
In its Sahelo-Saharan range, Denham’s bustard occupies open grasslands and lightly-wooded Acacia savanna. They appear especially fond of areas of dense mature seasonal grassland, a habitat that often abounds with crickets and grasshoppers.
Three subspecies occurring in three non-contiguous populations are recognized: N.d. denhami (Denham’s bustard) found from Guinea and Mauritania across to Ethiopia and northern Zaire, N. d. jacksoni (Jackson’s bustard) from Angola and southern Zaire east to Kenya and south to Angola, Botswana and Malawi, and N.d. stanleyi (Stanley’s bustard) from Botswana and South Africa.
Recent evidence from Chad indicates the Ouadi Rime-Ouadi Achim Game Reserve is an important wet season dispersal area for the nominate ssp., with densities between 0.16-0.25 birds/km² over substantial areas (Wacher et al., 2012).
Although widely distributed, Denham’s bustard has suffered population declines through much, if not all, of its range. Hunting is thought to be the primary cause of declines across the Sahel and throughout West Africa. The Rift Valley in Kenya was formerly regarded as a stronghold of the jacksoni ssp., but there are now probably fewer than 300 birds in Kenya. In eastern and southern Africa, hunting is also a problem, but the main threat in these areas appears to be conversion of grassland and light woodland to agriculture. Powerline and fence collisions in southern Africa take a heavy toll on birds.
The IUCN Red List of Threatened Species (2012) lists Denham’s bustard as Near Threatened as the species is estimated to be undergoing only a moderately rapid population decline across its range. In light of recent surveys, BirdLife International is leading efforts to see the species’ status evaluated and upgraded. This said, surveys in Chad in September 2011 (Wacher et al., 2012) found evidence of more than 400 birds in a sample of 6% of the 77,360km² Ouadi Rimé-Ouadi Achim Game Reserve. With at least half the reserve containing suitable habitat estimates could be as high as 1500-2000.
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