PAPER: African Harrier-Hawk biology2014-12-01 (600)
Smeenk C, Smeenk-Enserink N. 1983. Observations of the Harrier Hawk Polyboroides typus in Nigeria, with comparative notes on the neotropical Crane Hawk Geranospiza caerulescens. Ardea 71:133-143.
Abstract. In 1975 and 1976 we studied a pair of the Harrier Hawk Polyboroides typus in Pandam Wildlife Park, in the Guinea savannah of Nigeria. The breeding season coincided with the end of the dry season and the beginning of the rains. In the moister, more southern parts of West Africa the breeding season generally is earlier than in the more arid north, in agreement with the later onset of the rains at higher latitudes. Display occurred mainly in January and February and consisted of an undulating flight. The birds were quite noisy during the whole breeding season, especially near the nest. Characteristic was the 'blushing', in which the yellow facial skin turned deep red or pink; blushing was mainly connected with reproductive activities such as nest-building, copulation and exchange of prey. Copulation took place from late January till the completion of the clutch. Throughout the breeding season the female probably was fed by the male, but also hunted herself to some extent. Nest-building started late January and lasted about a month. Both birds participated in building. The two eggs were laid in the first week of March. The female did most incubating; the male was thought to have incubated one night, which is very rare among birds of prey. The incubation period lasted 37-38 days. After the loss of the chicks in 1976 two replacement clutches were produced. At the age of one week, the elder chick was very aggressive toward the younger; the growth of the latter had retarded; it was badly wounded and eventually succumbed. The first downy plumage was orange-buff, the head was decorated by a crest of long, silky feathers. The second down was white; at about six weeks the chick was completely feathered; the juvenile plumage is mainly brown. The young left the nest at about two months, but remained associated with its parents for another seven months. The Harrier Hawk hunts primarily in trees, extracting most of its food from holes and crevices, decaying wood, bird nests, etc. Foraging birds negotiate the trees by walking, clambering and clinging vertically or upside-down to stems and branches, keeping their balance with the aid of wings and tail. At Pandam the food largely consisted of lizards and birds, including nestlings. Polyboroides is best known for its habit of raiding weaver colonies; during this activity the birds cling to the nests, hanging upside-down and extracting the young weavers with their legs or beak. Young birds probably form a more important source of food during the rainy season further north than in the moist southern savannas. Polyboroides shows anatomical peculiarities which are to be regarded as adaptations to its hunting techniques: the broad wings and tail, and especially the long legs with very narrow joints; the intertarsal joint is able to over-extend by some 40 degrees. The neotropical Crane Hawk Ceranospiza caerulescens is very similar in this respect; Polyboroides and Ceranospiza therefore are sometimes regarded as close relatives. The genera, however, differ in a number of other characters not directly related to hunting methods, such as the covering of the legs and facial skin, downy and juvenile plumages, voice and colour pattern of the egg shell. The anatomical similarities therefore are the results of convergence rather than of relationship. Polyboroides possibly is related to Melierax; Ceranospiza is likely to be of Buteonine stock.
Although dated, this is one of the most detailed studies of the African Harrier-Hawk (or Gymnogene). The authors describe the biology, hunting and breeding biology of this species. The study site was in the southern part of Pandam Wildlife Park in Plateau State, Nigeria. Most of their observations were made on one breeding pair, from February 1975 and June 1976.
There was a colony of Village Weavers a few hundred metres from the nest, but it was not raided often.
The African Harrier-Hawk robs the nests of many weaver species; for a list see here.
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