Secretary birds

(Sagittariidae)

Class Aves

Order Falconiformes

Suborder Sagittarii

Family Sagittariidae

Thumbnail description
Very large predatory bird with hooked bill; long stork-like legs; bare facial skin orange; and long, black feathers forming erectile crest on nape. Pale gray and white but with black leggings and flight feathers; central tail feathers elongated

Size
49–59 in (125–150 cm); 7.5–9.5 lb (3.4–4.3 kg)

Number of genera, species
1 genus; 1 species

Habitat
Open woodland, savanna, and semi-desert steppe

Conservation status
Not threatened

Distribution
Sub-Saharan Africa

Evolution and systematics

The secretary bird (Sagittariidae) is sole member of a unique African family, a status shared only with the hammerhead (Scopus umbretta). Prehistorically, however, fossil remains of at least two secretary bird-like species with shorter legs are known from 20 million-year-old Miocene and even older Oligocene deposits in France. The special attributes of the leg morphology and karyotype of the secretary bird have always been recognized by placing it in its own family and suborder and sometimes even its own order. It is generally considered an aberrant bird of prey, in the order Falconiformes, but with some stork-like features that probably represent an earlier common ancestry with ciconiiform waterbirds, as is also shown by the New World vultures (Cathartidae). DNA-DNA hybridization studies also confirmed the diurnal birds of prey as nearest relatives. Proposed relationships with gruiform birds, such as seriemas (Cariamidae), cranes (Gruidae), or bustards (Otididae) seem to represent convergence of morphology for a terrestrial lifestyle rather than genetic relationship. Overall, the secretary bird behaves and has the skull anatomy of a large "marching" eagle, although aspects of its breeding biology are most similar to storks.

Physical characteristics

The secretary bird stands about 4 ft (1.2 m) tall on long pink legs. Stubby toes are armed with thick claws and used to kick prey into submission. Plumage of the upper parts is pale gray and of the underparts white, with the exception of the thighs and abdomen, which are black. Flight feathers on the broad wings are black, contrasting with the gray upperwing and white underwing and undertail coverts. Tail feathers are dark gray, with a broad black subterminal band and white tip; they are graduated in length from shortest on the outside to the exaggerated length of the central pair. The head is striking with a hooked, pale gray beak and broad cere; large bare areas of orange facial skin; and elongated, black nape feathers that form either a droopy crest or are erected to form a spiky halo. The iris is dark brown or yellow, although the geographical details of where this difference occurs are unclear. Sexes seem identical in size and plumage. Juveniles have a paler orange face, a brown wash to the plumage, fine gray bars across the white underwing and undertail coverts, and an iris that changes from brown to pale gray before attaining the adult color.

Distribution

The secretary bird occurs throughout sub-Saharan Africa except for areas of tropical forest along the West African coast and across the Congo River Basin. It is most common in areas of open savanna and steppe but wanders widely to open areas within woodlands of central Africa and less arid areas of southwest and northeast Africa.

Habitat

Open woodland, savanna, and steppe comprise the optimum habitat of the secretary bird. It is absent from stands of dense forest and woodland, although it may enter larger clearings, and it wanders into true desert only after exceptional rainfall. Its main requirements are a few low trees on which to roost and nest and an adequate food supply of small animals.

Behavior

In more productive areas, the secretary bird is resident and sedentary, but in areas with fluctuating conditions, it is a nomadic visitor during times of plenty. It normally occurs in pairs, each within a defended territory, at densities of 7.7 –193 mi² (20–500 km²) per pair depending on local conditions and abundance of food. At night, it roosts on top of a low tree, either standing or lying down on an old nest platform if available. At dawn, it flies to the ground to begin its daily walk across the veld. Depending on terrain and cover, it either strides briskly along and looks from side to side or shuffles slowly along and searches just in front of its feet. The normal rate is about 120 paces/min, which, with a stride of about 16 in (40 cm), covers about 2 mi/hr (3 km/hr). Prey is captured and eaten where- and whenever encountered. Members of a pair may hunt alone or together. Once satiated, the birds may rest or, using thermals in the heat of the day, rise up on broad wings, which span 7 ft (2.12 m), and soar to their nest, water, or alternative hunting areas. By dusk, they usually return to their roost site. Encounters with neighbors may lead to chases, bouts of kicking, or aerial pursuit, accompanied by deep croaking calls. For most of the day, however, the species is silent and pedestrian.

Feeding ecology and diet

Any small animals that can be kicked into submission are killed and eaten. These range in size from moths and grasshoppers to mongooses, hares, and gamebirds. Very small insects, such as termites or wasp larvae, may be picked up in the bill, but any large or active prey, including poisonous snakes, are killed after fast, active pursuit, kicking, and disablement. The bird will only stoop to pick up its prey after it is completely immobile. The exact diet depends on locality and availability: locusts and rodents predominate in one area with beetles and lizards in another. The hunting technique allows a wide range of prey types and sizes to be captured, most of which are swallowed whole or, more rarely, torn to pieces or cached under a bush for later consumption. Undigested remains are regurgitated as large pellets, 1.6–1.8 in (40–45 mm) in diameter and up to 4 in (100 mm) long, mainly below roost and nest sites, where they provide a quick indication of prey consumed.

Reproductive biology

Members of a pair share breeding duties, from building a nest of weeds, sticks, and grass on top of a tree; through incubation; to brooding and feeding chicks. Courtship includes high flights above the nest area, with pendulum-like displays of repeated diving and swooping up, accompanied by deep croaking calls. The nest is built into a stable platform of 3.3–6.6 ft (1–2 m) diameter, with a bed of dry grass in the

center to accommodate the clutch of 1–3 white eggs. Parents take turns incubating or brooding while the off-duty mate goes off to feed. Nest relief includes a greeting display. Once chicks have hatched, a cropful of food is regurgitated onto the nest floor for their consumption. Incubation takes 42–46 days and the nestling period varies from 65–106 days. Animal food is at first torn up and fed to small chicks, but within a few weeks of hatching, they are able to gulp down whole prey almost immediately. Where available, parents also swallow water before coming to the nest and dribble this into a chick's bill, along with partly digested food.

Breeding normally takes place during summer rains when food is most abundant but can occur at any time of year if prey numbers persist. Food availability seems to determine the number of eggs laid, rate of chick growth, age at fledging, and interval between broods. In some years, three broods may be attempted, in others none. A full brood of three chicks is raised only rarely since food shortages during breeding often lead to the starvation of the youngest and smallest sibling. Flexibility in breeding biology, along with nomadic movements to areas of abundant food, allow the secretary bird to be unusually productive for such a large bird in such a variable environment.

Conservation status

The secretary bird is widespread and common across its range in Africa, including larger national parks and other conservation areas. It has disappeared from areas of high human density or intensive agriculture, but clearing of bush and planting of pastures has extended its range. Its flexible breeding biology enables it to capitalize on good habitats and conditions, whereever and whenever available, although the low nest and terrestrial habits make it vulnerable to casual persecution.

Significance to humans

No particular significance seems to be attached to the secretary bird by Africans, although it is a much sought-after species by visiting birders. Its reputation as a predator of poisonous snakes often earns it protection.

Resources

Books

Brown, L.H., E. Urban, and K. Newman. The Birds of Africa. Vol. 1. London: Academic Press.

del Hoyo, J., A. Elliott, and J. Sargatal, eds. Handbook of Birds of the World. Vol.2, New World Vultures to Guineafowl. Barcelona: Lynx Edicions, 1994.

Sibley, C.G., and J.E. Ahlquist. Phylogeny and Classification of Birds: A Study in Molecular Evolution. New Haven and London: Yale University Press, 1990.

Steyn, Peter. Birds of Prey of Southern Africa, Their Identification and Life Histories. Cape Town, South Africa: David Philip, 1982.

Periodicals

Kemp, M.I., and A.C. Kemp. " Bucorvus and Sagittarius: Two Modes of Terrestrial Predation." Proceedings Symposium on African Predatory Birds, edited by Alan Kemp. Northern Transvaal Ornithological Society, Pretoria, 1977.

Kemp, A.C. "Aspects of the Breeding Biology and Behaviour of the Secretary Bird Sagittarius serpentarius near Pretoria, South Africa." Ostrich 66 (1995): 61–68.

Organizations

Raptor Conservation Group, Endangered Wildlife Trust. Private Bag X11, Parkview, Gauteng 2122 South Africa. Phone: +27-11-486-1102. Fax: +27-11-486-1506. E-mail: [email protected] Web site: <http://www.ewt.org.za>

World Working Group on Birds of Prey and Owls. P.O. Box 52, Towcester, NN12 72W United Kingdom. Phone: +44 1 604 862 331. Fax: +44 1 604 862 331. E-mail: [email protected] Web site: <http://www.raptorsinternational.de>

Alan Charles Kemp, PhD