Secretarybird

Captive secretarybird with two eggs in its nest

Secretarybirds form monogamous pairs and defend a large territory of around 50 km² (19 sq mi). They can breed at any time of the year, more frequently in the late dry season. During courtship, they exhibit a nuptial display by soaring high with undulating flight patterns and calling with guttural croaking. Males and females can also perform a ground display by chasing each other with their wings up and back, which is also the way they defend their territory. They mate either on the ground or in trees.[24]

The nest is built by both sexes at the top of a dense thorny tree, often an Acacia, at a height of between 2.5 and 13 m (8 and 40 ft) above the ground. The nest is constructed as a relatively flat platform of sticks 1.0–1.5 m (3–5 ft) across with a depth 30–50 cm (12–20 in). The shallow depression is lined with grass and the occasional piece of dung.[24]

Eggs are laid at 2- to 3-day intervals until the clutch of 1–3 eggs is complete. The elongated chalky bluish green or white eggs average 78 mm × 57 mm (3.1 in × 2.2 in) and weigh 130 g (4.6 oz).[24] Both parents incubate the eggs, starting as soon as the first egg is laid, but it is usually the female that remains on the nest overnight. The incubating parent greets its partner when it returns with a display of bowing and bobbing its head with neck extended. The tail is held upright with feathers fanned out, and the chest feathers are puffed out.[35]

The eggs hatch after around 45 days at intervals of 2–3 days.[24] Both parents feed the young. The adults regurgitate food onto the floor of the nest and then pick up items and pass them to the chicks.[35] For the first 2 or 3 weeks after the eggs hatch the parents take turns to stay at the nest with the young.[27] Despite the difference in nestling size due to the asynchronous hatching, little sibling aggression has been observed.[24] Under favourable conditions all chicks from a clutch of three eggs fledge,[36] but if food is scarce one or more of the chicks will die from starvation.[24] The young may be preyed upon by crows, ravens, hornbills, and large owls.[27]

Illustration of chick, from Faune de la Sénégambie (1883), by Alphonse Trémeau de Rochebrune

The young are born covered in grey-white down that becomes darker grey after two weeks. Their bare facial skin and legs are yellow. Crest feathers appear at 21 days, and flight feathers by 28 days. They can stand up and feed autonomously after 40 days, although the parents still feed the nestlings after that time. At 60 days, the now fully-feathered young start to flap their wings. Their weight gain over this period changes from 56 g (2.0 oz) at hatching, to 500 g (18 oz) at 20 days, 1.1 kg (2.4 lb) at 30 days, 1.7 kg (3.7 lb) at 40 days, 2 kg (4.4 lb) at 50 days, 2.5 kg (5.5 lb) at 60 days, and 3 kg (6.6 lb) at 70 days. The time they leave the nest can be anywhere between 65 and 106 days of age, although it most typically occurs between 75 and 80 days of age. Fledging is accomplished by jumping out of the nest or using a semi-controlled glide to the ground.[24]

Juveniles remain in their natal range before dispersing when they are between 4 and 7 months of age. The usual age at which they first breed is uncertain but there is a record of a male bird breeding successfully at an age of 2 years and 9 months, which is young for a large raptor.[37]

Juvenile with lizard kill at Namib-Naukluft National Park, Namibia

Unlike most birds of prey, the secretarybird is largely terrestrial, hunting its prey on foot. Adults hunt in pairs and sometimes as loose familial flocks, stalking through the habitat with long strides.[38] Prey may consist of insects such as locusts, other grasshoppers, wasps, and beetles, but small vertebrates often form main biomass. Secretarybirds are known to hunt rodents, frogs, lizards, small tortoises, and birds such as warblers, larks, doves, small hornbills, and domestic chickens. They occasionally prey on larger mammals such as hedgehogs, mongooses, small felids such as cheetah cubs, striped polecats, young gazelles, and both young and full-grown hares.[27][25][38][39][40] The importance of snakes in the diet has been exaggerated in the past, although they can be locally important, and venomous species such as adders and cobras are regularly among the types of snakes preyed upon. Secretarybirds do not eat carrion, though they occasionally eat dead animals killed in grass or bushfires.[24][41]

The birds often flush prey from tall grass by stomping on the surrounding vegetation. Their crest feathers may raise during a hunt, which may serve to help scare the target and provide shade for the face.[27] A bird will chase after prey with the wings spread and kill by striking with swift blows of the feet. Only with small prey items such as wasps will the bird use its bill to pick them directly. There are some reports that, when capturing snakes, a secretarybird will take flight with their prey and then drop them to their death, although this has not been verified. Even with larger prey, food is generally swallowed whole through the birds' considerable gape. Occasionally, like other raptors, they will hold down a food item with their feet while tearing it apart with their bill.[27]

Secretarybird skeleton; the feet are used for killing prey

Food that cannot be digested is regurgitated as pellets 40–45 mm (1.6–1.8 in) in diameter and 30–100 mm (1.2–3.9 in) in length. These are dropped on the ground usually near the roost or nest trees.[27] The secretarybird has a relatively short digestive tract in comparison to large African birds with more mixed diets, such as the kori bustard. The foregut is specialised for the consumption of large amounts of meat and there is little need for the mechanical breakdown of food. The crop is dilated and the gizzard is nonmuscular, as in other carnivorous birds.[42] The large intestine has a pair of vestigial ceca as there is no requirement for the fermentative digestion of plant material.[43]

Secretarybirds specialise in stomping their prey until it is killed or immobilised.[44] This method of hunting is commonly applied to lizards or snakes.[45] An adult male trained to strike at a rubber snake on a force plate was found to hit with a force equal to five times its own body weight, with a contact period of only 10–15 milliseconds. This short time of contact suggests that the secretarybird relies on superior visual targeting to determine the precise location of the prey's head. Although little is known about its visual field, it is assumed that it is large, frontal and binocular. Secretarybirds have unusually long legs (nearly twice as long as other ground birds of the same body mass), which is thought to be an adaptation for the bird's unique stomping and striking hunting method. However, these long limbs appear to also lower its running efficiency. Ecophysiologist Steve Portugal and colleagues have hypothesised that the extinct Phorusrhacidae (terror birds) may have employed a similar hunting technique to secretarybirds because they are anatomically similar, although they are not closely related.[44]

Secretarybirds rarely encounter other predators, except in the case of tawny eagles, which will steal their kills. Eagles mainly steal larger prey and will attack secretarybirds both singly or in pairs. Secretarybird pairs are sometimes successful in driving the eagles away and may even knock them down and pin them to the ground.[27]

The secretarybird is depicted on an ivory knife handle recovered from Abu Zaidan in Upper Egypt, dating to the Naqada III culture (c. 3,200 BC).[46] This and other knife handles indicate the secretarybird most likely occurred historically further north along the Nile.[47][48]

Secretarybird depicted as the Emblem of Sudan

The secretarybird has traditionally been admired in Africa for its striking appearance and ability to deal with pests and snakes. As such it has often not been disturbed, although this is changing as traditional observances have declined.[27] It is a prominent feature on the coat of arms of South Africa, which was adopted in 2000.[49] With its wings outstretched, it represents growth, and its penchant for killing snakes is symbolic as the protector of the South African state against enemies.[50] It is on the emblem of Sudan, adopted in 1969. It is featured on the Sudanese presidential flag and presidential seal.[51] The secretarybird has been a common motif for African countries on postage stamps: over a hundred stamps from 37 issuers are known, including some from stamp-issuing entities such as Ajman, Manama, and the Maldives, regions where the bird does not exist, as well as the United Nations.[52]

The Maasai people call it ol-enbai nabo, or "one arrow", referring to its crest feathers.[53] They have used parts of the bird in traditional medicine: its feathers could be burnt and the resulting smoke inhaled to treat epilepsy, its egg could be consumed with tea twice daily to treat headaches, and its fat could be boiled and drunk for child growth or livestock health.[54] The Xhosa people call the bird inxhanxhosi and attribute great intelligence to it in folklore.[55] The Zulus call it intungunono.[56][57]

The German biologist Ragnar Kinzelbach proposed in 2008 that the secretarybird was recorded in the 13th-century work De arte venandi cum avibus by Holy Roman Emperor Frederick II. Described as bistarda deserti, it was mistaken for a bustard. Frederick most likely gained knowledge of the bird from sources in Egypt. The 16th-century French priest and traveller André Thevet also wrote a description of a mysterious bird in 1558 that has been likened by Kinzelbach to this species.[47]

In 1968 the species became protected under the African Convention on the Conservation of Nature and Natural Resources.[58] The International Union for Conservation of Nature (IUCN) listed the secretarybird in 2016 as a vulnerable species and as endangered in 2020, due to a recent rapid decline across its entire range.[1] Although widespread, the species is thinly distributed across its range; its population has been estimated in 2016 to be anywhere between 6,700 and 67,000 individuals.[1] Long term monitoring across South Africa between 1987 and 2013 has shown that populations have declined across the country, even in protected areas such as Kruger National Park due to bush encroachment, an increase in the tall vegetation cover,[59] resulting in loss of open habitat that the species prefers.[60]

As a population, the secretarybird is mainly threatened by loss of habitat due to fragmentation by roads and development and overgrazing of grasslands by livestock.[61][62] Some adaptation to altered areas has been recorded but the trend is for decline.[60]

The first successful rearing of a secretarybird in captivity occurred in 1986 at the Oklahoma City Zoo. Although secretarybirds normally build their nests in the trees in the wild, the captive birds at the zoo built theirs on the ground, which left them open to depredation by local wild mammals. To address this problem, the zoo staff removed the eggs from the nest each time they were laid so that they could be incubated and hatched at a safer location.[63] The species has also been bred and reared in captivity at the San Diego Zoo Safari Park.[29]