The African Fish Eagle

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As the first hint of russet touches the horizon a ringing duet breaks the silence of the dawn. Soon there is an echoing carillon of voices across the dark sounding-board of water as neighbouring pairs of African Fish Eagles Haliaeetus vocifer add their clarion calls to the sunrise chorus.

The scene is set in the Okavango Delta in Botswana, but it could be mirrored in many other aquatic habitats in sub-Saharan Africa. The Bateleur may be lord of the continent's skies, but it is the African Fish Eagle's haunting voice that evokes the freedom of its wild places.

The African Fish Eagle belongs to the genus Haliaeetus and, although closely related to the Madagascar Fish Eagle H. vociferoides, it is the only representative in Africa of a group that comprises eight species worldwide, including the Bald Eagle H. leucocephalus, well known as the emblem of the United States of America.

This striking eagle is the most colourful member of its genus. The immature, as with most eagles, is drab in comparison to the adults and moults through several well-defined stages until it acquires adult plumage at the age of about five years. Some maintain that the sexes of the adults may be distinguished by the depth of the white bib - that of the female extending further down the breast - but it is doubtful whether this is a constant and reliable feature. However, if a pair perch together, which is often the case, the female is noticeably larger than her mate, in common with most eagles.

Adults are resident and spend 90-95 per cent of their day perched. The conspicuous white on the head, breast and mantle serves to advertise the presence of a territorial pair and usually this is sufficient to warn off intruders. However, on occasions they may be chased away and sometimes there is even physical contact between eagles in residence and those new to the area.

In optimum habitats these eagles may occur in high densities. In July 1971, along a 55-kilometre stretch of the Chobe River west of Kasane in Botswana, Carl Vernon and I counted 38 pairs of African Fish Eagles, an average of one pair every 1.4 kilometres. This is by no means exceptional and in some localities in East Africa concentrations may be considerably higher. On the Kazinga channel in Uganda, for example, pairs averaged 600 metres apart, while on Lake Naivasha some nests were only 160 metres apart and, in one case, only 54 metres separated two breeding pairs. However, such densities can only be sustained where there are sufficient nest sites and an ample food supply.

Despite their territoriality, African Fish Eagles may congregate in large numbers at a food source, for example at a drying pool or where fishermen are active. In one exceptional observation in Botswana, 189 eagles, mostly adults, were counted at a drying pool where they were feeding on trapped catfish. One wonders from just how wide an area such numbers were drawn. On Lake Malawi the eagles may follow trawlers like seagulls to scavenge offal thrown overboard; once, 61 were counted.

Although generally thought of as a specialist piscivore, the African Fish Eagle has a varied diet that includes birds, mammals, reptiles and, rarely, amphibians. Additionally it is an accomplished pirate and also feeds on carrion.

The time spent on fishing is minimal, usually one per cent of the day, despite the fact that only one in seven attempts may be successful. Common prey species are surface-feeding fishes such as catfish, tilapia, tigerfish and mullet. Mullet caught in the sea are sometimes snatched from a rising wave; it is interesting to reflect that this eagle was once known as the Cape Sea Eagle, and it is referred to as such in early editions of Austin Roberts' Birds of South Africa.

In a typical catch the eagle swoops down low over the surface of the water, throws its talons forward, and deftly snatches up a fish. The fluency and apparent ease with which this is accomplished is always a source of wonder to me. The capture is facilitated by long, sharp talons and spicules on the underside of the feet to assist in grasping the slippery quarry. Unlike the Osprey, which may become completely submerged, the African Fish Eagle never goes deeper than 15-30 centimetres below the surface.

One study calculated the lifting capacity of African Fish Eagles: fish up to two kilograms were easily lifted and carried to a perch, but those weighing over two but less than three kilograms were planed along the surface to the shore where they were eaten. Fish heavier than this (up to a maximum of 3.65 kilograms) were paddled to shore using the wings.

Mammalian prey is rarely taken but records include hyraxes on two occasions and once a scrub hare. In contrast, birds are regularly captured, or heronries are raided for their chicks. Waterbirds such as cormorants, egrets and herons are killed and eaten, and on the lakes of East Africa flamingos are often preyed upon. Sometimes the nestlings of sandbank-nesting species such as the African Skimmer are included in this eagle's varied avian diet. Birds may even be caught in flight; such records include a Red-knobbed Coot, Greater Painted Snipe and Cape Turtle Dove. On one occasion at sunset in the Okavango Delta I watched in astonishment as an eagle swooped in to snatch a weaver from a flock as they flew in to roost in a reedbed.

Another form of predation involves nest robbing. Immatures in Malawi were observed to pluck weaver nests from a reedbed, fly to a perch and tear them open to consume the chicks. African Fish Eagles have also been seen at Red-billed Quelea colonies where they must undoubtedly have been preying on nestlings.

Reptilian prey includes snakes snatched from the surface of the water, but in one record a snake was caught as it crossed a path. Crocodile hatchlings and occasionally terrapins are also eaten.

Piracy is regularly used as a means of obtaining food and the eagle's normal victims are piscivorous birds such as pelicans, cormorants and herons which are chased and harried until they disgorge their crop contents. Perhaps the most unrewarding example was observed in Kenya where one eagle specialized in robbing Pied Kingfishers of the fingerlings they had caught. However, such attacks do not only involve fish-eating species: a Martial Eagle robbed of the snake it was carrying and an African Hawk Eagle dispossessed of a Helmeted Guineafowl are but two examples that testify to the versatility and boldness of the African Fish Eagle.

In contrast to its dashing piratical attacks, the African Fish Eagle will readily feed on carrion, usually fish that have died when pools have dried up, or offal in the vicinity of fishing camps. However, one of my earliest memories was of a single eagle feeding with vultures on the putrefying carcass of a rhinoceros in the Hluhluwe Game Reserve in KwaZulu-Natal. It disillusioned my youthful image of the 'nobility' of eagles, but now I know that most eagles will feed opportunistically on carrion.

The eagles remain paired year-round and reinforce their relationship with frequent bouts of calling. The male's treble and the female's contralto notes are combined in the well-known syncopated duet emitted with the head thrown back, both perched and in flight - a characteristic of the genus. Except for soaring together, there are no aerial displays, which is not surprising in view of the close pair bond. The cartwheeling flight with interlocked talons immortalized in Walt Whitman's poem Dalliance of the Eagles in which he describes 'the rushing amorous contact high in space' refers to the American Bald Eagle. However, such behaviour previously interpreted as courtship in the African Fish Eagle has now been debunked. In a comprehensive analysis, Rob Simmons and John Mendelsohn have shown that cartwheeling flights with talons interlocked may almost always be construed as aggressive behaviour in raptors and not as courtship.

Nests are usually built within the canopy of trees but also in bare dead trees with no shelter from the elements. Sometimes they are constructed on an outcrop of rock or on a cliff face; one nest in the Matobo Hills in Zimbabwe was perched on top of a huge granite boulder near a dam.

The nest is always a substantial structure of sticks lined with dry grass, green leaves and sometimes even a few weaver nests. Some of the larger structures may measure 1.8 metres across and 1.2 metres in depth. Nests may be used repeatedly, for at least 21 years in one record, but alternative sites are also quite often utilized.

In southern Africa eggs are laid during the cool winter months, from April to June. The usual clutch is two (occasionally three) white eggs which become nest-stained as incubation progresses. The female undertakes the bulk of the brooding, but the male relieves her for short periods, usually after he has brought prey and she leaves the nest to feed on it.

The incubation period is approximately 42-45 days and the chicks hatch a few days apart because brooding begins with the first egg. Sibling aggression or Cainism, in which the older chick pecks its weaker sibling, has been observed but is not invariable as is the case with the Black Eagle. It also seems that the second-hatched eaglet may succumb to starvation if it is regularly dominated at feeding time by its older sibling. However, despite the occurrence of Cainism, two chicks are quite frequently successfully reared. Very occasionally, even three survive.

The young are initially closely tended by the female, and the male provides them with food. The length of the female's attachment to the nest depends on the site; in an unsheltered nest she needs to shade the nestling until its dorsal feathers emerge. The young leave the nest at the age of 70-75 days and remain under parental care for about a further two months. After this period they leave their natal area to join other young eagles in areas where food is relatively easily obtained, for example at a fishing camp or where fish are trapped in a receding pool.

All raptors are most vulnerable during their immature years and the African Fish Eagle is no exception. Intensive observations in East Africa, mainly on Lake Naivasha, by the late Leslie Brown, doyen of African raptorphiles, estimated that 94-96 per cent of young die before reaching adulthood. Even if one allows for certain imponderables in the calculations, there can be little doubt that the mortality of immatures is considerable.
It appears that throughout its range the status of the African Fish Eagle is relatively healthy. The construction of new dams, mainly in southern Africa, has even resulted in an expansion of its range, especially into some of the more arid western regions of the subcontinent. However, threats do exist, the most notable being the accumulation of pesticides such as DDT in lakes and dams. Being at the top of the food chain, the eagles absorb a cumulative dose with resultant eggshell thinning, as has been noted in eggs on Lake Kariba in Zimbabwe. Fortunately there is no present evidence of a dramatic population decline as occurred in the case of the Bald Eagle in the United States as a result of heavy pesticide contamination of their waterways, a situation that has now been largely reversed by strict environmental legislation.

For the foreseeable future the call of the African Fish Eagle will remain one of the most evocative sounds of Africa.

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