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Simon Thomsett on the African Crowned Eagle – Part 2

June 5th, 2011 · 6 Comments · Interviews

This is part 2 of Simon Thomsett’s writings about the Crowned Eagle. See here for part 1 and here for part 3

Male Crowned Eagle

Male Crowned Eagle, © Simon Thomsett

Replacement rate and longevity

If persecution was a factor then the breeding biology of the Crowned Eagle would predispose it, above all other raptors in Africa, to a gloomy future. While annual and biannual breeding has been recorded and its ecological perspective pondered, annual breeding is rare and probably related to the success, or not, of the previous season. Usually and possibly throughout its range it fledges about one chick every two years. The chick takes an inordinate time to reach independence and that alone is good enough reason for their slow reproduction. For a not-so-very large eagle it is hard to explain its slow fledgling and maturation period. I have seen begging 9-11 month old chick on the nest of an incubating pair, and also saw that same pair feed a 2-3 year old juvenile, possibly unrelated near their nest. That may suggest that recycling can be started before the chick fully disperses. I used to breed them in captivity and “pull” their chick at aged 4 months, to make them recycle for the next year. When they had egg losses (due to cobra and Honey Badger) they could lay again within weeks. It is interesting to note that on occasion Girl would lay two eggs. On one occasion when both hatched I saw no active Cain and Abel aggression other than the elder chick tweaking the toes of the younger. I did separate them lest one die of starvation.
The phenology of large raptors on the equator, especially those with stable prey base can hardly be expected to be precise. I’ve been uncomfortable with the dogma that there should be a season…simply because it is so in temperate places, but I must admit that come July, Aug, Sept my old pair would like to lay….sometimes irrespective of weather. Why I cannot fathom. But should I bump them into breeding in the first half of the year they would oblige (by giving them a stream of highly nutritious food (monkeys, springhares, small gazelles, etc)). In Kenya wild laying dates would seem to favour the latter 6 months of the year.
Female chicks take longer to fledge by about a week to a month and they take much longer to learn how to fly properly and hunt than males. While there may be a size overlap between male and female young (as well as adults) males are snappy, make their first flight and first kill quicker, certainly so in captivity. But in the wild I have also seen it so and presume that should a pair have a male, they may breed sooner the next year, than if they had a female chick.
The first year plumage is surprisingly varied. The most usual is the pure white fronted face, bib, chest and flanks and undertail as well as legs, often with a pinkish red wash on the upper chest. Slightly less common are those which one could easily have aged as two to three year olds. These just-fledged chicks, have darker patched faces, freckled bibs and slightly barred chests and spotted legs. While the pale ‘morph’ young just prior to leaving the nest usually have unmarked tarsus, they soon get spots on the front part of the tibio tarsal joint. I had at one time thought that there was a difference in the first year morphs between East and West Africa, but both Susanna Shultz and Guy Rondeau noted similar polymorphic first year birds in the west. By 4 months post-fledgling the inner thighs, previously poorly covered with downy type feathers, are covered with small feathers. The tibio tarsal pad is still bare and obvious up until it is a year old, whereupon it vanishes only to return to incubating females. Eye colour is variable too with some having khaki light brown just prior to fledging and others with adult-like yellow ochre eyes.
The plumage variations between sexes, as offered in some field guides is inconsistent and do not always apply. The various amounts of orange/rufous marbled patterning on the chest has been suggested as being sex linked. Some males have much orange on their chest and females have less, almost monochrome chests. But this does not always hold true. I suspect that most females have one less bar on the secondaries than do the males. While usually so it is not invariably the case. I have a suspicion that large females show female traits more strongly than smaller females; and small males show more male traits than larger males.
Plumage maturation therefore appears to depend upon sex, their size, and their particular first year morph, to say nothing of stress and variable nutrition. Of 9 young, 5 were male. Of these 2 males were larger and they matured slower than the 3 smaller males. It should be noted that in captivity moult can be accelerated or delayed by feeding regime and exercise. All these birds were trained, flown and hunted and therefore were most likely to follow a wild birds moult pattern. The females were less variable but all began their first flight feather moult a month or two later. They would half complete wing feather moult before the first deck feather fell. But not always. The first flight feathers were moulted out at 9 to 18 months, showing a marked lack of adherence to the calendar. Males are usually inseparable from adults at 4 to 5 years. Females (usually) taking half a year or more to mature. The only indication of juvenile feathers at this age are light crescents edging the distal part of each feather on the upper wing coverts near the carpals, obvious a year previously but at 4 to 6 years becoming very faint. Irregular moult can certainly be protracted due to stress, nutrition or work, and while it is usual for one moult will be overtaken by the next, some old ragged feathers may be retained. 3 and rarely 4 moults can be seen in an individual. Of particular interest is their ability to moult damaged feathers early. If any primary is fractured and left so (not imped), it will drop sometimes 10 months earlier than expected. My old male has a poorly healed humerus and fractures feathers on that side with frequency. He has replaced primaries in this wing many times more than on his good wing. At 32 years he developed a single white tipped greater upper wing covert, in mid wing, which increased in size at age 33. His mate a similar aged female also now sports a single white tipped feather on the same wing and same location. These maybe senile related.
Moult in Crowned Eagles is therefore a complex affair, but I suspect more temperate eagles would adhere to a calendar a little more fastidiously.

Behaviourally Crowned Eagles are nervous, constantly alert and on edge. Females are more phlegmatic but never docile. They are highly intelligent, cautious, independent and inquisitive, unlike African Hawk, Tawny or Verreaux’s Eagles. In their training and management they are more like goshawks than Aquila eagles. Being permanently curious and edgy would seem to be a common trait for most forest raptors. I understand that the Harpy and Philippine Eagles are much less nervous however. While I might be poo pooed for noting ‘intelligence’ I have no end of examples that place them in a unique position over all other African eagles, particularly when they hunt. They cannot be induced to hunt large prey by increasing their hunger, as would a Verreaux’s or Tawny Eagle. They may show moments of cowardice that while shameful, may expose a unique aspect of their hunting strategy. They are variable in temperament too as individuals to a degree greater than that found in most other raptors.
One thing is for sure, they are a highly variable species, in colour, size, maturation period, behaviour, prey preference, habitat use and requirements.

Crowned  Eagle

Crowned Eagle, © Simon Thomsett

Because of their low fecundity one would assume that Crowned Eagles were exceptionally long-lived. My male Crowned Eagle called Rosy was over a year when I got him in 1978. His mate Girl is about the same age. She is fine today and not a whit older, but Rosy had to undergo a bilateral cataract operation. Importantly it was not senile related and the eye surgeon was amazed to note that he had the immune response of a human child. They still have the will to breed, nest building and even mating. I would credit some wild Crowned Eagles in benign environments at easily reaching their mid thirties, and it cannot be impossible for some to reach 50 years or so, as can other large raptors. However when I visited Susanne Shultz in Tai Forest, Ivory Coast I was shocked at just how close those pairs were to each other, for there is one thing I know, battles between pairs are violent and life threatening, especially if you have those feet. I cannot imagine that those packed in, jowl to jowl, territories do not have murder as one of the main causes of death. Tai is a tough neighbourhood, whereas our nice clean community of more widely spaced eagles that we see on cosy wooded hill tops in Kenya, would be less likely to indulge in such behaviour. Mind you, given that our Kenyan territories are often fragmented patches, there may be deliberate take-over attempts for these isolated territories. Leslie Brown had birds that occupied one nest 9, to 13 years, but these do not discount territory moves which may well happen. I had one adult female who later died brought in with deep wounds that were certainly inflicted by another eagle. Another natural cause of death expected to be exceptionally high for this species is injuries incurred during struggles with heavy and well armed prey. In witnessing eagles killing vervet monkeys, a relatively small monkey, I have been impressed by the way some monkeys will fight back with their arms, hands and teeth. I once arrived just after a young female Crowned Eagle had attempted to kill a full grown female baboon and suffered the wrath of troop members. She would have been wounded or killed had I not intervened. I treated a wild female for an ocular inclusion in which debris had been injected into the eye and destroyed the lens. There was another lesion the same distance as the separation between Suni horns and I assumed that she had been head butted. Tom Butynski concluded that a monkey had harmed a chick in the nest that later died in S W Uganda. I have lost two released eagles, one to a leopard that surprised a male on a monkey kill in the rain, and the other from a crocodile that took a female as she ate a young bushbuck kill near the water’s edge. Other causes of natural death are probably starvation and disease. But trauma related deaths would seem to be high for this species.
Of deliberate human persecution I have personal recorded them being shot by an expat with Burmese cats in Karen, shot by a big cat conservationist for eating geese in Karen, shot by sheep farmers in Mweiga and Timau, shot by KWS rangers in Elgeyo Marakwet and Rombo, shot with arrows in Cheranganis, poisoned in Kibwezi and Maua Hills (Machakos) and Cheranganis, caught in snares in Cheranganis and finally nest trees deliberately cut down in Cheranganis and Mweiga. Another male was beaten to death in a corn house whilst killing a dog in Elgeyo Marakwet. Of accidental deaths I saw a juvenile female with and amputated wing severed by a snare set for ground game in Karen forest. One male at Ololua attempting to take domestic geese in a chicken wire pen in Leslie Brown’s old house was snagged but fortunately rescued.

Export and trapping

Markus asks if the bird trade is any threat. The amounts reaching their destination under permit may be sustainable, but those caught to supply them is a different thing. Given the amounts exported I would doubt if it would be a global threat, but I do consider their brutal trapping and subsequent terrible husbandry and resultant mortality a threat to local populations. The trade is of course an effrontery to conservation.
I recall two Crowned Eagles being exported from Kenya, one in the 1960’s by Cunningham van Someren, and another in the 70’s by Don Hunt. Both went to zoos. I suspect a few more were exported, but not many, chiefly because of the difficulty in handling them. From my experience in rehabbing raptors, I have been appalled at the handling and husbandry methods made by the public and trained government officials as well as most vets. I once saw a tethered Martial Eagle, meant for export in the 70’s with both legs tied with a piece of rubber inner tube, wrapped around an open fire in a hut. The children were busy taunting it with a stick. The trapper was a professional and I was unable to confiscate the bird. It must have died for it never reached the export company run by an internationally famous conservationist. I have seen many birds and raptors in capture holding pens, some feeding on each other, in conditions where mortality or irreparable damage is virtually guaranteed. From what I can glean from the highly secret businesses in Tanzania and Uganda, similar treatment is the rule. I once estimated, based on the trappers, and the company owner’s own admission, that some one in ten captured raptors would survive to get put on the plane.
I dread to think what lengths of brutality a trapper would go in restraining a Crowned Eagle, an animal that could maim or kill a human if given half a chance. I cannot imagine what sort of trap they would use, or how they would house it. But you can be assured that the process is cruel and life threatening. I would expect a higher mortality rate for Crowned Eagles than most raptors, simply because they are very awkward birds to handle and are easily stressed and likely to self mutilate against cage walls.
If for every one bird exported one must capture 10 birds, and each bird is an adult, the impact is considerable. This is an open question as no-one knows the details, nor is one allowed to know even when asked by officials to make an opinion.
The significance of the removal of adults, juveniles or chicks is not considered in the various regulations. Despite very good reasons to separate these age groups when it comes to a harvest quota, it is not done. Falconers of old never took haggards, or only rarely when they knew they were not breeding. They took passagers or eyasses. These juvenile birds experience such high mortality that it makes little difference to the population as a whole if some are removed. Established breeding adults are crucial to the survival of the species and represent a small fraction of the yearly crop of young and as such should be left alone. With this ancient wisdom, why is it that modern biologists have yet to enforce the same protocol in the raptor export business and within the CITES regulations? The absence of deferential treatment or harvest rates for age groups illustrates an oversight.
The removal of unfledged chicks or eyasses from the nest, if done smoothly and unseen can be least disturbing. This applies to nests with multiple young from which the adults are allowed to raise one or more chicks undisturbed after some have been removed. But Crowned Eagles raise one chick every two years, and their investment in that one chick and nest site is monumental compared to almost all other birds of prey. The removal of a single eyass out of a Crowned Eagle nest represents a total failure of not one but up to two years investment. It is a catastrophe to the adults, and may well make them move site.
If a delicate Cain and Abel rescue is made then one takes a chick that would otherwise die when it is aged only a few days. It demands well orchestrated teamwork with months of close watch, hides, an intrepid climber or two, incubator/brooder, 24 hour care, precise diet and foster parenting. Crowned Eagles in our latitude rarely have 2 eggs, and siblicide is therefore an unusual occurrence. I believe chicks have been taken by this method in Zimbabwe and South Africa, but I would doubt it being the method adopted in Tanzania or Uganda. There is of course the absolute certainty of human imprinting the day old chick so removed, unless one happens to have a captive pair of foster Crowned Eagles under which it can be raised.
A human imprint Crowned Eagle is never likely to breed (except by Artificial Insemination) and be a danger to people and is a lost member to its own species. There really cannot be much justification for having such an eagle.
I have never understood the need for exotic raptors in zoo, aviculture or falconry collections and think it unethical. I believe that the main rationale is not conservation but either egotistical or financial. The bigger and more complete the collection, the more kudos or visitors. I have argued aggressively against exotic species being used in falconry to the bewilderment of many, who think only in terms of sustainability.
I do see merit only in real conservation arguments that outweigh the loss of the birds from their home country. These could be life saving for the individual bird, treatment, captive breeding for re-introduction, or saving a species in captivity when its wild habitat is defunct. If there comes a time when captive breeding for re-introduction is deemed an option Africa certainly has the proven capability of doing it themselves. However permission is seldom if ever granted for domestic use, but readily granted for export.

Importers could and do argue that they need the birds for captive breeding, as if captive breeding is a panacea of its own. But I know of not one single incidence of an exotic raptor benefitting from this exercise. (Except for bonafide work by established conservation groups for E.G. for the Mauritius Kestrel). Should the breeder have a plan to export the progeny produced to augment our impoverished populations then that could be considered as an option. But no, the export of raptors from Africa is a one way ticket to oblivion from which nothing returns.

Tanzania is one of the few countries with a well established wildlife export trade and large eagles as well as the Crowned Eagle appear not infrequently as subjects for export. In theory, a legal harvest is dependent on a scientifically substantiated population estimate from which one can calculate a ‘take-off’ quota. No-body knows the sustainable harvest quota for Crowned Eagles and no-one knows the numbers in Tanzania, so it should be inappropriate to entertain export, but it happens in spite of pleas to reconsider. From its slow reproductive recruitment and slow maturation one would immediately impose a precautionary approach to their harvest, but this is not understood showing an absence of consideration for their unique biology.
Neil Baker’s invaluable compilation of data for the Tanzanian Bird Atlas has numerous advantages over Kenya in being contemporary and reliant on a larger informant network. From his work it appears that the Crowned Eagle is patchily distributed and nationally sufficiently rare to support a no export policy. The pattern of occurrence, density, threats and problems are not dissimilar to Kenya, but if one would include ‘export’ as a threat it has one more additional problem to contend with.
It is ironic that I have spent much time and money breeding and releasing captive bred Crowned Eagles on the border with Tanzania, to have the Tanzanians legally capture and export the very same species. It is poignant to note that I had painful bureaucratic hurdles to leap and still suffer from critics who disapproved of the release programme yet CITES and IUCN make it much easier to ship them out to zoos for profit. On one side of a border we consider such augmentation a worthy exercise, requiring at one time the full back up of KWS, volunteers, Finch Hattons Lodge, a project vehicle, a home -made aircraft, radio telemetry and running expenses. To have on the other side of the border a few kilometres away, someone ship them out to die overseas.
When we are asked on the raptor network to make a comment regarding a raptor species destined for export, we are obliged to remain unemotive and respectful of the country’s policies. Casting sentiments aside it is still extremely difficult to understand why one should export (or import) Crowned Eagles. Africa certainly exports raptors on license and with CITES permission, but there is no evidence to say that this exercise in any way benefits the wild resource. There is every indication to say that it harms it and every ethical perspective to say that it is wrong. Money is the only objective as there is no local conservation related obligation demonstrated by the receiver. I question the ethics of the importing country and individuals behind capturing Crowned Eagles for raptor exhibits in Europe, USA or Thailand (see Youtube for the latter!). There are a number of falconry trained Crowned Eagles in Britain and one even in Scotland, where Golden Eagles would be much better suited. If Crowned Eagles were common and stable, I still see no ethical reason why they should be harvested and exported. If the importer makes any claim that they are supporting conservation they have yet to demonstrate it. If some argue that they have secured the species future, albeit outside their doomed range, they have yet to propose any action to support a programme to achieve this objective.

Juvenile Crowned  Eagle

Juvenile Crowned Eagle, © Simon Thomsett

Future predictions for forest species

Recently I listened to Don Turner giving a talk on the status of birds in Kenya. He is a world renowned ornithologist and author with a precise analytical mind and spans pre and post colonial Kenya. He talked of the big picture, of global warming and of Kenya’s invigorated complacency and emerging lack of accountability for what we too happily assume to be only a western evil. Namely, Global Warming. (Africa burns the size of Australia each year, without guilt for carbon emissions, for EG.). Don spoke of Kenya, its 5.5 million inhabitants in 1955, and its over 41 million (published) today, showing little sign of abating and doubling in less than 15 years or so. That’s 20 million below 15 years old. (Even government admit to 71 million by 2030.NB. 4-6 times growth by 2100). UNEP had once calculated a maximum holding capacity for Kenya at 22 million, and we’ve doubled it already. It takes less than 3 years for us to add the same population it took humanity more than 3 million years to achieve by 1955. He then listed those bird species that have gone extinct, those that have not been recorded for many years and those we know will go extinct. Not surprisingly these were mostly forest birds. (NB. The only raptor to be officially extinct in Kenya is the other forest eagle; the Cassin’s Hawk Eagle). His frustration was evident and when questions came the unanimous consensus was that Kenyan conservation organisations and all the associated NGOs had failed shamefully at even raising the matter of “OVER POPULATION” and “UNSUSTAINABLE” land use practises and the devastating effect it has on our bio-diversity (ESPECIALLY FORESTS). All agreed the systems in place to evaluate threatened animals were disgracefully deficient. I thought his lecture refreshing because it did not sugar-coat the facts and moreover, was totally accepted by an entirely Kenyan audience. This audience had no ‘politically correct’ agenda and no hang-ups about putting the blame where it is due. I have to add than even in my most pessimistic mood the outlook was much worse than I had assumed. While a critic, I had also been subconsciously deluded by the current status evaluations persistently voiced by large NGOs. Perhaps I too have been successfully duped by KWS branding propaganda and national zeal that sells Kenya as a success. We all felt deceived and let down. The implications were clearly that we were facing a catastrophic unparalleled loss of forest species and bio-diversity and that there were no functioning actions in place that would mitigate these losses. This conclusion was applicable to all countries in Africa with similar human demographics and policies. It was probably worse in those countries considered unstable.

Two kinds of Crowned Eagles

Many authors have recognised the distinct difference in prey selection between Crowned Eagles living in forests and savannah biomes. Leslie Brown in Kenya, as well as observers in South Africa (Boshoff, A.F., Palmer, N.G., Vernon, C.J., Avery, G., Jarvis, M.J.F., Symes. C.T., Raath. A.) all noted a shift away from monkeys when living in “bush” environments and a shift in hyrax and antelope species. However these forests hardly compare to those of the central and west African types, and differ so markedly in species abundance and prey species ecology, height, quality and extent as to be incomparable.
These populations under study often occurred adjacently or within a small area sufficient for none to assume any real difference in the birds, only their prey. While I suspect that there are physiological and behavioural differences that define these two groups at the extremes of their occurrence it helps in estimating their status if they are so separated. Crowned Eagles are ‘true’ forest eagles apparently (thanks to recent DNA work) more closely related to old world Hawk Eagles than to any other. But the Crowned Eagle has been isolated from its closest relations long enough to have evolved independently and be unique in being that much larger and more powerful. (It is one reason why ‘hawk’ in Crowned Hawk Eagle has been dropped). It has the classic accipitrine body build with short wings, long tail configuration, the harpy-like facial ruff and crest, the deep brutish eyebrows (protection against violent collisions against brush), and a nervous disposition so similar to accipiters. These all support its forest roots. Perhaps monkey predation did originally drive its evolution by increasing its size and foot structure. The tarsus has tall ridges that run its length separating massive tendons and increasing the strength of the bone. The twist, said to offer a dynamic shock absorbing rotation is hard to understand, but certainly sets them apart in an area where raptors need it the most. All in all it is a surprisingly well designed eagle despite its somewhat primitive Neanderthal looks.
Despite having a typical forest adapted ‘flight envelope’ it does not oblige them to a life confined within closed canopy, but significantly they do use it to the exclusion of other large eagles and they almost certainly evolved within it. The other large eagles cannot penetrate their forest realm, but when the Crowned Eagle exits the forests they share their foraging ranges. Their evolutionary home would appear to be dense tall forests, and from that it has cautiously ventured out to less dense more open habitats. These eagles can be split into ‘Forest’ and ‘Bush’ eagles. While an artificial partitioning it is deserved, and needed for their evaluation.

The ‘forest’ eagle.

The supermarket of food in the African primary forests is mostly monkeys. A staggering 350 to 558 individual monkeys per square km is possible in these tropical forests providing an easy to see, if difficult to kill, food source. Killing monkeys needs two things, suitable weapons and strategy.
Of avian competitors in forests it has none. The Cassin’s Hawk Eagle is to all intense and purpose a thick-set Ayres’s Hawk Eagle or forest adapted African Hawk Eagle, certainly not a true Spizaetus Hawk Eagle or smaller direct relation to the Crowned Eagle. Its prey range barely overlaps (large squirrels) and it is not a serious competitor for food and an inconsequential problem with regard to nest sites. Monkeys are tough and have long limbs with powerful hands that grip and huge canines to bite. Very few eagles, even the (previously termed) Philippine “Monkey Eating” Eagle, or the Harpy Eagle prefer monkeys. While ‘bush’ eagles have much less monkeys on their menu, ‘forest’ eagle life revolves around them. The ‘forest’ eagle is numerically the most abundant and probably the most important medium sized carnivore in these forests, devouring the most amount of meat. Given their combined numbers, prey range and physical daily nutritional needs they consume more than any single predator such as the Leopard or Golden Cat. These forests are typically devoid of canid hunters/scavengers, hyenas, or terrestrial predators. The way things work in these forests is different from what we in savannah Africa are familiar. Here in the forest the Crowned Eagle is King, whereas the ‘bush’ eagle is not.

A titivating recent hypothesis is that a distant ancestor of the forest dwelling Crowned Eagle moulded and shaped the early monkeys. The “taphonomic” implications of Crowned Eagle predation on hominid evolution makes for compulsive reading. I have little idea what “taphomology” is all about but have no doubt it adds some credibility to whether Crowned Eagles moulded slow, small, daft arboreal proto monkeys into US? Alarms voiced by those early monkeys at seeing their main predator became complex and led to communication, group cohesion and delegated roles in their community. Even their physical agility and vision could, in part, be related to those pressures placed upon its evolution by an avian predator. An active arboreal life for an animal weighing over a couple of kilos is a diurnal one, for leaping into space on distal tips of branches in darkness would lead to an early demise, as gravity, darkness and weight are a bad combination. A larger body mass would therefore make you diurnal and vulnerable. You have to be smart and small but then a much larger body mass would make you less vulnerable as well. Predation by the ancient Crowned Eagle was most likely the pressing factor that increased primate body size as a way to beat predation. It came at the great cost of loosing access to the most nutritious distal leafy parts of trees. Some of the heavier monkeys descending to the barren, least productive forest floor and this obliged them to eat invertebrates and meat and predate. This created an omnivore with a smaller gut that also allowed quick bipedal movement, more time on its hands and the essential nutritional building blocks for a larger brain. Perhaps then as is true now, terrestrial predators in forests were few, but they would still retain an ability to climb trees as we do today. Perhaps the early proto Crowned Eagle could see the writing on the wall for it seems that one did its best in squeezing the life out of the Taung australopithecine child. That the Crowned Eagle is the only confirmed ‘juvenile man eating’ eagle today makes it highly likely that it was at one time a real threat to smaller early hominids juveniles. Sadly for the eagle, it may have contributed in transforming a tree dwelling small primate into us. Predation is a persuasive reason to evolve, and when one thinks about it no other predator could have been as important to early medium sized primates. I’d like to see one of those artist representations of early man evolution with each figure looking skyward waving a stick and hollering “watch out” in lesser and lesser degrees of intensity.
If this theory had any merit then why is it that the Harpy Eagle didn’t round up those backward new world monkeys and make those into terrestrial giants with brains? The Harpy’s main food is sloths so why in heck didn’t those evolve into huge beasts that had to descend to the forest floor. Oh, hang on, maybe they did, and we bumped them off.
Sometimes after having seen a truly monumental kill made by this medium sized eagle I shudder at just how life threatening they would be if they weighed only 2 to 3kg more. A Crowned Eagle weighing 12 to 18 lbs would be well within the weight range and flight envelope of a Harpy or Steller’s Sea Eagle. But it would be terrifyingly capable of killing humans. If it weighed say 25 to 30lb (such as the Haast’s Eagle of New Zealand), it would probably specialise in humans. I suspect many such ‘hazardous to human life large eagles’ were killed off by man, and you have to wonder if somewhere in the sub fossil strata of Africa’s rain forests such an eagle remains to be found.

I learned an enormous amount about the ‘true’ ‘forest’ eagles from Dr Susanne Shultz when I was asked to go to Tai Forest in Cote d’Ivoire for the Peregrine Fund to catch some of her study birds. I learned much too from simply looking around me in an environment, though tangibly similar, wholly alien. I found those ‘forest’ eagles as different and incomprehensible as a foreigner. They just were not the same as the ones in East Africa. One species can behave differently depending upon its environment and the distance between them. I was able to handle these eagles and compare them to the Kenyan variety and I did note subtle differences. And why not, for these eagles have been as separated from each other as have the Forest from the Savannah Elephants. I found them smaller but as large footed, thuggish in build with deeper eyebrows, noisy and pugnacious in character. I’d like to call them S. c. troglodytes ! Sadly these subtleties in structure are hardly quantifiable and unlikely to galvanise the ornithological taxonomists into desk-pounding proclamations of agreement.
One important thought occurred to me as I walked out across a broad lateral branch high above a green carpet of lower canopy trees in Tai, was the three dimensional extent of these forests, compared to those used by ‘bush’ eagles. One doesn’t have to sit through the agonisingly predicable “Avatar” movie to get a hang of what I am trying to describe, but it would help. It would help too if you saw it with 3D glasses, and not a pirated version on your laptop. The ‘forest’ eagle group lives in high, multi-layered canopy wet forests. The usable foraging area must consider these forests in terms of total surface area in the vertical aspect.
These horizontal multi-storied canopies and tangles of vertical growth harbour a whole world of epiphyte fauna from large to tiny sun and flying squirrels, tree mice, Pottos, galagos, Palm Civets, hyrax, a multitude of birds, tree pangolins, dormice, lizards, reptiles, hornbills and an unequalled array of monkeys. Literally tons of animals live in the trees and seldom descend. From the top canopy you look down upon the feeding animals, themselves high up on the tops of trees. On the ground surface dwell the mixed groups of Duikers, Suni, Bushbuck, Chevrotains, Liberian Mongoose, Cusimanse, Monitors, Dwarf Crocodiles, Bush pigs, Forest Guineafowl, Congo Peacock and Mangabeys. One underappreciated habitat is the tree buttress bases, limb falls, vine tangled glades, leaf litter and the upheaval of root structures that has no resemblance to tera firma, but to mouldy cheddar cheese. The terrestrial and arboreal species move together like gigantic communes. Food, fruit, rejects, faeces and litter is dropped from above and consumed below. Just as small bird feeding parties move together so do these and in so doing they move in and out of territories of Crowned Eagle pairs. These eagles must hit these parties as they pass through the neighbourhood like kids chasing an Ice Cream van. In other words there is a time of plenty followed by paucity and there must be plenty of territory infringements and disputes. As a result Crowned Eagle behaviour must to cater for this.

Here the monkeys alone amount to a weight of meat biomass available to Crowned Eagles in access of the meat found in the migrant ungulate ecosystem of the Serengeti. Little wonder that Crowned Eagle densities can be very high at 1 pair per 6.5km2 given that prey density! That’s less than 1.5km between each nest….a veritable colony of breeding eagles! But their actual foraging range is multiplied by the surface area that the multi-tiered canopy and vertical surfaces provide. One cannot compare the number of potential victims a serial killer stalks in a 5 sq/km quadrant of sky-scrapers in Manhattan as opposed to a similar size in single storied suburbia. In other words the actual surface area available to a forest eagle may be at least 3 times the 6.5km2 (some 20km2). The amount of suitable prey available is open to conjecture, but it is much more than in any other environment.
While there is a staggering supply of prey it isn’t as one may assume (for tropical forests) a steady and guaranteed uninterrupted year-round food supply. One aspect of possibly crucial importance to Crowned Eagle biology is the need for most of their prey species to forage together and move to seasonal fruiting trees. They move in noisy close-knit groups as much for detecting food as for mutual protection from Crowned Eagles. Although eagle territories can be small in these super-productive forests they may be more fiercely defended. I’d doubt any relaxing of boundaries for intruding eagles bent on following these feeding groups within neighbouring territories is permitted. Feeding groups of prey species must wander outside of a pair’s foraging range and thus often leave pairs with little prey, while other pairs have over-abundance. When the monkeys and duikers have left an eagle’s “patch” they must either rely on reserves or be able to kill larger or smaller-than-usual prey species that are left behind. They must have strategies that keep them from going hungry. I believe that their hording or “caching” ability may be one very well developed habit not entirely perfected to the same degree by any other eagle. This caching may help them through the expected time of paucity as well as be a more efficient use of hard-gained and large food. The dissection and transport of limbs, nearly innate even in captive bred Crowned Eagles, is only practical for large prey. I am unsure if any other large eagle has this habit. It also hints at some intelligence and forethought. One macabre anecdote is that while investigating an alleged kill of a human infant (4 year old girl) I was brought to the tree where her severed limb was found. The circumstances led to no doubt that the accusation was true for no leopard could have climbed that tree, and nor did the locals know that eagles cached limbs.
One plausible survival strategy when things look bleak is to hunt prey outside the normal prey range. I once found a trained male Crowned Eagle, left out for the night on a freshly killed dove early the next morning, and I have also flown other males at spurfowl and guineafowl with some success. Small males in particular can hunt birds quite frequently, and I knew of a pair in Mweiga that frequently took Kenya Crested Guineafowl and Leslie Brown once found a fresh Marabou Stork! Apart from birds they can also kill very large prey.
The Crowned Eagle would seem to be overly well-endowed with massive killing feet. None can talk of this eagle without reference to its extraordinary power and ability that, if it so wished, can kill animals 10 times its own body weight. If in doubt, Youtube the less powerful Golden Eagles killing Wolves and you’ll get confirmation that eagles can kill very large animals if they have to. The desperate hungry young of most raptors (both wild and captive) are reckless and capable of extraordinary feats of strength. One must put to bed the oft-repeated notion that eagles cannot kill very large prey even though it does not do them any favour amongst sheep farmers! Acknowledging that they can, opens up an intriguing debate as to why they do, but do so rarely. On occasion a Crowned Eagle can step into the mega-carnivore niche and this should surely have a survival benefit for when usual prey is temporarily unavailable? Larger forest animals such as large duiker and bushbuck may have less need to eat nutritious fruiting foods and thus are more sedentary and stay within confined territories. These species can be found alone and not be dependent on sentinels. In high latitude parts of the world when food is suddenly made scarce due to hibernation, snow, inclement weather or migration, food deprivation can drive an eagle to kill much larger prey. The seldom used reserve of immense power has an easily understood survival benefit in these circumstances. It does have its dangers of course in their being injured or exhausted to collapse. But it is an important, intriguing and yet curiously neglected part of many raptors’ biology.

Crowned Eagle nest

Crowned Eagle nest, © David Gulden

The status of ‘forest’ eagles can be made by quantifying the square kilometres of land under primary forest, and by subtracting most of those forests that have been poached out for bush meat. For an eagle it historically must have been surprisingly abundant, but few would argue that good quality forests with prey is anything other than a rarity and under severe threat. From a status point of view the ‘forest’ eagle occurs in those rapidly dwindling, formerly vast tropical rain forests of West and Central Africa. We can make fairly good guesses at their rate of loss if we know the rate of loss of these forests and their prey. Unbiased interpretation of satellite imagery should give one a good estimation.

The ‘Bush’ Eagle
‘Bush’ eagles are those that have ventured out of the wet jungles and colonised dry forests, moist central African-type isolated remnants, riparian and open highland woodlands from Ethiopia, southern Sudan, throughout Eastern Africa and down through the miombo belt to Southern Africa. In East Africa many forest patches were found clinging to the tops of isolated hills and mountains, and on inspection almost invariably had one pair of Crowned Eagles. These forests, respected by law and custom survived up until the late 1970’s when the human population seemed to tip the balance and lead to a country-wide loss of quality and extent of this eco-type. I believe it quite feasible to state that if a country has a certain human population density, some species can survive, but add on a few more million and they cannot. Those forest patches exceeded that threshold 30 years ago when we had less than half the human population.
Their day to day life, prey base, food security, hunting methods and competitors are different to the ‘forest’ eagle. I have watched soaring ‘bush’ eagles high above a thin riparian forest launch a successful attack against young warthogs feeding out in a dry desolate treeless plain. The flight, attack and location was atypical of the species, but it did not look out of place at all. These eagles hunt open area species small ‘plains game’ ungulates, carnivores, mustelids, viverids, primates, rock hyrax, hares (even Springhares) and so differ from the ‘forest’ eagles.
I was lucky to have sat with Leslie Brown looking out across the tiny forest patches of Ololua and Eagle Hill looking at his famous eagles and hear him speak of the changing menu of those pairs as forest prey species were lost and replaced by more open savanna species. Their adaptability within their own lifetime was impressive, with the Ololua pair changing its diet from diurnal Suni, duikers, monkeys and hyrax to nocturnal genets, mongoose, greater and lesser Galagos and hyrax in the 1980s. The change was forced upon them as poaching and disturbance obliterated the diurnal species. Notably they struggled to breed successfully for a decade before becoming only an occasional and non-breeding visitor. A change in diet such as this may be a pre-emptor to loss. These scrounging desperate eagles are picking the bottom of the barrel and Crowned Eagles that utilise unusual, domestic or nocturnal animals should not be thought of as successful.
These ‘bush’ eagles closest neighbours are not usually their own species, but other large eagles with some prey overlap. Aggressive encounters with their own are less likely but they must compete for space, food and sometimes nesting sites with other species such as Martial, Verreaux’s, Tawny and African Hawk Eagles and thus lose their monopoly. Having other species as immediate neighbours may help buffer encounters with other Crowned Eagle pairs who live the next street over.
The ‘bush’ Crowned Eagle’s distribution is not straight forward to predict. In Eagle Hill near Embu the famous Crowned Eagles lived in an area of some 10sq/km of forest patches interspersed with rocky out-crops and low thicket-clad hillsides. But other similar neighbouring hills, larger and as attractive did not have a pair. The wooded hillsides formerly so typical of mountains, rift escarpments and hills throughout Kenya were certainly likely to have pairs up until the mid 1970s, but it would be hard to estimate exactly how many without each being checked. Because each pair owned a forest patch and were separated from the next forest patch (10km to 40km) it was not possible to estimate density in terms of 1pair/??km2 in the same manner as those ‘forest eagles’ in contiguous forest. The Eagle Hill pair were alone throughout Leslie Brown’s 378sq/km study area during the 1970s for example, but that is not to say there is 1pr/378km2. The nearest pair I knew of was some 50km distant making the density 1pr/2500km2, or not. From a satellite photo these isolated and small locations would be tough to predict as Crowned Eagle habitat and one would be seriously led astray if one persisted in establishing a density figure for this habitat.
In contiguous forest Leslie Brown thought nests were separated in Kenya by 15km, and this may have held true in our highland forest where biomass is less abundant. I encountered 4 nests some 5 to 6km apart in the Aberdare’s Salient, but worried about being so bold as to make a density estimate. For one thing, each nest tended to be in a valley and pairs would utilise that valley in long winding corridors. In the adjacent valley, over the ridge another pair would operate 5 to 6km distant. While nests may have been close, actual territory use was linear or wedge shaped and separable by ridges. Short of radio telemetry work establishing territory size in East African fragmented forests is tough. Because of “edge effect” particularly where park boundaries meet densely populated rural farmland communities with small livestock, I suspect that nesting sites and territories require a buffer of at least 2km. But in more tolerant areas without high level of persecution they can nest within (see Ololua forest pair in ‘Urban Eagles’), some 100m of human habitation.

It could be supposed that a population that lives in drier less productive areas would have less biomass available and less density. Within these restraints ‘bush’ eagles, particularly at higher latitudes would be expected to reproduce less, mature slower, live longer and have heavier body weights. Perhaps they do.
The status of the ‘extralimital’ ‘bush’ eagle population is probably as important as the forest eagles, if not so dense, it is wide spread. However statistics on the presence or removal of suitable small forest and woodland habitat across this region is worse than that of the rain forests, as it is not seen as an eco-logical crisis worthy of investigation. Dry woodland loss is deemed less noteworthy of international concern, difficult to qualify and quantify on satellite imagery and frustratingly difficult to assert as suitable for Crowned Eagles. Leslie Brown used to wonder why it was that seemingly less suitable sites would have many eagles, yet others larger, and to us, better sites did not have nearly as much. It is therefore not very easy making accurate judgement of a forest from afar, and footwork is required.

The urban Eagle
No summary of the Crowned Eagle would be complete without reference to their ability to nest within very close proximity of sub-urban humanity. In Kenya the famous Ololua pair that nested within sight of Leslie Brown’s office window in Karen was testimony to their tenacity. That same site was known to the Bursell family back in the 1930s, and it survived regularly producing young until the late 1980s. It was one of the last pairs to go from a list of 23 known to me. Nesting activity ceased in 1994 and by 2005 Crowned Eagles were very rarely heard or observed in the area. An adjacent pair nested on a Cape Olive only 120m from the road I used to take to school. The site was within Nairobi National Park, but they must have used residential suburbia to hunt. Today that pair seems to have moved 500m to a small eucalyptus plantation, incongruously still within the park’s boundary but adjacent to a main road. Yet another adjacent pair nest in a croton in a newly formed sanctuary near a mortuary, racecourse and show ground. I recall incidences of direct persecution of these eagles, one shot by the secretary to the Elsa Trust, another shot for eating Burmese cats, another shot in Mwitu Estate for again eating cats. These individuals, despite an educated upbringing and very aware of conservation issues epitomise the NIMBY (Not In My Back Yard) sector, that is all too prevalent in Kenya.
In Durban such a pretty scene is repeated. Pairs nest in river gorges that have on either bank a suburban setting. Pairs nest elsewhere not in protected indigenous forest parks but in forests sometimes dominated by plantations of exotics.
These pairs have one thing in common seldom if ever encountered elsewhere. These territories lie within the affluent, well educated elite communities who raise Swarovski binoculars at them, not (usually) shotguns. It is absurd to assume that this pattern of occurrence is replicated anywhere else where no such veneration will excuse the eagles of their depredation of livestock or bush meat.
The forests near Nairobi have governance and protection seldom afforded any other forest (other than those protected in well managed national parks). There was for example dead timber on the forest floor, suni droppings, minimal livestock, minimal snaring and low human disturbance. Whereas other forests, far from the capital city, even in much less populous areas have as a rule a rural community to sustain with firewood, livestock grazing and other natural harvestable produce collection. Incidentally those pairs in suburban Nairobi forests are by no means secure and are probably under immense stress and should not be held as examples of the success of the species.
Previous to the aforementioned gum tree nesting pair I never knew or heard of these eagles using anything other than old mature dominate native trees within native forests. I recalled seeing gum tree leaves in a nest near Mweiga in 1978-79, and speculated then if they knew the insecticidal properties of this (then locally) tough to find exotic. Crowned Eagles will of course use eucalyptus and other exotic trees to perch in wherever these trees mingle with indigenous forests or bush. For the vast majority of plantation exotics in Kenya they are all characterised by sterility and as much biodiversity as a wheat field. These exotic forests have not had the time to stabilise and slowly allow habitation by enterprising wildlife pioneer settlers. Again these exotic forests support rural communities on their fringes and within their core who disturb the forests and probably keep such pioneer species out. I hear for example, that the Red breasted Sparrowhawk is very much at home in exotic plantations in South Africa, whereas in Kenya is has yet to be recorded in anything other than indigenous and now rare type of highland forests.


6 Comments so far ↓

  • John Huntsberger

    Learning about the African Crowned Eagle and the habitat in which they are found is an entirely new ‘birding experience’ for me as I’ve no knowledge of the bird or its habitat. Mr. Thomsett’s writing is not only informative, but captivating to experience, holding my attention throughout.

  • John Huntsberger

    Learning about the African Crowned Eagle and the habitat in which they are found is an entirely new ‘birding experience’ for me as I’ve no knowledge of the bird or its habitat. Mr. Thomsett’s writing is not only informative, but captivating to experience, holding my attention throughout.

  • Rob Davies

    Well I have just managed to finish reading parts 1 and 2 of this treatise and I eagerly await part 3. I had to print them out to get through that info but brevity is not a requisite when it would preclude the sharing of this immense knowledge of a vulnerable and difficult species. Well done Simon and I hope this is the early makings of a very valuable monograph. I hear your comments about the status of Black / Verreaux’s but maybe that is something we could discuss elsewhere and I’m sure we shall chat about it. It is indeed sad to hear of the loss of these eagle pairs from high population areas and it is difficult to establish status across the whole region when most population estimates of eagles are still not much more than guesses in the IUCN categorisation. I hope we can make those guesses more educated based on habitat extent and human impact and your account of the Crowned Eagle goes a long way to enabling that process for this big forest eagle. I have really enjoyed the evolutionary insights in your account and it is rare to come across some writing that delivers so much new and deep knowledge about an African bird of prey. Wonderful and a detailed record of the history of this species in your ‘neck of the woods’. Leslie Brown would be saddened by what has taken place for this bird but he would be very happy that you have carried the torch for them so ardently. My favourite bit in your writing is how you convey the relationship of this bird to the different treed habitats in an evolutionary and behavioural context, including our own evolutionary context! The concept of the rain forest as a rich 3-d surface area for these birds through which the smaller prey species follow the fruiting makes so much sense and explains the nature of those forest eagles. on the urban eagles, we had a lady in Pietermaritzburg (near Durban) who phoned us to complain about a Crowned Eagle which was devouring her pure-bred Siamese Cats and dropping some of their hind-quarters back into the garden when the meals were finished. She said she had hung the washing out over the garden to scare off the birds but that didn’t seem to be working! I was horrified to read of your ‘conservationists’ who actually shot these eagles. Let’s hope that a culture of tolerance can prevail but first up is obviously saving the habitat.

  • Steve Bergh

    I am completely engrossed with this part 2. So much conflicting data in comparison to what I have personally seen. I had a imprinted female perigrine that would lay eggs march 28th every year. Not the 27th or April 1st always March 28th no matter what the weather. I find it amazing the variance of the breeding times the Crowned has.

    I have always wanted to fly a Crowned in falconry but have changed my mind with this part2. I am a long winger at heart and the Crowned is defiantly a huge Goshawk. The examples of behavior are so accipiter like I find it amazing.

    I have a Golden Eagle I have had for falconry that is 28years old. I have had him for 22 years now and is about a calm a bird I have ever had. Very Aquila in disposition. Being a terciel is a plus. He is imprinted and tries to copulate on me this time of year but no way to use the seman for breeding because of the laws in the USA. He has a liking to kill dogs so I had to stop flying him because I fear a law suit was imminent. In Alaska where I live there is a huge stronghold of Golden Eagles. A doctrinal paper was written by Carol MacIntire on the Golden Eagles in the Denalie park area. The game is so plentiful that less than 50 miles of valley from the visitors center to the Toklat river she documented around 50 active nest sites. These aeries were producing many times triples. No Cain and Abel issues. I believe it is because there is so much food being provided by the parents that the competition goes down. Their is minimal crabbing among nest sites for defense of territory unlike what is described by the Crowned. Totally amazing to me to see the inconsistency between species.

    My old falconry apprentice for years went to work for a man in the US that had a large raptor program. They had a female Harpy they obtained years back and he was one of the main people to work with this bird. I went down to see him and was able to observe the bird close up and found it amazing how calm she was. She was easy to handle and not accipiter like at all. Very Aquila like and even calmer than most female Goldens I have handled.

    The Crowned is one of the most impressive birds of all the raptors I believe. I have always thought this. As a falconer I can see those meat hooks doing some incredible damage to prey . The plumage is spectacular and stunning. Having breed raptors in captivity myself in understand the commitment and dedication this requires. My hats off to you Simon for a great job well done. I attended the breeding seminars the P-fund put on 25 years ago when they moved to Boise and people like Cal Sanford have changed the ability to guess about things to actually allowing people be successful at breeding. Still I am in awe you have captive breed this magnificiant species.

  • Kevin Bestwick

    Fascinating detail. Thank you Simon for sharing your insights and experiences. I have the privilege of having an active nest at the bottom of my garden in Westville near Durban, South Africa, which I am able to monitor and photograph daily. See my blog

  • joey mokoena

    thanks for the information, i leave in the city and the only time i see this eagle is on tv. Now i have a better understanding and respect for crowned eagle

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