In the late 1970s I went with Leslie Brown and Peter Davey to the famous Eagle Hill in Embu where 8 species of eagles all nested. It had been studied since the 1940s, making it then the longest study of eagles anywhere in the world. Eagle Hill was no different from the multitude of other hills that dotted the landscape on the eastern lower slopes of Mt Kenya from Machakos to Meru, though it did have a higher density than usual. In those days there were still rhinos in the hills and lions in the scrublands below. The people there, the Wa’Emberre, had no problem with eagles, being so familiar with them that when asked, they would show you an eagle nest, know its species, sex differences and when it last raised young. Two Ayres’s Hawk Eagle nests had been found by honey hunters and were part of Leslie Brown’s study group.
Among all the big African eagles there was the small Ayres’s Hawk Eagle, the most delicate and the rarest of them all. It was a favourite of L. Brown who would comment on its terrible nesting success in his books as being the reason for its rarity. This was a weird twist in the understanding of reproductive biology I couldn’t then understand, for no species would be a klutz at breeding to its own detriment, but in the intervening years of watching nest failures, for the silliest of reasons I had to concede he was right...as always. Even a Grey-headed Bushshrike killed their chick once. Their nests had a predilection for falling down and we saw the remains of a large fledgling and its mother, killed by Verreaux’s Eagle Owls. Elsewhere they seemed to fail much of the time and be accident prone. Nesting failure and high mortality dogged the species.
On a hill called Kiangungi
On a smaller hill lower down in the dry acacia plains, then devoid of any obvious sign of human habitation other than a few smart circular thatched Wa’Emberre homesteads, with fine wicker-work chicken pens on stilts behind neatly made lion proof thorn walls was a hill called Kiangungi. It had a Martial Eagle pair and a pair of Ayres’s Hawk Eagles. In those days it was quite feasible to throw a sleeping bag in the car and drive from Nairobi to Embu, sleep the night under the stars listening to lions, and get back the next day counting Wahlberg’s Eagle nests all along the way to Thika. I went once with Peter Davey without Brown, the purpose being to renovate the study hides at both eagle nests. I was young and fit and clambered around trees while Peter gave orders and smoked his infamous pipe. Embu was as big an adventure as Tsavo except that you messed about in it, talked to the people there who honestly knew where everything was, climbed hills and fell out of trees and slept with trepidation in rhino paths. Ayres’s Hawk Eagles bring back that strong sense of nostalgia whenever I see them.
I found myself having a few hours to kill alone in the Ayres’s Hawk Eagle hide with my camera. The hide was made of hessian sacks, with brush sewn on the outside, placed on a small cliff under a dark forest canopy. Less than 100m away in a large croton tree was the nest at a lower angle down the slope below. You learn a great deal in a hide, seeing and hearing the surroundings in a manner no other method, no matter how high tech, can reveal. You sense the reasons for their caution, actions and behaviour. Perhaps it is invading safari ants, bees, a mongoose, suni or large eagle. You see the prey delivery and where the male prepares the food and the fussing of the female feeding her young and the small details of their domestic life. When the adults are away you panic when something threatens their chick and you get out and shoo it away. You recognise individuals and learn their nuances and characters. They are not stupid for they know you are in the hide, but that hide and researchers had been there since their grandparents and they had grown used to it. No one bothered them. The occasional local medicine man or honey hunter searching the hills knew they never took chickens, and that they conducted their affairs above ground. In those days there was no communication, no phones, no radios and nothing to distract and as a result we were fine tuned to nature.
I took a photo that turned out to be the first colour photo ever taken of the pair and their chick! It wasn’t a good photo. I used print film and had what would today be laughable equipment but back then it was something to be proud about. It upset Brown, for his were all black and white and that was a score in itself.
Embu, Kiangungi. Circa 1977. Male with a typical white forehead and highwayman’s mask, and female with typical dark helmet. The young are variable too with either a cream ground colour on the chest or a rufous wash. The species is certainly “polymorphic” with dark morphs as well. There is some debate as to whether or not there are truly melanistic individuals or simply very dark, dark morphs.
The Ayres’s unique bird-hunting technique
We watched the eagles hunt on a number of occasions. They would soar up on very broad wings looking languid and buoyant and not the bird hunting specialist at all. But in a few turns they would be very high up even on grey days. Then, after you had lost sight of them, from out of nowhere the small birds below would all erupt in panic and flee. From the epicentre of this expanding ring of panic an Ayres would emerge carrying a bird. Once on Eagle Hill, while standing on a Peregrine cliff an Ayres stooped past us and down into the top of bare croton trees and took a green pigeon as it jumped off the branch. Looking from above we got to understand that the Ayres prefers to take birds from tree tops. There is no aerial pursuit or twisting chase and minimal energy wasted in flapping flight. If they missed they missed, and they would set their wings again to mount up on a thermal and vanish overhead. Later I saw Ayres hunting doves and Quelea with Peregrines and Lanners at waterholes in dry arid areas. Recently while buying hardware on a dusty road side in Elementeita I saw an Ayres circle high above. The moment I forgot about it I saw it pull out of a terrifying dive between power lines and hit a domestic pigeon out of a group that had just taken off in panic. Not a single person saw it despite the noise and the floating feathers. These eagles seem to have a special ability to catch birds in the act of fleeing from a perch.
This photo is an attempt to portray the last thing that crosses a dove’s mind, or the first thing that confronts a marauding eagle owl. One of the defining features of the Ayres’s Hawk Eagle is the white “landing lights”. The purpose of which is unknown. I believe these are ocelli (or ocellus), artificial “eyes” that present a threatening face. Booted Eagles have them too and we see similar patterns in the frontal portrait of Gyps vultures though their “eyes” are on the upper chest.
The Ayre’s Unique stooping flight
Their stoop of the Ayres is comparable to a large falcon though with a smoother trajectory and less twists and turns in mid stoop. Seen from the back the wing tips are just short of the tail and the shoulders are broad, very similar to a large falcon. What is different is the length of the secondaries that extend so far down the primaries. These secondaries are long, giving a big surface area allowing the Ayres to soar.
The similarity in the wing tip to tail ratio, and broad shoulders to a large falcon is readily seen in the left photo. On the right is a fuzzy photo of a space shuttle re-entry or a stooping Ayres’s Hawk Eagle?
Ayres’s have a “heart shaped” stoop well known in guide books, with the tips of the primaries held at the base of their tail and the elbows and hand pushed away from the body. Large falcons, eagles and buzzards do this too but it’s not their speciality. The alula, literally their thumb, hosts a small winglet and on the Ayres it is supported by a longer and stronger bone than any raptor I know. It is on close inspection about the same size as an accipiter’s alula, but the feathers themselves are tougher. This winglet or bastard wing as it is properly called, is common to all birds and provides a slat that allows clean air flow over the top of the wing at a high angle of attack. It is used in the flare-out landing to slow a stall. On the Ayres this winglet in the nose down stoop is very conspicuous and it moves in and out at a higher rate controlling the descent more than in other stooping raptors. (Kestrels by the way also stoop with the winglets out, but I suspect they are used to control and break the fall). In a stoop the alula clearly isn’t functioning as an anti-stall device and serves another function. As the Ayres flattens out from the stoop hitting speeds well over 150kph these winglets can support their body weight until the speed trails off to about 50kph, when the wings come out. There is minimal form and induced drag and it shows as it hurtles through the air in a vertical to horizontal closed wing stoop. In the impact flare the massive feet that look like under-slung torpedoes are suddenly lowered and the toes opened wide, the wings form a rounded parachute profile, the chest is pushed forward of the head and in a microsecond the foot grabs the bird in mid air. Importantly the bird is then carried away. Very seldom do they land close to the impact site and consume their prey. Once I saw an Ayres on the Naivasha lake shore on a small duck in the reeds, and another on a cricket pitch in Nairobi with a Crowned Plover, both on the ground, but this is unusual.
The Ayres is an extreme flyer, at the edge of its flight envelope and as refined as a Peregrine. It has evolved away from other eagles, filling a niche over woodlands and forests, and it copies neither a goshawk, large falcon nor typical eagle. It is unique in Africa although the Rufous-bellied Eagle from Indian and the tropical Far East would be a direct (though unrelated) counterpart. The Booted Eagle, to which it is often compared, also has an impressive stoop but its feet are much smaller. The Booted is a bird hunter too but its feet suggest prey of a different kind and perhaps the need to land and kill the prey.
Photo from Timau showing an Ayres melanistic morph. This is often considered absent in Kenya in guide books but it is present. One would expect to find all morphs in Kenya because the species is a nomad or sometimes intra-African migrant. Note how broad the centre part of the wing is.
“My! What big feet you have!” said Little Red Riding Hood (Worst raptor biologist of the century).
“All the better to kill you with dear”...
Looking hard at the Ayres's Hawk Eagle’s feet one is immediately impressed by the length of its middle toe and the very long inside and hind talon. A 1000 gram female Ayres has as long a hind talon as a 2500 gram female Tawny Eagle, a species that can kill gazelle fawns. The female also nearly copies the foot mass and span (of 134mm) of the male African Hawk Eagle, a very powerful eagle.
Extraordinarily large feet on other raptors mean they kill things heavier than themselves. But the Ayres does not. Looking to other species with massive feet that kill things smaller than they, like the Fiji Peregrine or Orange Breasted Falcons that kill parrots over forest tops, the Verreaux’s Eagle snatching a hyrax off a cliff or even the Harpy Eagle that kills sloths, macaws or monkeys, they have as a common habit the ability to completely immobilise their prey in mid air. They kill above the forest canopy or in the trees or above ground and do not need to descend to the ground to administer the coup de grace. They need big feet to immediately incapacitate their prey. Parrot killing falcons have huge feet and bills, because parrots are dangerous if not killed quickly and one must wonder how important parrots are on the menu of the Ayres’s Hawk Eagle in the Central, West African forests where parrots and hornbills are a major component of avian biomass. Curiously the middle toe does not have the fleshy protuberance just aft of the talon that serves as an extra gripping toe. You would have thought that over a few million years of being a specialist bird hunter they would have evolved this...after all even the very unrelated large falcons have done so. It does have however an unusually long and curved middle talon that provides that extra span of grip.
The Ayres’s foot is compared to the rear and front toe and talon of a Black Sparrowhawk female (Amputated by an overzealous individual ...but survived with a prosthesis for many years). It is similar in having an elongated hind toe and talon (hallux) although the foot mass is much smaller.
Note absence of the fleshy protuberance on the Ayres’s left middle toe. Compare this with the picture on the right, clearly showing the protuberance on the middle toe of a sparrowhawk. Also note the entire length of the sparrowhawk’s upper middle toe is scaled, like the belly of a snake, uniform plates, proximal to distal. The Ayres has the first 4 scales near the talon which then transform into fine pentagon/hexagon scales like most eagles, the Martial and African Hawk Eagles especially.
Indicator of mate-quality?
I’ve been hatching a theory that raptors may sexually select mates with big feet. The bigger the feet the more likely you are to support another (a mate and young) with food and defend the territory and nest-site. It makes a lot of sense to show off this with an exaggerated show of weaponry. Gelada baboons display teeth that are massive...although this species eats herbs and is very peaceful. Those teeth are important nevertheless. Smilodon Sabre Tooth carnivores had teeth that appear to impede and perhaps sexual selection and social hierarchy had something to do with it? In Nepal I stared with disbelief at the feet of the Mountain Hawk Eagle. They would cruise by with their massive feet hanging low, as if showing off. And in the cold mountains this costs a bit in terms of calories to do. When we caught them for tagging I was a bit disappointed in seeing that they had a long middle toe and were primarily bird hunters. So why the overt display of force?
The Ayres’s Hawk Eagle isn’t that far off the Mountain Hawk Eagle in foot mass and design though its flight, build and ecology differ hugely. When looking at the Crowned Eagle foot one has to stifle a growing sense of panic, they are that intimidating. They are not inconspicuously painted either: feet are yellow, eyes are yellow. Yellow stands out and is a sign of health because it can fade when ill. I suspect the Ayres, Mountain Hawk Eagle and Crowned Eagle may win over mates by showing off their feet. I don’t like brooding on a second or complementary theory and that is one of intra/inter/extra-specific fighting. Sadly eagles do fight more than most biologists think. Fish Eagles are notoriously aggressive for example, and their feet need not be so big to catch the fish they do. Small footed Fish Eagles would not be able to defend themselves as well as large footed combatants. Few have thought about raptors evolving large feet to fight for mates and protect territories, but I’m sure that once they have armament to kill prey it’s only a matter of time until they use them to maintain a territory from rivals and ensure the affections of their mate.
The Ayres’s bill is small and neat like a true bird hunter and the cere retreats into the top of the skull at a higher level than most eagles. The line of the pupil is lower in relation to the top of the cere than most eagles. It does not have that long drawn out big eagle cere and bill. It’s shorter, and you wouldn’t really know it unless you put a falcon (not an eagle) hood on it or tried to draw them. From the bites it inflicts there is good reason to suppose it can also use its bill to sever the neck of its avian prey. The shorter the bill the higher the torque, albeit over less surface area.
Last eagle standing
Ayres’s Hawk Eagles were so rare, so special and seemed, even way back in the Eagle Hill Embu days, as approaching extinction. Brown then noted that the Ayres’s Hawk Eagle, Ovambo Sparrowhawk and especially the Taita Falcon were contenders for conservation attention, something we have overlooked since. Vultures and large eagles and large falcons were so abundant it was almost absurd to suggest they would decline, but as he noted shortly before his death in 1980, there was a sudden shift in human perceptions and tolerance, a sudden tangible change in environment quality and of course a staggering increase in human density incapable of living with or able to sustain even small wildlife let alone eagles. Ironically the Ayres’s was the last eagle standing on Eagle Hill by the year Brown died*, following a collapse in the eagle population in less than 3 years.
*Thomsett, S,. Eagle Hill, Kenya: changes over 60 years, Scopus 34: 24–30, January 2015.
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