Crowned eagle

Appearance

The crowned eagle is a very large eagle. Measuring from 80 to 99 cm in length, it is the fifth longest extant eagle in the world. The female, at a weight of 3.2–4.7 kg, is around 10–15% larger than the male, at a weight of 2.55–4.12 kg. An average body mass of 3.64 kg was given in one account. Elsewhere, an average of 3.8 kg has been claimed. Slightly smaller weights were found in a South African survey where 5 males averaged 2.65 kg and 8 females averaged 3.71 kg. Overall, they are the 9th heaviest living eagle species. The wingspan typically ranges from 1.51 to 1.81 m. The largest authenticated wingspan for a female was 1.9 m, with a claim of wingspans of up to 2 m needing confirmation. This eagle's wingspan is quite short for the bird's size, being around the same mean width as that of a tawny eagle or a short-toed snake eagle, species that weigh about half as much as a crowned eagle. However, the somewhat boxy and rounded wings are quite broad, being broader than, for example, the much longer-winged golden eagle. The wing morphology of the species gives it maneuverability in its densely wooded environment. The wing chord measures 445–532 mm, with a median of 467 mm in males and 512 mm in females. In South Africa, 5 males averaged 475.2 mm in wing chord length and 7 females averaged 506.9 mm. While it, on average, is less heavy and has a smaller wingspan than the often sympatric martial eagle, its average total length exceeds that of the martial eagle thanks to its much longer tail. The crowned eagle's tail is from 300 to 410 mm long, with a median of 315 mm in males and 348 mm in females. South African males averaged 320.4 mm in tail length in a sample of 4 and females averaged 352.4 mm. The bill is of a medium size relative to its body size, with one large museum specimen's bill measuring 55 mm in length from the gape, 45 mm in culmen length and 33 mm in bill depth. In South Africa, culmen length averaged 50 mm in 4 males and 54.9 mm in 7 females, with a range in both 46.5 to 61.4 mm.

The tarsus is of a modest length for a raptor of its size, at 8.5–10.3 cm, and is clearly shorter than that of martial eagle. However, the feet and legs are visibly thicker and heavier than those of the martial eagle and the talons are apparently quite massive in both length and width. While few comprehensive measurements of the talon size of wild crowned eagles are known, one female museum specimen reportedly had a hallux-claw of 6.2 cm, while another female was measured at 5 years of age, the age of sexual maturity, 5.74 cm in the hallux-claw and an adult male measured 4.9 cm. In South Africa, hallux-claw length averaged 52.4 mm in 5 males and 60 mm in 7 females with a range in both of 48.6 to 61.4 mm. These figures put their talon size as around the same size as the largest golden eagles and half the size of a harpy eagle. Some captive crowned eagles have been credited with a hallux-claw length of up to 10 cm, although, much like a single report of captive harpy eagles with a 13 cm hallux-claw, no such outsized talons are known to have been confirmed. In a small sampling of large, forest-dwelling raptors, the front-left talon of the crowned eagle, at 4.74 cm, was around one cm less than that of a harpy eagle or the huge, recently extinct Haast's eagle and slightly smaller than those of the Philippine eagle. Considering a big female of these species can weigh up to twice as much as an average crowned eagle may illustrate the relative largeness of the crowned eagle's talons.

The adult crowned eagle is quite strikingly plumaged. Its crown is dark to rufous-tinged brown with a prominent, oft-raised black-tipped double crest, which can give the head a somewhat triangular appearance. The upperparts of an adult are a blackish brown-grey color, with a variable tinge of blue. The throat is brown while the belly and breast are white overlaid densely with blackish bars and blotches, variably marked with cream or rich buff-rufous coloration. The wing primaries are white at the base, broadly tipped with black and crossed by two black bars. The tail is black with brownish-grey bands. The thighs and legs are barred and closely spotted with black and white. The underwing coverts of adults have a bold chestnut coloration, spotted lightly with black. The adult crowned eagle has eyes that can range from yellow to almost white, a cere and feet of an ochre-yellow color and black talons. In the wild, misidentification of an adult is improbable thanks to the species' bearing and voice. The strongly barred outer wings and tail are all diagnostic in flight. Further simplifying identification, details such as the crest, the bird's upright perching posture and large size are unique to this animal. While they do differ somewhat in size, the genders' sexual dimorphism by size is relatively modest and eagles are unlikely to be sexed by this alone. However, the male may be distinguished by his more rapid wing beats from the more sluggish female.

As seen in about half of the "booted eagle" group, the juvenile crowned eagle has strikingly different looking plumage compared to the adults. Much variation occurs as the maturation process occurs. A great majority of juveniles have a white head and underside, which contrast with the thighs and legs, which are heavily spotted with black. The juvenile eagle's back is light brown or grayish-brown, with pale feather edgings that often give the back a scaled appearance, especially on the upper-wing coverts. There is often a pinkish red wash on the upper chest. Just-fledged chicks tend have dark patched faces, freckled bibs and slightly barred chests and spotted legs. Less common juvenile crowned eagle plumages, possible even when they are under a year of age and still under parental care, may include eagles so stripy that they which one could easily have aged as two- to three-year-olds. The tail of the juvenile is black with three pale bars and a narrow black tip. The juvenile eagle's cere is grey and the feet are dull yellow. By 4 months post-fledgling, the inner thighs, previously poorly covered with downy type feathers, are covered with small feathers. 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. 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 color is variable too with some having khaki light brown just prior to fledging and others with adult-like yellow ochre eyes. Up to 15 months after leaving the nest, the immature eagles more closely resemble the plumage they have at first independence than the adult's plumage. The juvenile may be confused with the similarly colored juvenile martial eagle, especially in flight. It is distinguished from the martial species in having a much longer, more heavily barred tail, much shorter wings and spotted thighs.

Distribution

The crowned eagle is found only on the continent of Africa. In East Africa, the crowned eagle's range extends from central Ethiopia, to Uganda, forested parts of Kenya and Tanzania to as far south as eastern South Africa, with a southern distribution limit around Knysna. In western and central Africa, the crowned eagle's range extends through much of the vast African rainforest. They may be found from Senegal, The Gambia, Sierra Leone and Cameroon, where they inhabit the Guinean forests, to the Democratic Republic of the Congo, where they live in the Congolian forests, and down south to as far Angola. Despite its large distribution there, the crowned eagle is now rare in many parts of West Africa.

Status

The crowned eagle is fairly common in suitable habitat, though at the population level, its numbers have shown a decline in sync with deforestation. Declines appear to be widespread and may be increasing due to the often fevered pace of clear-cutting. This species main habitat is rich, high-canopy forest, which is a major target of timber companies, agriculturists, palm oil and biofuel plantations and miners as well as slash and burn farmers. A charcoal-based economy outdoes mineral-based economics in Democratic Republic of the Congo and Zaire, both fuel wars and makes inroads deep into previously virgin forests. As two of central Africa's largest businesses, this has a devastating effect on forests and wildlife. Charcoal taken from Kenya and Ethiopia is often thought to finance Somali warlords. Tanzania, which is more heavily developed for agriculture than adjacent Kenya, has even more reduced forest habitat. The crowned eagle is far more common in protected areas and reserves than elsewhere in its range, though is still recorded consistently outside of these areas. Biologists in Africa now suspect that the crowned eagles adaptability to small, fragmented tracts of woodland has been exaggerated in the past. Some habitat losses have been offset by the establishment of exotic tree plantations, where this species can nest, but which generally lack a sufficient prey base. The crowned eagle in Ethiopia is certainly at very low density and restricted to protected areas. It may be obliged to utilize exotics stands, but it is unlikely to ever be capable of surviving in the complete absence of indigenous forests. Certain southern African countries, such as Zambia, Zimbabwe and Malawi, have almost no extensive stands of native forest today, while other countries such as Namibia were never heavily forested. In 2012 the species status was changed to Near Threatened by the IUCN. Like the martial eagle, the crowned has throughout modern history been persecuted by farmers, who maintain that the bird is a threat to their livestock. In fact, both the crowned and martial eagles only rarely attack livestock. In some cases, however, crowned eagles have actually been killed while attempting to hunt domestic animals. Another cause for persecution of the species is that crowned eagles are considered competitors in the illegal bushmeat and poaching trades. Within the forest land-locked countries of Africa, the bushmeat trade is the largest source of animal protein for humans. It is a multibillion-dollar business with some 5 million tons being killed each year. In just 500 million acres of the Congo Basin owned by 8 countries the weight equivalent to 40.7 million humans is removed each year. The effect of this unsustainable culling is to severely depress or remove the large, medium and small wildlife species of the forests. Crowned eagles require some 430 kg of "bush meat" a year and thus directly compete with the industry. The red colobus, a monkey that typifies the optimal forest quality and is a main food species for crowned eagles, has been singled out as one of the fastest declining and most endangered monkeys in the world due primarily to the bushmeat trade. In some cases, crowned eagles have reportedly even been shot by primate conservationists in a misguided attempt to mitigate their predation of declining primate species. It is estimated that 90% of the global distribution of the crowned eagle may be subject to habitual persecution or is even killed and eaten itself as bushmeat. On the other hand, some educated foresters and fruit-growers actually encourage protection of populations, due to the controlling effect crowned eagles have on populations of potentially harmful mammals. In April 1996 the world's first captive-born crowned eagle hatched at the San Diego Zoo. Among ISIS registered zoos, only San Diego Zoo, San Francisco Zoo, Los Angeles Zoo, Fort Worth Zoo and Lowry Park Zoo house this species. Several wildlife rehabilitation centers in Africa house crowned eagles. Due to their high-strung dispositions, tendency for aggression towards humans and resistance to hunting prey via coercion and hunger, the crowned eagle is often considered to be poorly suited for falconry. However, there are several eagles of this species used as such in England and sometimes in Africa, where they have been reportedly used to cull locally overpopulated feral dogs.

Habitat

The crowned eagle inhabits mainly dense woodlands, including those deep within rainforest, but will sometimes also be found in relict patches, wooded escarpments, riparian strips of "Acacia", heavily wooded hillsides, and rocky outcrops throughout its range. The crowned eagle may be found from an altitude of sea-level to at least 3,000 m. Owing to lack of current suitable habitat, the eagle's range is often somewhat discontinuous. In the Democratic Republic of the Congo, the crowned eagle has been confirmed to survive at relatively high densities in protected areas that maintain dense, old-growth rainforests. In Kenya, 84% of the crowned eagles range is within rainforest with an annual rainfall amount of more than 150 cm. Around stretches of East Africa where protected areas mostly consist of fairly open habitat, crowned eagles usually live in wooded areas of rocky hills and narrow riverine strips, only rarely ranging into savanna surrounding the hills. Southern Africa has been subject to the most comprehensive study of crowned eagle habitat, largely since many areas there would seem inhospitable to a large raptor often associated with old-growth forest. In southern Africa, its distribution south of the Limpopo River coincides largely with montane forest, although it is not restricted to that habitat and may range secondarily into plantations, usually of eucalyptus. In South Africa, it occurs in both lowland and montane evergreen forest, dense woodland, and forested ravines and gorges in open savannas and thornveld. In Zimbabwe, the crowned eagle can be found in quite open woods with "Adansonia" trees and may occasionally forage in savanna and secondary growth. In Malawi, highland birds forage in lower miombo woodland, and lower altitudes, breeding occurs in deciduous forest, more locally in dense miombo, tall riparian woodlands, and in remnants close to cultivation. Crowned eagles in Zambezi, occurs in evergreen forest in the eastern highlands, in rugged, hilly terrain over the central plateau, in hills and escarpments in the southeastern portions of the central watershed, and in riparian habitat along the larger rivers.

Reproduction

Crowned eagles have one of the most prolonged breeding cycles of any bird. It is common for raptors that live around the tropics to have a relatively elongated breeding period. Crowned eagle pairs breed once every two years; a single breeding cycle is approximately 500 days in duration. Most other eagle species complete a breeding cycle in under six months, or in about 35% of the time it takes the crowned eagle. While the incubation and nestling stages are about average for a tropical eagle, about half the weight of this species, has an incubation/nestling cycle of a similar length), it is the extraordinary post-fledging period of 9–11 months that makes the crowned eagles' breeding cycle so long. In harpy and Philippine eagle, although these are less extensively studied, it may take a similar or even longer amount of time for the young to attain full independence. A case of crowned eagle pairs that reportedly bred every year in South Africa are unsubstantiated but may be due to an apparently high population loss rate among juvenile eagles near areas that are heavily populated by humans. Breeding can occur almost year-around throughout the range, though egg laying seems to peak roughly around the end of the African wet season or the early dry season, from July to November. Territories or home ranges are maintained vigorously. In Zimbabwe, individual home ranges can vary from 140 to 200 km2 in size. Near the city of Nelspruit in South Africa, home ranges averaged 30 km2 in size. In southern Africa, the mean distance between active nest sites can range from 2 to 19.5 km.

After engaging in the breeding display described above, the pair collaborate in building a massive nest in a fork of a large forest tree, typically from 12 to 45 m above the ground. While the female fetches more nesting material, the male tends to be more active in nest construction. In East Africa, many nests appear to be close to a forest river. Generally, crowned eagles seem to be attracted to the taller trees in the forest. On the Nyika Plateau in Malawi, the favorite nesting trees are the large emergent "Aningeria adolfi-friederici" and "Gambeya gorungosana", and a pair in the Lower Shire used a "Sterculia appendiculata" tree. In Zimbabwe, "Newtonia buchananii" are reportedly one of the most used tree species for nesting. Exceptional crowned eagle nests have been observed on sheer cliff faces. In southern Africa, the species nest in drier and denuded terrain than expected such as "Adansonia" stands on semi-arid hillsides. Despite the relative sparseness of this habitat, these sites have a varied and convoluted terrain, with nooks and crannies, valleys, overhangs and hideaways that allow a crowned eagle to exercise its particular hunting skills. In Kenya, similar fractured landscapes can also be utilized by crowned eagles, such as the black gigantic volcanic rubble fields of Tsavo West National Park, the lower Chyulu Hills, Kibwezi and Soysambu Conservancy. These are jungles of boulders covered with low growth interspersed with high trees. A nest built from scratch may take up to 5 months to construct, however existing nests are often repaired and re-used during successive breeding seasons, a process that can take as much as 3 months. It is typical for an eagle pair to use a nest for more than five years and, unlike several other booted eagles, crowned eagle pairs rarely build more than one nest for alternative use. Most large eagles build a very large nest and the crowned eagle is no exception, as it builds one of the largest nests of any eagle. In the first year they build a nest, it may measure 1.5 m across and 50 cm deep. However, a larger nest, usually after several years of usage, may measure up to {{convert|2.5|m|ft|abbr=on}} across and up to 3 m deep. The nest consists of both dead and greener branches and has a light coverage of leaves and animal matter. Copulation takes place in the nest, several times a day. Reportedly copulation can occur up to a year before laying, although these may be exceptional cases of mating for non-fertilization purposes. A pre-copulation display typically occurs, wherein the male runs repeatedly around the crouching female with wings upraised, which displays the chestnut of the under-wing coverts and beautiful barring.

In South Africa the crowned eagle lays its eggs from September to October; in Zimbabwe, it lays from May to October; mainly nearer to October around the Congo River; anywhere from June to November in Kenya, with a peak in August through October; in Uganda from December to July; and in West Africa, laying peaks in October. The clutch of the crowned eagle either contains 1 or 2 eggs. Often in East Africa, just one egg is laid. Eggs are usually just white, though may sometimes be overlaid with sparse red-brown markings. The eggs are moderate in size, averaging 68.2 mm × 53.6 mm, with ranges of 60.9–75.5 mm in length and 50.8–57.9 mm in width. When a natural disaster befalls a nest, a replacement may be made in 2 months time. Incubation lasts for approximately 49 days. 80–90% of egg incubation is done by the female during the day. Food is mainly brought to the nest by the male in the early stages of breeding, though sometimes both sexes may deliver food. The male brings food to the incubating female every 3 to 5 days. When they initially hatch, the young tend to be quite quiet. If two eggs are laid, the younger one dies by starvation after being outcompeted for food by the older one or even directly killed by its older sibling. No nest of wild crowned eagles has been known to successfully produce more than one fledgling, though in captivity two have been known to survive with human assistance. In cases where the older nestling dies, the younger one may be fed more regularly and survive.

After hatching, the male kill rate rises to one kill every 1.5 days. Pair behavior while raising chicks is very variable, some males are very attentive to their young, while others leave virtually all brooding to the female. After 40 days of age, the young is capable of feeding itself, though is often still fed. The first feathers through the white down emerge when the crowned eagle chick is 40 days old, with the feathers ultimately covering the down in 76 days. After 76 days, the main feather development is in the tail or the wings. Wing flapping begins at 45 to 50 days, increasing after around 75 days. The young fledge at 90 to 115 days, with an average 110.6 days and any period of time less than 100 days is considered unusually soon. On average, male chicks tend to be more active wing-flappers and usually will first fly around 10 days earlier than female chicks. After fledging, females are attentive 95% of the day and brood 50–75% of the day, the amount decreasing slightly with each day. The female does much of the prey capture and a majority of the nest defense after the young fledge. After fledging, the young remain in the neighborhood of the parent's nest and are fed every 3 to 5 days by either parent for their first 270–350 days of life. The rate of food-delivery varies from several times a day to every 3 days on average during the post-fledging period. The fledged juvenile will solicit adults for food but does not actually take the prey unless this occurs around the nest site. The first recorded kill for a young crowned occurred 61 days after fledging, although this is considered exceptionally early by the standards of this species. Flights increase incrementally through the post-fledging period, although the young do not engage in rising flights until they are fully independent. Independence appears to be triggered by the increased indifference of parents to bringing food. Due to the loud vocal interplay between the parents and the fledging eagle, the adults seem to take it as a sign that their offspring has sought independence if they return to the nesting area and hear no begging auditory response. The young eagle usually remains in the care of its parents for a total of up to 11 months after it fledges, longer than is known in almost any other raptor. The advantage of this prolonged stretch to independence is that it may make for a stronger young eagle when compared to other accipitrids which have almost no post-fledging dependency period. In 34 possible cases, 18 resulted in eggs being laid. Fledging success is approximately 83% and almost all young that leave the nest also reach independence.
It is estimated that most crowned eagles will reach breeding maturity at around five years old, as is typical for other large eagle species.

Food

The crowned eagle is often described as the most powerful raptor in Africa, even more so than the two slightly heavier species endemic to Africa, the martial eagle and the Verreaux's eagle. One listing included the crowned eagle as the only bird in a ranking of the 10 strongest living land creatures. Elsewhere, the harpy eagle is listed as the overall most powerful living eagle and bird of prey. Since there are no known actual tests in any African raptor of the pressure exerted via their grip, as has been done with some other large eagles, their power has been estimated from the size of the feet and talons and from the prey they typically select.

The crowned eagle's staple diet is mostly mammalian. One estimate of the typical prey range posited that the weight range is from 1 to 5 kg, which is, based on the species ecology in Kenyan hillside woodland. This prey weight range is roughly the same weight range that's typically attributed to martial or Verreaux's eagles. Perhaps unsurprisingly, that is the weight range of the rock hyrax, which all three large eagles are known to hunt regularly in East Africa. In a large collection of bones in the South African Museum, it was found that 51.2% of the bones collected from Nature's Valley in South Africa were from smallish species which weigh under 2 kg in adulthood, 26.3% from "medium-sized" species with an adult weight of 2 to 20 kg and 22.5% were from larger species with an adult weight of over 20 kg. However, about 91% of 87 bones that were from the relatively large antelope species, amongst those that could be accurately sized, were from juvenile specimens. On the other hand, in the rainforest community of Taï National Park in the Ivory Coast, the estimated average weight of prey for crowned eagles was clearly higher at 5.67 kg. Of all other living eagles, only the female harpy eagle has been credited with an average prey weight range that is comparably high and, at the species level, the crowned eagle's prey size from the latter study is the largest of any known for all extant accipitrids. The crowned eagle is perhaps the only living accipitrid to routinely attack prey weighing in excess of 9 kg. According to some authorities typical prey sizes for crowned eagle may be considered to range to at least 20 kg in body mass. Reportedly, the crowned eagle can lift more than its own body weight in flight, though verified accounts of this are sparse.

In a deep forest, an adult eagle may cover a hunting range of up to 6.5 to 16 km2, with the home ranges being smaller for those that inhabit rocky hills and cliffs abounding in hyraxes. Eagles start hunting soon after dawn and mainly kill early in the morning and in the evening prior to sundown. Being a forest-dwelling species, the crowned eagle has no need to travel great distances to hunt, nor employ a great deal of active hunting flight. Rather, it tends to hunt passively. Crowned eagles may locate a suitable hunting spot by listening or watching for prey activity, though may also use habitual hunting perches where they've previously had hunting successes. Although this behavior is unconfirmed, some crowned eagles have been reported to let out a soft whistle, unlike their other vocalizations which, for some reason, is attractive to monkeys and will then attack the first monkey to enter their line of sight. These eagles often still-hunt, wherein they drop or stoop onto prey from a branch perch. Following the sighting of suitable prey, the eagle quickly and stealthily maneuvers itself through the forest towards its prey, a certain element of surprise inherent in its final approach. A majority of the crowned eagle's kills are made on the forest floor. Arboreal prey may be forced to the ground during an attack. The sharp, powerful talons may produce sufficient force to kill the prey on impact; if not, death from trauma or asphyxiation soon follows. Several prey items have been killed by ramming the talons into the skull and penetrating the brain. Having killed on the ground, it has the ability to fly almost vertically upwards to a branch while carrying its prey before feeding, though it will tear up prey into manageable pieces on the ground when it is exceedingly heavy. While they both attack somewhat similar prey in often similar habitat, the considerable difference in body weight and wing-loading between crowned and harpy eagles have been attributed to load-carrying while hunting, as harpies tend to capture and carry off most prey in active flight rather than an attack on the ground and dismember if necessary.

On rare occasions, crowned eagles may also hunt on the wing, flying slightly over the canopy and causing a cacophony amongst monkey groups until they detect and capture their prey, often a monkey or tree hyrax. Crowned eagles are believed to take uneaten portions of prey up into the trees to cache around the nest or habitual perches so that the pieces can be consumed over the course of the next several days. If the prey is too heavy for taking flight with, even after dismemberment, for example, a bushbuck, crowned eagles have been known to cache food at the thickly vegetated base of a tree and only carry limbs to the nest. Pairs may collaborate in capturing prey, with one bird flushing the prey so the other can glide in unseen and ambush it. Female eagles may target male monkeys more often than males, which are more likely to hunt female or young monkeys. In one case, a female crowned eagle stalked a bushbuck calf over the course of two days but was repeatedly foiled when it went in for the attack, either by the mother bushbuck or an associating troop of yellow baboons. However, the crowned eagle one day assaulted the bushbuck calf quickly, leaving it with a gaping wound on its flank, and flew off to observe from a distance. Within a few more days, the bleeding, wounded calf was unable to keep pace with its mother and was tracked and killed by the assaulting eagle. Another assault, this one on an adult vervet monkey apparently had a similar outcome as the bushbuck attack. This type of strike-and-wait hunting technique may be used by diverse predators, from Komodo dragons to great white sharks, which tend to track their victims by scent after biting them rather than sight and sound, but is virtually unprecedented in birds. Crowned eagles have been recorded to consume carrion but this behavior has only rarely been observed.