The white-tailed eagle (Haliaeetus albicilla) is a very large species of sea eagle widely distributed across temperate Eurasia. As are all eagles, it is a member of the family Accipitridae (or accipitrids) which includes other diurnal raptors such as hawks, kites, and harriers. One of up to eleven members in the genus Haliaeetus, which are commonly called sea eagles, it is also referred to as the white-tailed sea-eagle. Sometimes, it is known as the ern or erne (depending on spelling by sources), gray sea eagle and Eurasian sea eagle.

While found across a very wide range, today breeding as far west as Greenland and Iceland across to as far east in Hokkaido, Japan, they are often scarce and very spottily distributed as a nesting species, mainly due to human activities. These have included habitat alterations and destruction of wetlands, about a hundred years of systematic persecution by humans (from the early 1800s to around World War II) followed by inadvertent poisonings and epidemics of nesting failures due to various manmade chemical pesticides and organic compounds, which have threatened eagles since roughly the 1950s and continue to be a potential concern. Due to this, the white-tailed eagle was considered endangered or extinct in several countries. However, some populations have recovered well due to some governmental protections and dedicated conservationists and naturalists protecting habitats and nesting sites and partially regulating poaching and pesticide usage, as well as careful reintroductions into parts of their former range.

White-tailed eagles usually live most of the year near large bodies of open water, including both coastal saltwater areas and inland freshwater, and require an abundant food supply and old-growth trees or ample sea cliffs for nesting. They are considered a close cousin of the bald eagle (Haliaeetus leucocephalus), which occupies a similar niche in North America.

Description & appearance

The white-tailed eagle is a very large bird and one of the largest living birds of prey. It is the largest of the dozen species called eagle to be found in Europe and is the largest eagle across its distribution, excluding the Russian Far East and during winter in Hokkaido where it co-exists with its larger cousin, Steller's sea eagle. The white-tailed eagle is sometimes considered the fourth largest eagle in the world and is on average the fourth heaviest eagle in the world. The only extant eagle species known to be more massive in mean bulk are Steller's sea eagle, the harpy eagle (Harpia harpyja) and the Philippine eagle (Pithecophaga jefferyi). The white-tailed eagle measures anywhere from 66 to 94 cm (26 to 37 in) in total length with a typical wingspan of 1.78 to 2.45 m (5 ft 10 in to 8 ft 0 in). This species may have the largest wingspan of any living eagle. The Steller's sea eagle, which is larger in weight, total length and non-wing standard measurements, may be the closest rival for median wingspan amongst living eagles.

Average wingspans are not known for the Steller's species, however white-tailed eagles do appear to outsize the average wingspan of the wedge-tailed eagle (Aquila audax), which is sometimes also titled the largest winged extant eagle (however one wedge-tail did possess the largest wingspan ever verified for an extant eagle). In one sample from Norway, 5 male white-tailed eagle were found to average 2.26 m (7 ft 5 in) and 8 females were found to average 2.37 m (7 ft 9 in). In another sample of wild birds of unspecified origin, 5 males were found to average 2.1 m (6 ft 11 in) and 7 females averaged 2.3 m (7 ft 7 in). Record wingspans have included a specimen from Greenland which measured 2.53 m (8 ft 4 in) while another specimen apparently spanned 2.6 m (8 ft 6 in). The bald eagle broadly overlaps in size with the white-tailed eagle. In direct comparison, the white-tailed eagle averages somewhat larger in body mass than the bald eagle and may be marginally larger in bill and talon size although these linear aspect can be quite similar between the two species. However, the white-tailed has a significantly larger wing chord and average wingspan. On the other hand, the bald eagle usually possesses a longer tail length on average, which imparts a somewhat longer total length that the white-tailed eagle, and a longer mean tarsal length.

Size variation is generally a clinal trend, these figures usually being measured in standard measurement such as wing, tail and tarsal length, or body mass rather than wingspan or total length. As expected for many widely distributed animals of varied lineages, the white-tailed eagle conforms to Bergmann's rule in that more northerly birds tend to outsize those found relatively closer to the Equator. Somewhat less predictably, size seems to decrease from west to the east as well. The largest white-tailed eagles appear to be found in Greenland, which are just slightly larger than those from Scotland and Scandinavia and notably larger than eagles from central Europe, especially in proportions of the wing area. Meanwhile, those from the southerly portions of their breeding range, such as Asia Minor (principally Turkey), southern Kazakhstan and Korea Bay appear to be the smallest-bodied population, but this is complicated by the fact that there's nearly been no comprehensive measurements or published weights known for these extremely sporadic and rare Asian populations of eagle. Furthermore, weights of fully grown eagles from Greenland are not known. Unlike many accipitrids, in white-tailed eagle (and seemingly other sea eagles as well) juveniles are often of similar weight to adult eagles, whereas in most the juveniles will usually weigh somewhat less. However, more typically, juvenile eagles have somewhat larger average wing and tail lengths than adults.[3][38] In the white-tailed eagle, body mass can typically range from 4 to 6.9 kg (8.8 to 15.2 lb) in females.

The slightly smaller male may typically weigh from 3.1 to 5.4 kg (6.8 to 11.9 lb).[9] Average weights in European white-tailed eagles can range from 4.02 kg (8.9 lb) in 5 males and 5.11 kg (11.3 lb) in 9 females to (from the reintroduced birds of Scotland of Norwegian stock) 4.98 kg (11.0 lb) in 39 males and 6.06 kg (13.4 lb) in 43 females. In comparison, the weight ranges for white-tailed eagles from northeast China were claimed as only 2.8 to 3.78 kg (6.2 to 8.3 lb) in males and 3.75 to 4.6 kg (8.3 to 10.1 lb) in females. The heaviest female white-tailed eagles can apparently scale up to 7.5–8 kg (17–18 lb) and even males can sometimes weigh up to 6.5 kg (14 lb), which would make the largest males perhaps the heaviest recorded modern male eagle as male harpy and Philippine eagles (being more sexual dimorphic in favor of the female) are not known to exceed 5 kg (11 lb) (the highest weights for male Steller's sea eagle are not known). The global mean body mass of white-tailed eagles is estimated at approximately 5 kg (11 lb). The average female Steller's sea eagle may weigh just under 25% more than the average female white-tailed eagle (the average weight of male Steller's is not known) while the average European golden eagle weigh about 11-12% less than the average European white-tailed eagle and the bald eagle species as a whole about 10% less than the white-tailed eagle species.

Voice, singing & call

The white-tailed eagle is considered a very vocal bird of prey during the breeding season, although some authors consider their voice "not loud or impressive for the size of the bird". The male call is oft transcribed as gri-gri-gri or krick-krick-krick, while the female is a deeper gra-gra-gra-gra or krau-krau-krau-krau. These will increase in tempo and pitch, with about 15-30 calls in a sequence. Often pairs will duet during early spring, in flight or from a perch. When perched, the male calls kyi-kyi-kyi-kli-kliek-yak with the head thrown back and upwards in the last call ended with a lower ko-ko-ko, the perched call of females is similar but deeper, a krau-krau-krau-uik-ik. Typically, the perched version of their calls tend to be shriller and higher than those issued in flight.

In courtship display, male calls krau-krau-krau-uik-ik-ik answered by females with a lower ra-rack-rack-rack-rack. Young in nest call a shrilly piieh-piieh, while the female when receiving food from male calls out with tschie-tschie or vueee-vueee. Single or repeated krlee or similar component of calls used in other circumstances, but this can be very variable. Alarm calls tend to be 3-4 short, loud klee or klek notes. Sometimes a different call of alarm or anger, a deep gah-gah-gah or jok-jok-jok, similar to alarm calls of a large gull, is also uttered when a nest is approached (usually recorded while directed towards humans). The young let out a monotonous veee-veee when hungry (or "bored") which intensifies if the eaglets are not fed or brooded immediately.

Habitat

White-tailed eagles may be found in varied habitat but usually are closely associated with water and generally occurs in lowland areas. Although mainly a lowland species, the species is known to live at elevations of 1,500 to 2,300 m (4,900 to 7,500 ft) so long as there is water access in some parts of Central Asia and Siberia. In coastal areas, the species may range from high sea cliffs down to low-lying islands and archipelagos. Especially in winter, many white-tailed eagles often frequent low coastal spots, estuaries and coastal marshes. Several studies have supported that coastal areas are preferred when available during winter. In many areas, white-tailed eagles can seem to switch freely between usually cliff habitat and wooded spots for nesting sites and the center of their home range habitat.

In some areas, such as Japan, this species may occur in regions with intensive human fishing activity and they may become unusually partially habituated to this human presence. Inland, white-tailed eagles usually require secluded woods, forested areas or groups of trees with tall mature trees and access to freshwater wetlands such as lakes, river systems, marshes or extensive, low-disturbance farmland. In the alluvial wetlands of Croatia, 95% of nests were found within 4 km (2.5 mi) of deep freshwater. In some areas, white-tailed eagles readily visit commercial fish farms, carp ponds and similar areas with easily accessible food but they will usually avoid areas where human disturbances (especially noisy varieties such as construction, water sporting and heavy boating activities and hunting) commonly occur. However, forestry activity and resulting lessened numbers of tall mature trees and large tree stands in Estonia was found to affect breeding white-tailed eagles less so than it seemed to affect breeding black storks (Ciconia nigra). On the other hand, from studying wintering white-tailed eagles in partially or heavily disturbed wetlands in parts of the Netherlands shows that such areas cannot support the eagles for any long-term period and may only be visited for a day or two by individual eagles.

Hunting & diet

The white-tailed eagle's diet is varied, opportunistic and seasonal. Prey specimens can often include fish, birds and, mostly in a secondary capacity, mammals. White-tailed eagles are powerful predators and capable of attacking large prey of considerable sizes but, like most predators, prefer prey that is vulnerable and easy for them to capture. Especially during the winter (and opportunistically in all seasons), many birds of the species live largely as scavengers, usually by coming across available carrion or watching for the activity of corvids, vultures or other raptors. White-tailed eagles in northeastern Germany were shown to hunt mostly from perches, in a “sit-and-wait” style, usually from a prominent tree perch. Like other sea eagles, they can only capture fish normally in the littoral zone, seldom hunting fish when they exceed a water depth of 1.5 to 2 m (4.9 to 6.6 ft). In addition to trees, they may also use crags, hillocks or high grassy tussocks as hunting perches so long as the perch provides a good overall view of the environment. Fish tend to be grabbed in a shallow dive after a short distance flight from a perch, usually with the eagles only getting their feet wet.

Occasionally, though, white-tailed eagles have been recorded plunging right into water, usually while hunting on the wing at a height of at least 200 m (660 ft). In Norway, plunge-diving is considered rare. At times they will also fish by wading into shallows, often from shores or gravel islands. The species will at times variously follow fishing boats, readily exploits commercial fisheries, stocked lakes, carp ponds and the like, and scavenges dead fish or fish-offal in a wide range of situations. When it comes to non-fish prey, it has been said that white-tailed eagles often hunt by flying low over sea coast or lakeshore and attempt to surprise victims. However, the hunting success rates on healthy birds can be low as revealed in studying wintering eagles in Sweden attempting to hunt mallards (Anas platyrhynchos). White-tailed eagles also regularly pirate food from otters and other birds including cormorants, gulls, ospreys, corvids and various other raptors. Carrion is often the primary food source during lean winter months, with fish and ungulates preferred but everything from cetaceans to livestock to even humans being eaten after death. From studies of captive white-tailed eagles, daily food requirements were estimated at 500 to 600 g (1.1 to 1.3 lb), which is equivalent to about 10% of the birds' body weight, with crop contents commonly of 190–560 g (0.42–1.23 lb). Some semi-captive juveniles on the isle of Rùm, Scotland could eat up to 1.4 kg (3.1 lb) in one sitting. However, in Norway, it was estimated that a family of wild white-tailed eagles including each adult and 3 fledglings were consuming on average up to 625 g (1.378 lb) per bird each day. Furthermore, one male consumed an estimated 2 kg (4.4 lb) in a single meal upon capturing a large fish. The crop can bulge to the size of a small grapefruit after they've consumed a large meal.

Breeding & mating

The breeding season is from January to July in the south of the white-tailed eagles range, and from April to September in the northern part of their range. They pair for life, though if one dies replacement can occur quickly. A bond is formed when a permanent home range is chosen. White-tailed eagles frequently engage during early spring variously in soaring, sky-dancing and other aerial displays, all with much loud calling, often performed by pair members together, including spectacular mutual cartwheel downwards where talons touch or interlock. Nuptial displays may occur almost year-around in some parts of the range, increasing after young disperse in autumn, thence at rare intervals until spring when such behavior, of course, peaks. Courtship often begins with a bird, often the male, "sky pointing" or "long calling" by throwing their head back. Soaring by pairs follows, with either partner leading by 1 to 6 m (3.3 to 19.7 ft) or soaring opposite directions. One may swoop upon the other who responds by tilting to one side or may roll over to touch talons momentarily before separating. This may be repeated or gain intensity until they are talon-grappling or "mutual cartwheeling", consisting of the pair locking talons mid-air and whirling earthwards in series of spectacular cartwheels. When doing this, the pair may stop only a few feet above the ground. Talon grappling is usually associated with territorial clashes in most accipitrids, especially between males, but in Haliaeetus such behaviour also seems to be related to courtship and is engaged by pairs as well as males attacking intruding males.

When locally common, 2-3 pairs can be seen displaying in the sky quite near each other but each pair in fact are within a border of a well-defined territory. In Greenland, recorded pair densities were 0.3-0.6 occupied territories per 100 km2 (39 sq mi).[208] In eastern Germany, densities were reported as 1.6-2 pairs per 100 km2 (39 sq mi). Norwegian nests appear at no closer than 1–2 km (0.62–1.24 mi); territory size on Norwegian coasts were reported as roughly 6 to 9 km2 (2.3 to 3.5 sq mi). Fischer noted 8-9 pairs on the small Norwegian island of Fugloy due to its huge seabird colonies, with late summer numbers boosting to 75 eagles, all within an area of only 22 km2 (8.5 sq mi). 5 occupied nests were noted on an 80 km (50 mi) stretch of the Russian Yenisei River. Mating chiefly occurs between mid-March and mid-April in most parts of the range. The pair may copulate be about once every 20 minutes for several hours. Prior to mating, a more skilled and graceful flying than usual may be undertaken by the male as the final stage of courtship. After his final display, the female crouches low, almost flat, with her head and neck outstretched, wings half open, and tail held level with the rest of the body. Mating has been recorded in white-tailed eagles to occur almost anywhere including a low perch, on the nest, on the ground or even on frozen lake surfaces, usually close to the nest at least but also at as far away from the nest as 3 km (1.9 mi) in Norway. Sometimes both members of the pair will assume the female's typical mating position simultaneously side-by-side until he jumps on her back. Once the male mounts he often calls loudly and flaps both wings to maintain his balance. After about 12 seconds he dismounts and sits quietly, ultimately will do so about 5 to 6 in 1.5 hours and at nearly any time of the day.

White-tailed eagles most often nest in large trees, with perhaps coniferous trees preferred and nests may be in a high main fork, on the canopy or a large side branch. Access to food is a key consideration in the nest's location in white-tailed eagles.

Nests are usually huge, constructed of sticks and branches, averaging roughly 1 m (3.3 ft) across and up to 2 m (6.6 ft) deep, but can be several metres in width & depth, lined variously with moss, greenery, seaweed or wool. The nest must have easy access for the eagles with a clear view of the surrounding environment, shelter from the elements and protection from nest predators.

Individual white-tailed eagles tend to be remarkably consistent on egg-laying times from year to year, seemingly regardless of surrounding weather conditions.

Chicks, juveniles & raise

In 1983, it was listed that brood size is one 56.5% of the time and two 40.8% of the time in 8 studies from different parts of the range; only 2.5% of these studies had a brood size of three and only 1 from Norway was a brood size of four recorded. From 14 studies, the mean number of chicks in a brood may range from 1.1 in western Germany to 1.9 in southern Kazakhstan, with average overall brood size of 1.52. However, these datasets may be skewered lower than natural brood size as a majority these are from the mid to late 20th century when pesticide use considerably lowered the mean number of hatchlings in many parts of the range. New hatchlings will weigh about 90 to 100 g (3.2 to 3.5 oz) but in Hokkaido new hatchlings were surprisingly notably heavier at 110 to 115 g (3.9 to 4.1 oz).

The sex of nestlings can be identified using field methods, or using DNA. Initially, the hatchlings have a creamy white down which is longest and whitest on the head and often dirty grayish on wings and rump. Before they start walking about the nest, their underside may have several bare patches. The nestlings first become audible at around 2 to 3 days and have become active enough to move around the nest and excrete over the nest edge by 10 days old. The initial down is replaced by a thick woolly coat of longer, coarser grayish down, which is usually darker on the crown, underparts and flanks. The legs and cere at this young nestling age can range from pinkish to pale yellow. By about 30 days the first feathers poke through the down. The eaglets can feed themselves starting at 35–40 days. At 6 weeks (40 days) they are more firm on their feet and between this and the following week feathers take over the down, with patches of down remaining but usually gone by week 7. Wing flapping begins only when the wings are partially feathered at 42 days. Around seven weeks of age, the eaglet is more alert and stronger and frequently manipulates sticks and walks more so. At 8 weeks only the long feathers of the wing and tail have yet to develop fully and eaglets tend to start exploring the surrounding branches. The eaglets will attempt their first flight at about 70 days and will usually be flying well by about 90 days of age. Fledgling occurs around mid- to late July in Norway, and about 3–4 weeks later in Russian White Sea.

The female of the white-tailed eagle pair seems to do all the brooding early on and will be especially reluctant to leave the nest as well. Thereafter, at 14 to 28 days, brooding behaviour by the female gradually declines. The male may start to brood occasionally around this time period but will not do so at night. Females may sit on the nest or shelter the fledglings from rain even 28 days after fledging but usually such behavior is much reduced. Chicks may be fed as many as 11 times in 24 hours by the female who usually dismantles prey brought by the male. To 28 days, male continues to do most of the prey capture but thereafter female does much of it and both parents begin to leave kills on the nest for the eaglets to consume. As the chicks grow older, favored food often switches from fish to birds to meet their increasing food requirements as well as prey behavior as waterfowl may be flightless during eclipse.

Caches are often depleted quickly late into nestling development. At start of fledgling, 4-5 carcasses may be brought in a week but by the end only 1 to 2 are usually brought, probably to encourage the young to start their own prey captures. After leaving the nest, the young usually stay nearby for another 35–40 days and may still be largely fed by their parents but gradually learn to take their own prey. In continental Europe, the young eagles are gone from the nest as early as early July to about August 10 and fully independent by late August. In this late summer stage, they may learn quickly to feed on stranded fish or to capture ducks flightless in eclipse. Juvenile eagles may remain for a long time on their parents range and apparently are not resented or repelled even to the ages of 1 to 3 years. However, usually by their first winter they will have congregated with other unrelated juveniles. Communal juvenile roosts in Norway can maintain 30-40 white-tailed eagles usually in trees or steep slopes of offshore islands.Sexual maturity is reached at 5 to 6 years of age.