Golden eagle

Name and taxonomy

Carl Linnaeus described the bird in 1758 as Falco chrysaetos. Mathurin Jacques Brisson moved it to the new genus Aquila in 1760. Aquila is Latin for eagle, and may come from a word meaning dark. The species name joins the Greek words for gold and eagle. The type locality was later fixed on Sweden.

Six living subspecies are usually recognised. They differ in size and shade, and the differences run in a gradient. The Himalayan form, called the berkut, is the largest. The Japanese form is the smallest and the darkest. Some genetic work proposes cutting the list down to two.

Evolution and relatives

The golden eagle belongs to a group called the booted eagles. Every bird in that group carries feathers over the tarsus. Most other members of the hawk family have bare legs. The genus Aquila occurs on every continent except South America and Antarctica.

Genetic work groups the golden eagle with Verreaux's eagle of Africa. Gurney's eagle and the wedge-tailed eagle sit in the same clade. Bonelli's eagle turned out to belong there too, which surprised ornithologists. The imperial, tawny and steppe eagles form a separate close group. Larger fossil golden eagles are known from Ice Age France and Crete.

Appearance

Adults are mainly dark brown, and grey shows on the inner wing and on the tail. The crown and nape carry the pale golden patch that names the bird. Leg feathers are paler than the rest, from light gold to white. The bill is dark at the tip and horn coloured at the base, with a yellow cere. The bare part of the foot is yellow, as in many hawks.

The body runs 66 to 102 cm long, with a span of 1.8 to 2.34 metres. Males of the European form weigh 2.8 to 4.6 kg. Females of that form weigh 3.8 to 6.7 kg. A female banded in Wyoming in 2006 weighed 7.7 kg, the heaviest wild bird known. One authenticated wild female spanned 2.55 metres across the wings.

The wing chord measures 52 to 72 cm and the tail 26.5 to 38 cm. The hind claw can reach 6.34 cm, longer than a bald eagle's. Young birds are darker and show white over about two thirds of the tail. White patches often appear on the wings, and they vary a great deal. Full adult plumage arrives when the bird is about six years old.

Habitat and range

Golden eagles need open or half open ground where they can see prey. They avoid towns, farmland and heavy forest. Most live in mountains today, and hunt and nest around rock. They will still breed in lowland country where the habitat suits them. Animal Diversity Web records them from sea level up to 3600 metres.

In Europe the bird holds the Pyrenees, the Alps, the Carpathians and the Caucasus. Scottish and Irish birds live on upland grassland, blanket bog and crags. Across Siberia they occur thinly in lowland taiga where the trees thin out. In Asia they nest above the tree line at more than 2,500 metres. One bird was recorded circling at 6,190 metres in Khumbu in 1975.

In North America they range from Alaska to central Mexico. Plains, prairie, sagebrush and oak woodland all hold pairs. They avoid true desert, such as ground with under 20 cm of rain a year. The species no longer breeds in the eastern United States. An isolated population survives in the Bale Mountains of Ethiopia.

Diet and hunting

Small and medium mammals make up most of the food. Hares, rabbits, marmots, prairie dogs and ground squirrels lead the list. Birds, reptiles and fish appear in smaller numbers. Large prey is taken now and then, including seals, deer, coyotes and badgers. Geese, cranes, owls and falcons have all been caught in the air.

A grown eagle needs about 230 to 250 grams of food a day. Life runs in cycles of plenty and hunger, though. A bird can go a week without food and then eat 900 grams at one sitting. In Idaho, 20 per cent of 115 hunting attempts ended in a catch. They rarely store prey for later.

The bird hunts by day, mostly by quartering low over open ground. An unhurried soar runs at 45 to 52 kilometres per hour. A hunting glide can reach 190 kilometres per hour. In a full dive the legs come up and the wings close against the body. Pairs sometimes hunt together, with one bird tiring the prey for the other.

Behaviour and social life

A pair holds one of the largest home ranges known for any bird. Most ranges cover 20 to 200 square kilometres. In San Diego County the average was 93 square kilometres. Where jackrabbits are plentiful in Idaho, a range can shrink to 4.85 square kilometres. The smallest ranges on record are in the Bale Mountains of Ethiopia.

Most of the day passes quietly on a perch. Male eagles in Idaho sat still for about 78 per cent of the daylight. Females spent about 85 per cent of the day perched or on the nest. Cold weather can push the usual solitude aside. In eastern Idaho, 124 eagles once perched along a line of 85 power poles.

Most populations stay put all year. Birds raised north of 60 degrees usually migrate south for the winter. Young Finnish eagles move 1,000 to 2,000 kilometres due south. About 4,000 birds may pass Mount Lorette in Alberta each autumn. Scottish birds barely move, and banded birds averaged 44 kilometres from the nest.

Communication and senses

For a bird of this size the voice is oddly weak. Writers have called it high, shrill and puppy-like. Golden eagles stay quiet for most of the year. Most calls belong to the nesting season. Nine distinct calls were noted in western Montana.

Calls mainly work as contact between birds that know each other. Chicks beg, and parents answer at the nest. The species does not use its voice to mark a territory. It uses flight instead, in an undulating display along the range edge.

Body language carries meaning at close quarters. A threatened bird stands upright, raises its head feathers and opens the bill. It may roll onto its back and present both sets of talons. A bird on the nest may instead lower its head and freeze.

Reproduction and life cycle

Golden eagles usually mate for life. Courtship involves undulating flights by both birds. The male drops a stone or a stick, dives, and catches it in the air. He repeats that trick several times over. The female does the same with a clump of earth.

A pair builds several nests inside the territory and uses them in turn. Cliffs are the first choice, but trees, banks and towers all serve. Nests sit anywhere from ground level to 107 metres up. Building or repair takes 4 to 6 weeks. The largest nest on record stood 6.1 metres tall and 2.59 metres wide.

The female lays 1 to 4 eggs, and two is usual. Eggs come 3 to 4 days apart, and she starts sitting on the first one. Incubation runs 35 to 45 days, averaging 42. The chicks therefore hatch several days apart. The older, larger eaglet often kills the younger one.

The male brings most of the food during the early weeks. A chick reaches around 500 grams and starts standing at about 20 days. Young birds leave the nest at 66 to 75 days in Idaho. Scottish birds fledge a little later, at 70 to 81 days. They stay dependent for weeks and first breed at four to seven years.

Predators and threats

A grown golden eagle has almost no natural enemy. Only the wolverine and the grizzly bear are on record as taking nestlings. Most fatal attacks on eagles probably come from other golden eagles. There is no confirmed record of another bird raiding an occupied nest.

Prey occasionally kills the hunter. One eagle died from the quills of a porcupine it had attacked. Red deer on the Isle of Rum have trampled eagles that went for a fawn. A great blue heron and the eagle that seized it both died of their wounds. A fulmar once killed a Scottish eagle by fouling its feathers with oil.

Starvation is probably the most under-reported natural cause of death. Eleven of sixteen dead young eagles in Denali National Park had starved. Of 266 recorded deaths in Spain, only 6 per cent lacked a human cause. Power lines, traps, poison and wind turbines all kill birds today. Eagles also abandon a nest readily when people disturb them during incubation.

Population and conservation

The IUCN Red List rates the golden eagle as Least Concern. The world total is put at 170,000 to 250,000 birds. Breeding pairs are estimated at 60,000 to 100,000. The range covers about 140 million square kilometres. Only the osprey covers more ground among birds of its order.

Sport shooting and commercial stock farming turned people against the bird. Firearms and industrial poisons made killing it easy. About 20,000 eagles were shot in the United States before 1962, many from aircraft. Farmers blamed them for lamb losses, though studies found rabbits filled most stomachs. Federal law has protected the species in the United States since 1962.

British numbers fell hard under persecution. The RSPB reports around 48 golden eagles in southern Scotland today. The bird is a Schedule 1 species there, and nests are still robbed and poisoned. England has funded work towards a return after about 150 years away. Eight areas were marked as suitable for that project.

Relationship with humans

Falconers have flown golden eagles for centuries. Kazakh and Kyrgyz hunters of the Eurasian steppe still do. A Kazakh eagle falconer is a burkitshi, from the word for the bird. Khitan hunters in Manchuria used eagles more than a thousand years ago. A Song dynasty painting shows them riding out with the birds.

Bayan-Olgii Province in Mongolia holds an annual Golden Eagle Festival. Judges award prizes for hunting skill, obedience and traditional dress. A documentary of 2016 followed a girl called Aisholpan at that festival. Researchers worry that tourism now shapes the practice. Some birds are hand-raised for visitors rather than for hunting.

Modern uses go beyond tradition. In 2017 the army of France put golden eagles to work catching drones. Utah names the golden eagle as its state bird of prey. Ranchers still lose the occasional lamb to an eagle. Some researchers argue the birds help farms by holding down rabbits.

In culture and myth

Early cultures across the Northern Hemisphere treated the golden eagle with respect. In Mexica belief, the sun god told the people where to build their city. They were to look for an eagle perched on a cactus. That scene founded Tenochtitlan and now appears on the flag of Mexico.

Rome made the eagle a badge of the empire and of the army. Each legion carried one eagle standard. The soldier who carried it was called the aquilifer. Its place on the field showed where the troops were. Losing a standard brought disgrace, and armies fought hard to win one back.

In Kazakh and Kyrgyz tradition, an eagle falconer holds real standing. The old Turkic title kush begi, lord of birds, went to a khan's close advisers. In British tradition the bird stands for wild high ground. Its return to the Scottish glens is probably read as a measure of recovery.