Golden eagle

• 1. Introduction

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  The golden eagle, known to science as Aquila chrysaetos, holds a particular distinction among the world's birds of prey: it is the most widely distributed eagle species on earth, with a range estimated at 140 million square kilometers. Yet despite that vast presence, much of what this bird does remains hidden from human eyes. It spends the better part of every daylight hour simply sitting still. In Idaho, adult males were observed perching for an average of 78% of the day; adult females, near the nest, for about 85%. What does a creature capable of diving at speeds between 240 and 320 kilometers per hour do with all that stillness? And how did a bird of open skies come to occupy the coats of arms, battle standards, and mythologies of civilizations separated by thousands of kilometers and thousands of years? Those are the questions this documentary sets out to answer.

• 2. A Name from Two Languages

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  Carl Linnaeus first described the golden eagle in 1758, placing it in the genus Falco in the 10th edition of his Systema Naturae. He called it Falco chrysaetos. At that time, birds were grouped largely on superficial resemblances, which is why so many raptors of different lineages ended up clustered together under Falco. The type locality Linnaeus recorded was simply "Europa"; that was later narrowed down to Sweden.

  Two years after Linnaeus published, the French ornithologist Mathurin Jacques Brisson moved the species to a new genus: Aquila. The word is Latin for eagle, and is possibly derived from aquilus, meaning dark in colour. The species name chrysaetos comes from Ancient Greek: khrusos for gold and aetos for eagle. The bird's name, in other words, carries the same meaning in two classical languages at once.

  The golden eagle belongs to a group called booted eagles, defined by the presence of feathering over the tarsus - the lower leg - unlike many other accipitrids, which have bare legs. The genus Aquila extends across every continent except South America and Antarctica. Genetic research has since revealed some surprising relationships within that genus. The golden eagle belongs to a clade that also includes Verreaux's eagle in Africa, the Gurney's eagle, and the wedge-tailed eagle of Australasia - a grouping that had long been suspected from physical similarities among these large-bodied birds.

• 3. Six Faces of One Species

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  Six subspecies of the golden eagle are recognised today, and they span an enormous range of size and coloration. At one extreme sits the berkut, or Himalayan golden eagle (Aquila chrysaetos daphanea), the largest subspecies on average. One captive female berkut had an authenticated wingspan of 2.81 meters. Its back is generally blackish, with a rich brown-red nape, and it ranges across central Kazakhstan, the Himalayas from northern Pakistan to Bhutan, and into parts of China.

  At the other extreme, the Japanese golden eagle (A. c. japonica) is by far the smallest subspecies. Males of this form weigh approximately 2.5 kg and females around 3.25 kg. Even adult Japanese golden eagles often retain extensive white mottling on the inner tail feathers - a pattern that in other subspecies signals a juvenile bird.

  The North American subspecies (A. c. canadensis) is the one featured on the coat of arms of Mexico, where it is known as the "Mexican eagle". It holds the largest breeding range of any subspecies and is probably the most numerous. A massive wild female of this form, banded and released in 2006 near Wyoming's Bridger-Teton National Forest, became the heaviest wild golden eagle on record, at 7.7 kg. She surpassed the previous record holder, a European specimen of the nominate subspecies that weighed around 6.7 kg.

  Some recent genetic studies have proposed collapsing the six subspecies into just two: A. c. chrysaetos in the west and A. c. canadensis in the east, with the remaining four forms absorbed accordingly. The fossil record adds further depth: large Middle Pleistocene golden eagles from France are referred to a paleosubspecies, and specimens from the Late Pleistocene Liko Cave in Crete have been assigned the name Aquila chrysaetos simurgh. An ancestral golden eagle with a heavier skull, broader wings, and shorter legs was also found in the La Brea Tar Pits of southern California.

• 4. Speed, Silence, and the Art of the Stoop

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  Golden eagles are sometimes regarded as the finest fliers among all raptorial birds. Their wings are broad and long, with finger-like indentations at the tips. One characteristic sets them apart from every other species in their genus: they habitually hold their wings in a slight upward V-shape, a posture called a dihedral, which adds stability in soaring flight.

  At a casual, unhurried soaring speed they travel around 45-52 km/h. When hunting or in territorial display, gliding speeds can reach 190 km/h. When stooping - that steep, committed dive - the eagle pulls its legs against its tail, folds its wings close against its body, and can reach between 240 and 320 km/h. That makes the golden eagle one of the two fastest living animals. In those measures, the golden eagle appears to match, and possibly exceed, the stooping speed of the peregrine falcon.

  For all that aerial power, the golden eagle's voice is startlingly modest. In western Montana, nine distinct calls were catalogued - a chirp, a cluck, a wonk, a honk, and a hiss among them - but ornithologists have described the overall voice as weak, high, and shrill, calling it "quite pathetic" and "puppy-like". The bird communicates primarily through contact calls between individuals, including adults to offspring and occasionally territorial birds to intruders.

  Hunting success is lower than the bird's fearsome reputation might suggest. A study in Idaho recorded 115 hunting attempts; only 20% resulted in captured prey. A fully grown eagle requires about 230-250 g of food per day, but eagles have been known to go without food for up to a week and then consume up to 900 g in a single sitting.

• 5. Territories and the Life of a Pair

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  Home range size in golden eagles is one of the largest recorded for any bird species, though it varies enormously. Across most of the range, territories run from 20 to 200 km2. In San Diego County in California, recorded home ranges averaged 93 km2. In southwestern Idaho, where jackrabbits are abundant, some territories have been as small as 4.85 km2. The smallest known home ranges on record sit in Ethiopia's Bale Mountains, where they span only 1.5 to 9 km2.

  Golden eagles typically build several nests within a territory, returning to the same sites over multiple breeding seasons. Courtship is elaborate. The male picks up a piece of rock or a small stick, drops it, then enters a steep dive and catches it mid-air, repeating the maneuver at least three times. The female performs a parallel display with a clump of earth. Mating appears to follow around 40-46 days before the first egg is laid. Copulation typically lasts only 10-20 seconds.

  Hatching is a slow process. A golden eagle chick can be heard calling from within the egg 15 hours before it begins to break through. The first chip in the shell is followed by roughly 27 hours of inactivity. Hatching activity then accelerates, with the shell broken apart in around 35 hours and the chick fully free in 37 hours. After hatching, 80% of food items and 90% of food biomass is brought to the nest by the adult male.

  Fledging occurs at 66-75 days of age in Idaho and 70-81 days in Scotland. By about 60 days after fledging, young eagles can gain height nearly as efficiently as their parents. In Cumbria, young golden eagles were first observed hunting large prey 59 days after fledging. Full independence generally arrives 75-85 days post-fledging. Young eagles then wander widely for years, typically establishing their own territories at four to five years of age.

• 6. Migration and the Largest Gathering on Earth

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  Most golden eagle populations are sedentary, but the species is a partial migrant. Eagles raised at latitudes above 60 degrees north are almost always migratory; those breeding near 50 degrees north may undertake shorter movements. In Finland, most banded juveniles move between 1,000 and 2,000 km due south while adults remain on territory through winter.

  At Mount Lorette in Alberta, approximately 4,000 golden eagles may pass through during a single fall, making it the largest recorded golden eagle migration on earth. The mountain ranges there provide reliable thermals and updrafts that make long-distance soaring flight practical. Birds hatched in Denali National Park in Alaska traveled anywhere from 818 to 4,815 km to their winter ranges.

  Eagles breeding in northeastern Canada's Hudson Bay area reached their wintering grounds in 26-40 days, arriving from November to early December. The departure dates from wintering grounds show almost as much variation. In southwestern Canada, birds leave their winter ranges between the 6th of April and the 8th of May, with an average departure of the 21st of April. In southwestern Idaho, wintering birds depart between the 20th of March and the 13th of April.

  In Scotland, among banded golden eagles that were recovered, the average distance between where they were ringed and where they were found was 44 km - 63 km in juveniles and 36 km in older birds. The vast majority of the species, including all breeding birds in the contiguous Western United States, all of Europe outside northern Scandinavia, North Africa, and most of Asia, are non-migratory and remain near their breeding territories year-round.

• 7. Symbol, Falcon, and Soldier

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  Human fascination with the golden eagle reaches back to the beginning of recorded history. In pre-Hispanic Mesoamerica, the eagle held central importance in Mexica (Aztec) cosmology. The tribal and sun god Huitzilopochtli had told his people to build their city, Tenochtitlan, where they saw the sun in the form of an eagle perched on a cactus whose red fruit was shaped like a human heart. That image - shown in early manuscripts, carved in sculpture, and carried forward to the present-day Mexican flag - carried astronomical, geomantic, and mythological meaning simultaneously.

  The Roman legions organised their military around the eagle standard. A soldier called the aquilifer, or eagle-bearer, carried one eagle per legion. The position of these standards on the battlefield communicated troop movements during engagements. To lose an eagle standard was considered deeply shameful; legions made extraordinary efforts to recover them when lost or captured.

  For centuries, the golden eagle has also occupied a central place in falconry, where it is considered one of the most highly regarded birds ever used in the practice. The berkut subspecies has been particularly prized by falconers in Central Asia. In 2017, the French Army took a more modern approach, training golden eagles to intercept drones.

  The golden eagle is also the official state bird of prey of Utah. And a conservation initiative in England, backed by one million pounds in funding, has identified eight areas as potential sites for reintroducing the species, which has been absent from England for around 150 years.

• 8. Decline, Range, and What Remains

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  The golden eagle once bred widely across temperate Europe, northern Asia, North America, North Africa, and Japan. Today, the global population is estimated at between 170,000 and 250,000 individuals, with breeding pairs estimated at 60,000 to 100,000. On a global scale, the IUCN does not consider the species threatened.

  The species holds the largest known range of any member of its family. Within its taxonomic order, it ranks second only to the osprey in breadth of range. Yet that broad range masks localised losses. In much of what was once golden eagle territory in Europe and eastern North America, the birds have been extirpated or reduced to very low numbers. The Eastern United States lost its last known nesting pair in Maine in 1999; the species is now absent as a breeding bird east of a line running roughly from North Dakota through West Texas.

  Human activity has been the dominant driver of those losses. After the Industrial Revolution made sport hunting widespread and commercial livestock farming common, golden eagles came to be viewed as a threat to livelihoods. Firearms and industrialised poisons made killing the birds far easier than before. In December 2016, the US Fish and Wildlife Service proposed allowing wind-turbine companies to kill golden eagles under permit, with those permits potentially lasting 30 years - six times the length of the then-current five-year permits.

  In Spain, a study of 266 golden eagle deaths found that only 6% could not be directly attributed to human activity. Near a wind turbine facility in west-central California, telemetry of 257 individual eagles found that first-year birds had an estimated survival rate of only 84%. The estimated survival rate for juvenile eagles in their first 11 months from Denali National Park ran from 19-34%. The oldest confirmed wild golden eagle was banded in Sweden and recovered 32 years later; in North America, the record stands at 31 years and 8 months.