When night has passed and the cool earth is gradually warmed by the morning sun, broad-winged birds of prey suddenly appear among the mountains, and swiftly slip into gently rising air currents. With their great wings beautifully outstretched to a span of up to 1.5 meters, they glide up into the heavens with imperturbable self-assurance.

On a clear day in Taiwan's countryside one can often see crested serpent eagles circling over the ridgelines. Against the backdrop of the emerald-green mountains they ride the winds in leisurely style, occasionally turning back into the wind to dive, and often giving out long, shrill calls, "kweeee-kweeee-kweeee-kwee-kweeee-kwee," with which they proudly proclaim their mastery of the skies.

Touch the sky

The crested serpent eagle (Spilornis cheela) is found throughout much of south and southeast Asia and in large parts of China, but the subspecies Spilornis cheela hoya is unique to Taiwan. It grows to around 70 centimeters in length, weighs over 1.5 kilograms, and is chunkily built. Of the 28 members of the eagle family (Accipitridae) found in Taiwan, only Hodgson's hawk eagle is larger.

The crested serpent eagle's whole body is covered in gray and brown feathers, but striking bands of white run across the wings and the middle of the tail feathers of the adult birds, making them easy to identify from the ground when they circle in the skies. In the past, people of the Hengchun peninsula in southern Taiwan called them "deer-pattern" eagles, because the white spots that speckle their bellies like snowflakes resemble the markings on the body of the Formosan sika deer. Whenever a crested serpent eagle senses danger or is excited, the black-and-white crest on its head naturally rises up, giving it an imposing and ferocious air.

Crested serpent eagles are found in mountain areas up to 2000 meters above sea level, mostly in broadleaved forests. Around noon on sunny days with gentle winds, in suburban areas such as Wulai, Hsintien and Shihting in northern Taiwan, one can see crested serpent eagles sailing majestically through the heavens.

The eagles' large size makes flying by the strength of their own wing muscles very strenuous, so after nine or ten in the morning, as the sun shines into the mountain valleys and the birds' sensitive wings "detect" the buoyancy of the air, they take the opportunity to hitch a ride on air currents that carry them high into the skies, where they stretch their wings and enjoy the pleasure of flying. Because they rely on thermals to ascend, they can only fly between around 10 a.m. and 3 p.m. each day, and they make the best of this flying time. "They can play in the air one to two hours at a stretch," says nature photographer Lin Hsien-tang, who has been traveling the mountains of northern Taiwan alone for many years to "capture" crested serpent eagles in flight.

The birds' size also makes it difficult for them to fly or hunt among the trees of the forest, so they mostly patrol the skies at a great height, looking out for prey. When they see something moving, they come down and land in the top of a tall tree with a good view, to wait their chance. They have tremendous powers of concentration, and will wait alertly for half an hour on a treetop without moving, rain or shine, like a monarch keeping a lonely vigil.

Expert snake catchers

In warm, wet subtropical Taiwan, the mountains are alive with snakes. The crested serpent eagle is most snakes' nemesis, and snakes account for the greatest part of its diet. Lin Hsien-tang estimates that seven out of ten times when a crested serpent eagle goes after prey, it has a snake in its sights. Even the Asian green snake, which is well camouflaged in the bamboo forests, cannot escape its eagle eye. Many areas on the edges of Taiwan's forests have been cleared and planted with bamboo, and these plantations are infested with Asian green snakes. Such areas also offer the eagles a broad, unobstructed view of movements on the ground. For these reasons, crested serpent eagles like to frequent these partially cleared areas of forest.

When prey appears, the eagle plunges down and grasps it by the head and body with its sharp talons, then carries it away to a nearby treetop to eat at its leisure. The eagle usually first eats the head, then rips the body into little pieces and swallows them.

The reason crested serpent eagles have such a predilection for snakes can be explained by the reptiles' size, which makes them much more suited to the birds' large appetite than rodents, worms or frogs. Apart from the Asian green snake, almost all the other snakes to be found in Taiwan-such as the Chinese habu, Taiwan beauty snake, rat snake, stink snake, Taipei ground snake, and Taiwan kukri-also find their way onto the eagles' menu. But on rainy days and in winter, when snakes become less active, crested serpent eagles also fill their gizzards with lizards, skinks, worms, frogs, toads and other ground-living animals.

Every April to May Asian green snakes rapidly increase in number, and with this ample supply of food available, the crested serpent eagles also enter their breeding season.

The eagles normally range the mountain forests alone, but in late February to March they begin the prelude to their breeding season, with gatherings that make the broad mountain valleys seem suddenly crowded. Author Su Yun describes a flock of crested serpent eagles flying in a space encircled by a ring of mountains: "The sky seems filled to overflowing, and the forest trees seem to flail in all directions."

The mating game

"This is when the eagles get down and boogie." Lin Hsien-tang, who has seen up to a dozen crested serpent eagles at one of these "socials," explains that such get-togethers are once-a-year occasions, with the express purpose of choosing a mate.

The courtship rituals continue for one to two weeks. During this time, no matter whether a large crowd of birds gathers to size each other up, or there is only one marriage partner in prospect, the male birds always pull out all the stops to show the females what they are made of, performing all kinds of aerial stunts to prove their flying prowess.

Wing quivers, sideways rolls, 360-degree forward and backward flips, and rapid hunting dives with wings pulled in, are all regular elements of the male birds' body language in the mating season. The variety of their antics causes birdwatchers to cry out in delight, and the female birds watching from the sidelines do not act coy either. They keep their eyes firmly fixed on the males, watching their every move, ready to reward them for their efforts by choosing the partner that pleases them best.

When the male and female birds come to fly together, they give a dazzling doubles performance. With talons locked together they tumble through the air like flying-trapeze artists, as they build mutual confidence high in the sky.

Crested serpent eagles are territorial birds, and by the end of this short courting period of about ten days, they have each claimed their own patch of the nearby mountainsides, to which they disperse. Even those birds that do not find a suitable mate depart alone, and the raucous skies suddenly fall strangely silent. In northern Taiwan, birdwatchers once observed how a ten-kilometer stretch of mountains was divided up between ten crested serpent eagles. After four of them had succeeded in finding mates, the pairs kept themselves to themselves in their irregularly shaped territories, building their nests around a kilometer apart.

Journey of life

After the commotion of courtship, the breeding eagle pairs become "stealthy and furtive," as Lin Hsien-tang describes their cautious and nervous manner. They prefer to build their nests in Formosan koa trees, whose fluttering leaves create dancing shadows that make the birds very difficult to spot from the ground. Crested serpent eagles are active over large areas, and are highly secretive in their breeding activities, so that many aspects of their life remain a mystery. According to the conservation department of Yangmingshan National Park, which has conducted Taiwan's only preliminary study of the birds' natural history, all the nests that have been discovered so far have contained only one eaglet. The existing literature also states that the female birds lay only one egg per clutch. Therefore to increase their chances of breeding successfully, the adult birds are particularly cautious.

Lin Hsien-tang says that crested serpent eagle eggs are slightly smaller than hen's eggs, and the female birds incubate them for about a month. The eaglets begin to hatch out in May. They are similar in size to domesticated hen chicks, and are covered in white down. Until the eggs hatch, the birds' division of labor is thoroughly traditional: the female birds stay at home, sitting on the nests to incubate the eggs, while the males are responsible for finding food and standing guard.

The male birds are extremely cautious. When one brings prey home, he will first stop some distance away from the nest to look around; only when he is sure that all is well will he bring the food into the nest. If the male discovers anything moving, he will eat the prey himself there and then, to avoid giving away the nest's location. In everything, the eaglet's safety comes first.

The parent crested serpent eagles are highly protective of their young. They are also the Taiwanese raptors that most often communicate with their young by voice. When an eaglet begs for food it gives out piteous squeaks, and if the male bird is away from the nest too long, the female will call to him with piercing cries. Compared with other Taiwanese raptors such as the crested goshawk and Indian black eagle, the calls of the crested serpent eagle are more complex and varied, and have many forms. When a raptor gliding through the skies gives out a succession of long-drawn-out calls, you can be sure it is a crested serpent eagle. Lin Hsien-tang, who is trying to sort out when exactly the eagles use their different calls, describes them as "the raptors with the best vocal cords."

Flight training school

When an eaglet leaves the nest, it begins a difficult journey of learning. A month after it hatches, its white down has been replaced by adult plumage, and its coloration is almost indistinguishable from that of its parents. It has all the look and demeanor of a bird of prey. The mother bird also no longer remains on the nest all day, but scouts around nearby. At the same time she trains her chick in survival techniques, such as how to flexibly use its claws and beak to rip up prey, and most importantly the skills of flight.

When the young eagle's wings become sufficiently stiff, it begins to continually flap them in the nest to strengthen its wing muscles, then unsteadily leaves the nest and practices flapping while gripping a branch. After that, it practices short hops and jumps, gradually increasing their length, until it finally takes the plunge and makes its first short flight through the forest.

During this process, the adult birds continually pass on their own flying experience. Because it demands a great deal of skill to swoop through the sky, or dive onto prey by suddenly drawing in the wings, at this time one can see the parent birds flying in front to demonstrate these moves, while the already adolescent juvenile flies along behind, watching. The Yangmingshan National Park authorities have treated several crested serpent eagles that have crashed into electricity pylons, which goes to show that they sometimes fail to master the skills, or make mistakes.

In early July, after two-and-a-half months under the protective wing of its parents, the young eagle responds to the call of the skies and leaves the nest in search of its own airspace.

Grass Mountain eagles

In the wild, crested serpent eagles have no specific natural enemies, but the challenges of nature fill their lives with danger. In the wet and foggy seasons, snakes become less active, and life becomes tough for the eagles.

But the biggest threat to the eagles' survival comes from human activities. Because the birds like to hunt in areas cleared for farming, they often come close to human habitations, and often suffer disturbance, or are caught by hunters. Some are also injured in collisions with motor vehicles: one of the two crested serpent eagles released by the Yangmingshan National Park authorities early last year had been hurt in this way. In the past the park has also rescued crested serpent eagles that have been abandoned after being kept in captivity.

Two years ago, a documentary film about crested serpent eagles entitled Eagles Fly over Grass Mountain, commissioned by Yangmingshan National Park, received a Golden Horse Award for best documentary. As the film suggests, even on Grass Mountain (an old name for Yangmingshan) with its heavy pressure from tourism, if only people take care to properly manage this green area, its diverse ecology can support healthy crested serpent eagles. The sight of the eagles hanging in the skies, and the sound of their long resounding calls, will tell us that the land below is still in good health.

Whether crested serpent eagles can continue to freely roam the skies is indicative of whether human civilization can value other forms of life, and will also decide whether mankind can continue to live rich lives.

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The crested serpent eagle lives naturally in the space between heaven and earth, freely riding the wind. Lin Hsien-tang, who for many years has been roaming Taiwan's wild places alone, captured this view of the raptor's heroic bearing in the mountain forests. A juvenile bird learning to fly among the branches also seems to have heard the call of the skies.

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On sunny days when gentle breezes blow, crested serpent eagles ride thermal currents high into the sky. Above the ridgelines of Taiwan's mountains at low to medium elevations, one can often see them gliding with outstretched wings.

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(above) The crested serpent eagle is at the top of its food chain, and only lays one egg at each brood. (facing page, bottom) The crested serpent eagle feeds mainly on snakes. Pictured here, an eaglet that has not yet left the nest is taking large bites from an Asian green snake brought home by its parents.

(facing page) Crested serpent eagles like to hunt in areas where the forest has been artificially cleared. But they themselves are threatened by human hunters, and by the pressure of overdevelopment.