By Alexander Ladygin
Published at BBCWildlife, #10, 1995
A shadow slid slowly across the snow-covered birch as the huge, piebald bird landed heavily on a branch. For an instant it was lost again, swallowed up by the forest's protective mosaic of black and white. But the flash of its startling chrome yellow feet and powerful talons revealed it as Steller's sea eagle, the first one I had ever seen at the nest.
That sighting marked the beginning of my study, eight years ago, of one of the world's least known birds of prey. My wife and I had set up home in a remote, long-deserted fishing village on the Kamchatka Peninsula, 9, 000 kilometres away from our friends and metropolitan lifestyle at Moscow State University. For me, being in this wilderness was the realisation of an ambition to follow in the footsteps of my geologist-father, who had spent 10 years studying Kamchatka's volcanoes. Here, in our cabin on the edge of a large coastal lagoon, we enjoy the simple life, waking and falling asleep to the sound of the ocean, and sharing the traditional haunt of several pairs of Steller's sea eagle.
Weighing in at nine kilos, and with a wingspan of around two and a half metres, Steller's is one of the biggest birds of prey in the world. For sheer size, it outmatches its American cousin, the bald eagle, and most other birds, save the Andean condor. Compared with the bald eagle, Steller's sea eagle is also much rarer and more poorly known. Its entire world population comprises only about 4, 000 pairs, whose breeding range is confined to a narrow strip along the coast of Russia, from the Bering Sea south to the Sea of Okhotsk.
This restricted range is explained by the eagle's strong dependence on the lakes, rivers, and streams that supply its staple diet of salmon. Throughout most of Kamchatka, except in the north (where the eagles are nomadic), the eagles' home ranges are tied year-round to these rivers, which run cold and clear from the volcanic slopes down to the east coast. Various species of salmon are virtually the only fish these rivers support, but they are immensely prolific, and the smallest of streams has its spawning beds. In the breeding season, the eagles snatch migrating and spawning
salmon from the rapids and shallows. Salmon skin is notoriously tough, so much so that native Kamchatkans use it for making clothes and shoes, but it is no match for the eagle's stiletto talons and massive, yellow axe-head of a bill.
In winter, when the rivers and lagoons are frozen, the eagles resort to the coastal deltas, which are relatively ice-free except for broken ice-floes driven ashore by wave action. After spring storms, an abundance of food - sea urchins, starfish, sea cucumbers, squid and other molluscs - is also stranded on the shoreline among the kelps. In early spring, before the salmon start running, the eagles are attracted to this bounty of jetsam, and from my cabin window I have watched them rummaging all day long in the seaweed. The eagle's powerful bill is an excellent weapon for breaching the armour of hard-shelled sea creatures.
Heavy snow, rain and squally winds are hallmarks of the Kamchatka climate, and, in winter, the eagles' tree-top eyries are buried under thick domes of snow. Under this burden, a gale sometimes proves too much for the supporting birch branches, and the nest may collapse after a season or two. This hazard may explain why the nest of Steller's sea eagle is relatively small for such a large bird. By contrast, a white-tailed eagle's nest I saw in central Kamchatka was about 5m high and 2.5m across, probably representing more than 100 years of tenancy and building activity. When measuring the nests of Steller's sea eagles, I have noticed that the foundation is usually spread across several branches to reduce the risk of being destroyed by the weather. Nevertheless, many nests succumb. One day, when I climbed to a birch-tree eyrie, a sudden squall collapsed the nest, pitching a well-grown eaglet to the ground. A second eaglet was left sitting on a side-branch, uttering heart-rending cries. I collected the fallen youngster in my sack, and climbed back up to replace it in the tree.
If the nest is irreparably destroyed, the pair constructs a new one nearby, sometimes within tens of metres. In the event of such a disaster, they may also switch to an alternative nest which they keep constantly ready for use. Nests are generally built close to areas guaranteeing a plentiful supply of salmon. I once found 10 pairs nesting in only about 10 hectares among the braided channels of a salmon river valley.
The nest is, however, not always built in a tree. Kamchatka's rocky coast plunges steeply to the ocean. In spring, sunshine and salt-spray quickly free the cliffs of snow, making the cliff-tops attractive nest-sites for the eagles. These are not always on the coast, and one nest I had under observation was on cliffs several kilometres inland. Steller's sea eagles use the updraughts on ocean-facing cliffs to soar, but unlike other eagles, this technique is not used for searching for food. Perhaps it has some territorial function, but this can only be ritualistic, because Steller's sea eagle has a very small hunting territory in the breeding season. Moreover, a territory is not exclusive: different breeding pairs may use it and even hunt together, though, other than in winter, they do not feed side by side. I believe it is the abundance of the salmon food supply that does away with the need for strictly defended hunting territories.
• Home on the range. A pair of eaglets wait to be fed in their nest on a rocky cliff. The nests, also built in trees and bushes, soon become infested with parasitic insects.
• Preying on the predator. Apart from man, the eagles' only enemies are crows, which wait until the nests are unattended before swooping in to drink the eagle eggs.
In Kamchatka, the onset of spring is very different from the one we had been accustomed to in middle Russia, where rapid snow-melt triggers a sudden explosion of life. Spring in Kamchatka is imperceptibly slow, and for a long time the only sign of change is the lengthening days. Finally, after an endless succession of blizzards, the curtain of cloud which mantles the peninsula for most of the year lifts grudgingly half-way, flooding the normally steel-grey Pacific with deep blue. Suddenly, the eagles begin indulging in spectacular aerial pairing displays over the ocean, in which, like other large birds of prey, the male and female whirl downwards, joined together by their talons. At the very last moment, just above the water, they separate and pursue one another across the surface, periodically disappearing from view behind foaming wave crests.
The approaching summer will be short, and so the eagles start piling new branches on their nests before all the snow has melted. Winter has scarcely loosened its grip in April when the female lays two (more rarely one or three) white or bluish eggs. From this moment on, I begin to make detailed observations from tent-hides sited near nests. One such hide was situated on the steep side of a deep river valley, level with the top of an eagle's nest. For another, I constructed a log platform in the canopy of a birch. From here, 1 enjoyed a panoramic view embracing the lagoon, separated from the ocean by a sand-spit, the marshy river basin fed by willow-edged streams from the surrounding hills, and, away to the north, snow-clad volcanoes.
Steller's sea eagles are exceptionally wary and distrustful at the nest, and to minimise disturbance I try to enter my hide early in the morning before sunrise, and leave it late in the evening. In early spring one year, when the ice began to break up, I decided to find a route across the lagoon to start my observations on a remote nest. I set off in my canoe early in the morning, picking my way and sliding silently through the ice-floes. Far away, on a small island in the middle of the lagoon, several eagle silhouettes were visible. One made a shambling, waddling run into shallow water and grabbed a flatfish. At this time of year, the eagles will take even tiny sticklebacks, catching them with remarkable adroitness for a bird with such a huge bill.
Landing on the island, I followed a bear track across the wet tundra. As I neared the eagles' nest, local crows announced my arrival with shrill calls. Alerted, the incubating female eagle jumped across to a nearby branch to join her mate. Seen together as a pair, the difference between the two sexes is striking, the female being one-and-a-half times the size of the male. Having spotted me, both eagles flew off. Unlike falcons, they never attack a human intruder, and defend only a small area (less than two hectares) around the nest against neighbouring eagles and other birds. I entered my hide to await the eagles' return, and with temperatures around two degrees below zero for the rest of the day, wrapped myself in a down sleeping-bag and settled in comfort to watch and photograph. A cold wind from the ocean heralded another snow-storm that would last several days. As the strengthening wind shook the trees and snow deepened around us, the female rose with difficulty from the nest and shook off the snow melting on her wings before snuggling down again.
Irrespective of the weather, incubation lasts 36 days. But icy winds and regular snowfall can persist until the beginning of June, making incubation the most hazardous period in the breeding cycle. If the eggs are left for even a short spell, they quickly cool and can perish. So the incubating eagle sits it out through all weathers. Around hatching time, I had to examine the nest contents, but needed to do so as briefly as possible, because the newly hatched eaglets are so small and vulnerable. Luckily the bent, gnarled birch was ideal for climbing. Even so, some old nests are so massive and overhanging that, to peep inside, I had to use a mirror attached to the end of a stick. This may reveal the first egg already hatched into a tiny, white, downy ball, crouching and cheeping plaintive!) in the nest-cup. The second egg, which the female continues to incubate, usually hatches t few days later. When they are close to hatching the young can be heard calling from inside the egg, a ventriloquist's act which threw me the first time I encountered it.
After inspecting a nest, I make a hasty exit because ever-watchful crows will be waiting tc pounce. Once, one landed on the nest, and it wai with relief that I watched the female eagle makt it back in time to catch the crow's tail in her bill The outcome is not always so fortunate. I have seen crows pecking small eaglets or supping the contents of eggs when the parents were absent.
Sometimes the eagles are the vandals. On day, in mid-May, I was sitting in a small cabir near a nest, observing the eaglets' behaviou and their interactions with the female. They ha( been awaiting the male's return with food al day, but he had tailed to show up. Peeping witt hunger, the eaglets craned their empty crop: over the side of the nest. Finally, at sunset, the female could stand it no longer and flew over to a magpie's nest near my hide and began to destroy it. Ignoring the mobbing by the magpie parents, the female ravaged the nest within five minutes, killed three magpie chicks and carried them, one after another, back to her own brood.
By the beginning of summer, the spectacular mass salmon run up Kamchatka's streams is under way. Hunchback is the first of the salmon species, followed a little later by Siberian (sham), steelhead and sockeye. The first sign that the salmon have returned is the presence of seals, which pursue them into the lagoon. From now on, the Steller's sea eagles feed entirely on salmon, converging on the lagoon from surrounding nest-territories. On any one day, I have seen more than 10 breeding pairs together in the lagoon, fishing, feeding and interacting. Among them are immatures with pale bills and drab brownish plumage marked with white. They will pass through several intermediate plumage stages before acquiring the full adult livery at five years old.
At the lagoon there is a natural cycle of activity. The start of the flood tide, when salmon shoals move from the sea into the lagoon, signals the best time for fishing. At peak times, there are so many fish in the shallows that my canoe paddle regularly strikes the lithe, slippery bodies of salmon. Until this moment of plenty arrives, the eagles are content to wait patiently on a sand-bar at the river mouth, or any suitable raised area in the middle of the lagoon, confident in the knowledge that a bonanza is imminent. On hot days the atmosphere of this primordial scene is heightened by the lagoon steaming like a boiling pot, creating mirages which make observation difficult. At other times, everything is blotted out by thick fogs rolling in from the ocean.
One calm, sunny morning, with only the occasional splash of a large salmon disturbing the smooth surface of the lagoon, I almost dozed off in the hide. Suddenly, I was jolted out of my reverie by one of the two eagles I had been watching, taking off and heading purposefully for a stream. He flapped several times above the water, then dropped like a stone. Other eagles immediately shrugged off their drowsiness to focus on the lone hunter. A few seconds later, he emerged laboriously from the water with a silver salmon gripped firmly in his talons. Burdened by the weight, he struggled to the nearest shallows and pinned the still-squirming salmon to the ground with his feet. Then, with several powerful blows of the bill, he tore off hefty chunks of flesh and swallowed them.
After a quarter of an hour the eagle was satiated, unable to finish the fish, which must have weighed 3-5 kilos. This was the cue the other watchful eagles had been waiting for. In a flash they homed in, attacking from the air, and stole the remains of the fish from under the owner's feet. Because a large salmon generally exceeds the needs of its captor, such piracy is widespread and an integral pan of relationships in the Steller's sea eagle community, just as it seems to be among bald eagles. I have watched Steller's sea eagles fishing hundreds of times and am convinced that piracy features in half of all captures. It is tempting to speculate that the highly conspicuous plumage pattern of Steller's sea eagle (unlike the camouflage of other eagles) is an adaptation enabling eagles to spot one another easily, the mutual attraction being beneficial for exploiting large and superabundant salmon.
In this particular encounter, the winner extricated himself from the melee and made off with his spoils on the regular flightline over the marsh towards his nest. There, his two offspring were impatient to be fed. The elder eaglet stuck its head out from under its mother's wing, while she used her bill to try to nudge the younger sibling back beneath her. The thin down on the youngsters' heads was covered with drops of dew. Now about a month and a half old, the elder one was already standing up and performing daily gymnastics, jumping in the nest and noisily banging its wings. Its sibling, its crop completely empty, half-rose, moved to the sunlit nest-rim, and screamed for food.
It was now very warm in the sun and the eaglets were tortured by clouds of mosquitoes. With binoculars, I could clearly make out scores of mosquitoes settling on the eagles' eyes, bills, foreheads and naked areas of the body. (These importunate blood-suckers also detracted from my pleasure in witnessing the idyll of eagle life, as the dried mosquito corpses littering the sheets of my field notebooks constantly remind me.)
At last, the familiar greeting call of the male was heard from the depths of the wood. He alighted on a perch, tossing his head back as he uttered a shrill 'kah-kah-kah'. Usually, Steller's sea eagles are silent, but now the female answered in kind, whereupon her partner moved with his prey to the nest and began tearing off small pieces of flesh. A female, however, always prefers to feed the young herself, and so she bit off her own pieces for transfer to their greedy bills. After a quarter of an hour, the feeding session was over, the eaglets' swollen crops dangled to one side and a satisfied silence descended on the nest.
At birth, the chicks weigh only 150g each, but they grow by leaps and bounds, and weigh 40 times as much only 45 days later. By the time they leave the nest, their gourmet diet of pure salmon has transformed them from small fluffy balls into one of the largest, most magnificent birds in the world. •