Fire, Ice and Eagles
By Alexander Ladygin
Published at Natural History, 1994, #2
Winter nights are long on Russia's far eastern Kamchatka Peninsula, but moonlight brightens the landscape when it reflects off snow some six feet deep. I leave my log cabin before dawn and ski toward Kuril Lake, hoping to elude detection by crows, ravens, and eagles, the better to observe their natural habits. The temperature is barely 0ш F, and steam rises from the lake. From the "window" of my second cabin, one that I have built of snow, I see eagles that have left their night-time communal roost and are soaring over the lake in search of a breakfast of salmon. The eagles are the reason I spend winters in southern Kamchatka, sitting all day in an igloo, brushing snow from my notebook, and hoping my camera will still work despite the frigid temperatures. Although cramped and uncomfortable, my snow cabin, one of many I have built on the very edge of the lake, gives me a view of a teeming oasis in the midst of a white desert.
Kuril Lake, near the southern tip of the peninsula, is the largest sock-eye salmon spawning ground in Asia. Traveling from the Pacific Ocean, through the Sea of Okhotsk, and up-river to Kuril Lake, some eight million salmon arrive annually near the place where they hatched some four or five years earlier. Even though the spawning season is unusually long—from July to March—at peak times the huge numbers of fish pack not only the feeder streams but also the shallow edges of the lake itself. Spawning, the laying and fertilization of eggs, takes place over and over again at the same sites. The pile-up of eggs and the abundant bodies of adult salmon, which die after reproducing, are the foundation of the winter life of Kuril Lake.
My study area is within the Kronotskiy State Biosphere Reserve, about 2. 5 million acres in area and one of the largest in Russia. Kamchatka itself is a land of glaciers and active volcanoes. Some thousand feet deep, Kuril Lake is of volcanic origin and is fed by creeks and springs. The sheer volume of water and the influx of relatively warm spring water keeps the lake from freezing over completely in winter. Until they begin to hibernate in late December, bears are active fishers of salmon, and the resident foxes, wolverines, otters, and even shrews take advantage of the spawning frenzy.
Thousands of birds of various species are also able to remain all winter because the lake is ice-free. Gulls feed on decomposed salmon carcasses and caviar; common golden-eye ducks and mallards gather dead eggs from the bottom of the lake-shore; mergansers capture young smolts (salmon hatchlings); and swans and mergansers dig up salmon nests and devour the eggs. Even perching birds not usually associated with fish, such as woodpeckers and willow tits, can be seen making a meal of washed-up remains of salmon and eggs. Crows, ravens, golden eagles, and white-tailed eagles also vie for a living on the lake—scavenging carcasses and pirating fish from other birds.
The most impressive of the birds that take advantage of this winter bounty, and the subject that I have studied for more than ten years, is the Steller's sea eagle. True fishing eagles, closely related to North American bald eagles, these birds are named after Georg Steller, the eighteenth-century Russian naturalist who explored Kamchatka, Alaska, and the Aleutian Islands. Steller's sea eagles are characterized by their bright white foreheads, shoulders, and tails, which contrast with their brownish black bodies. Their beaks are massive, deep, and strongly arched. But the most remarkable aspect of these eagles is their size; Steller's sea eagles can weigh up to twenty pounds, about twice as much as a bald eagle, and can have a wingspan of some seven feet. Also known as the white-shouldered eagle, this bird breeds only in Russia; of the total world population of 4, 200 breeding pairs, 1, 200 pairs nest on the Kamchatka Peninsula. In the winter, some of the birds migrate to Japan and Korea, but about 1, 000 individuals, or one-eighth of the world's population of Steller's eagles, remain at Kuril Lake to feed on its riches.
Unlike bald eagles of North America, which have attracted the attention of biologists, conservationists, and ecotourists, Steller's sea eagles are little known and are studied today by only a handful of scientists. The haunts of the bird are remote, and this may account for its extreme shyness with humans. No roads lead to Kuril Lake, and the nearest village lies some sixty miles away. A scientific station has an outpost on the one river that flows from the lake to the sea. The limited access to the region and the bitter weather make for hard living conditions for scientists in winter. But like other visitors to this area, we enjoy plenty of fresh salmon and caviar.
Among themselves, Steller's sea eagles are extremely gregarious. Even in the breeding season, when many species of birds forgo flocks for family groups and hunt singly or in pairs, Steller's eagles tend to feed communally. This habit is related to their specialization as fish eaters; fish, their main food year-round, tends to be concentrated in lakes and streams. Most Steller's sea eagles in Kamchatka breed along the more northerly coasts of the peninsula. Beginning in late March, the eagles begin to refurbish their huge nests, which they use year after year. The usual clutch consists of two eggs, and the parent birds raise the eaglets on chunks of freshly caught fish until the young birds fledge by summer's end. As early as September, the leaves fall, the icy winds of winter begin, and the eagles' lives change dramatically. The lakes in northern Kamchatka freeze over, locking up their food supply. Adults, sub-adults (eagles less than five years old), and the young of the year wander southward and congregate in large groups, becoming even more social than in summer. Of the thousand or so eagles that take up winter residence on Kuril Lake, I have seen more than four hundred gather on one feeder stream choked with salmon. As soon as one eagle finds a carcass, other eagles quickly gather. The evolution of this intensely social foraging system, and the central role it plays in the birds' general ecology, is the focus of much of my winter work.
I believe that the size of their prey explains why feeding Steller's eagles attract one another and, indeed, rarely feed independently, even when food abounds. It certainly contributes to the varied interactions of Steller's and other species of eagles. Adult sock-eye salmon average about the winter, some of the birds migrate to Japan and Korea, but about 1, 000 individuals, or one-eighth of the world's population of Steller's eagles, remain at Kuril Lake to feed on its riches.
I believe that the size of their prey explains why feeding Steller's eagles attract one another and, indeed, rarely feed independently, even when food abounds. It certainly contributes to the varied interactions of Steller's and other species of eagles. Adult sock-eye salmon average about six pounds and are sheathed in tough skin. Unless a salmon is dead and decomposing, this hide is difficult for birds other than Steller's eagles to penetrate. The golden and white-tailed eagles that live at Kuril Lake may take hours to pry an opening around a salmon's gills, front fin, or anus, and for the most part, they depend on the massive-billed Steller's eagles to open a fish carcass. Salmon is unusual prey for white-tailed and golden eagles, which in most of their range, and in summer in Kamchatka, prey on other birds and on mammals. They, have no specific adaptations for capturing large live salmon and tend instead to scavenge dead fish on the gravel bars of the lake or feed on the leftovers when the Steller's eagles have had their fill. The existence of the golden and white-tailed eagles on the salmon spawning ground is attributable to the presence of the more brawny, dish-eating specialists, the Steller's sea eagles.
In contrast, Steller's sea eagles are active predators on the spawning ground. They can catch and pull live salmon from the water, but sock-eye salmon carcasses are simply too heavy for even Steller's eagles to carry away, and they more often feed on dead fish deposited on the gravel bars and icy edges of the lake. One salmon is more than enough to satiate several eagles. The birds seldom bother with rotting fish being picked apart by other species of raptors. While golden eagles form small feeding groups of three or four members, and white-tailed eagles tend to hunt alone, wintering Steller's eagles are attracted in great numbers to other Steller's eagles. The degree of attraction and interaction reaches a peak when dozens of birds converge on a mound of dead salmon—often ignoring other carcasses—and harass and fight one another in an attempt to steal the spoils.
From my snow cabin, I have witnessed some impressive squabbling from just ten to twenty yards away. Although physical injury, or even contact, is rare, the eagles use a number of ritualized displays to convey dominance, submission, and a variety of moods. Wing, tail, and head displays are most common. Sometimes one or more eagles will stretch out their wings and wave their tails to signal their determination to feed on a particular fish. Steller's eagles and their cousins the bald eagles regularly force other birds to give up prey, as when a bald eagle harasses an osprey into dropping its catch. Because of their penchant for feeding together, Steller's eagles also often engage in piracy and steal fish from one another on the lake-shore. Piracy takes place only when the fish is sizable; small fish are not worth the energy expended in a fight or are simply consumed too quickly to allow piracy to occur. Moreover, even though its massive beak enables a Steller's eagle to snatch and swallow large chunks of fish, eating a salmon takes a long time; before it has finished eating, any eagle partaking of such a banquet is likely to be seen by another hungry eagle.
For a long time I wondered why the eagles preferred robbing one another to feeding independently, especially when the lake-shore teemed with living and dead salmon. I now believe that even for such a mighty bird as the Steller's sea eagle, opening large, tough-skinned carcasses is a challenge. Cashing in on another eagle's work is quicker and easier than ripping open a fresh carcass and is even worth the energy lost in displaying and squabbling. Sub-adults, which are not yet adept at manipulating salmon, must either steal part of another bird's fish or resort to eating soft, rotting carcasses. The dynamics of the Steller's eagles' strategy are not those of classic piracy, in which an entire prey is appropriated. Rather, piracy and scavenging are combined. Because a typical salmon provides more than enough food to satiate a single eagle, intruding birds do not so much steal as use the valuable, surplus salmon. Group feeding may be beneficial to the species because large, unwieldy windfalls of food are ultimately shared by many eagles.
I was surprised to find that conflicts reached their peak in frequency and intensity when food was most abundant. Conflicts between two individuals were rare, but when group size increased to five, the number of conflicts rose exponentially. A major factor affecting the makeup of feeding groups is the age of its members. Adult eagles more often attacked feeding birds and were more successful at piracy than subadults.
As has been suggested for herons, storks, and gulls, the color of plumage may play a role in the formation of Steller's eagles' feeding groups. Subadults must wait five full years before they attain fully mature plumage, with the striking white head, tail, and shoulders. Younger birds are dark brown with a few white spots, and their beaks are pale, lacking the bright orange of their elders'. The contrast between white and deep brown in the adults makes them easy to spot at a feeding site and, I believe, gives other eagles a powerful visual signal of a particular bird's place in the feeding hierarchy—in which adults take precedence. I think this holds true not just on the wintering grounds but also on the breeding grounds, where Steller's eagles tend to nest near one another along salmon rivers and where several nesting pairs may share a common hunting area.
According to my best estimates, each Steller's eagle consumes about fifty fish a season at Kuril Lake. In other parts of the Kronotskiy Reserve where no spawning grounds exist, eagles may die in winter. But on Kuril Lake they tend to gain weight. I was even able to catch some by hand on the ground because, after gorging on several pounds of salmon, the eagles were unable to fly away. Of the seven winters I have spent on the lake, the one exceptional season was the winter of 1992-93. During weather that was unusually harsh, even for Kamchatka, ice covered the spawning grounds, making fish inaccessible to the eagles and all the other birds that rely on salmon for their winter livelihood. Far fewer eagles congregated on the lake. Perhaps the next couple of winters will reveal whether this is a short-lived phenomenon or a climatic trend with greater, and grimmer, implications for the wildlife of the area.
As the spawning season winds down and March approaches, most adult salmon have reproduced and died. Food now becomes scarce. During this time, the communal roost of the Steller's sea eagles, which is located in stands of birch trees some three to six miles from the lake, becomes particularly important as an area where eagles exchange information regarding the location of food. When one scouting eagle finds a spot with a few salmon left, its soaring confreres will readily find and join it. Eagles flapping in a particular direction will soon catch the attention of the birds still in the roost, and the "word" will spread. This continues until the lack of salmon and the hint of spring send the eagles north to nest again.
In the middle of March, when the eagles begin to return to the northern coasts, I too leave Kuril Lake. I board an orange polar helicopter and rise above the deep, bright water. From the air I can see the single river that connects the lake to the sea, the one artery that brings life to Kuril Lake in the form of millions of spawning salmon.