We were walking along Newbury Road in Sutton last week when we saw a large bird making its way up the Lane River valley against the wind. My companion asked, “Is it a heron?” And she meant a great blue, because it was large. But herons row through the air on elegantly arched wings, while this bird was moving over the broad wetland by simply and subtly altering the contours of its flight feathers. When I raised my binoculars to my eyes, its identity was unmistakable. Roger Tory Peterson called it a species that was “all field mark”: it was a bald eagle.
In the 1970s, when I began watching birds, bald eagles were still quite rare. Their range spanned the entire continent and in the 18th century they may have numbered between 300,000 and 500,000. This may seem like a lot of eagles, but compare them to, say, the modern estimated population of the humble eastern phoebe, which is 32 million. And then contemplate the fact that in the 1950s the population of bald eagles in the lower 48 states was down to 418 pairs, due to decades of shooting and habitat loss, but more recently because of the widespread use of dichloro-diphenyl-trichloroethane (DDT).
The banning of DDT, enforcement of prohibitions against raptor shooting, and environmental regulations enacted in the second half of the 20th century caused the recovery of eagle populations. But the recovery was not rapid. The “Atlas of Breeding Birds in New Hampshire” was published in 1994, and the bald eagle was not included. According to the Bald Eagle Project of New Hampshire Fish & Game, in 1988 a pair began building a nest at Umbagog Lake in Coos County. Until 1996, this was the only known breeding pair in the state. As of 2017, New Hampshire had 59 territorial pairs of bald eagles. Of those, 38 successfully fledged young.
Our excitement at the sight of a bald eagle was therefore understandable. Just after the eagle disappeared over the hills to the northeast, we met a local resident and told him of the sighting. He was impressed; he had not heard of one south of Keyser Lake.
My mind went to zoologist Paul Colinveaux’s book “Why Big Fierce Animals are Rare” (1978; Princeton University Press), which is clear, often funny explanation of Charles Elton’s “pyramid of numbers.” In the 1920s, the English ecologist noticed that throughout the natural world, progressively larger animal species were correspondingly less common. The insects that warblers eat are much more common than the warblers. Herring are more abundant than sharks. There are more gazelles than lions. Elton suggested that small animals were more abundant because they produced more young, but he was mistaken.
As Colinvaux, a member of the University of Michigan faculty, pointed out: the number of professors is not determined by the graduate schools’ production of doctorates, but by the number of faculty positions available. In other words, population numbers are determined by the amount of resources available, not by reproductive rates. Phoebes are more common than eagles because there is more for them to eat, not because they lay more eggs.
Bald eagles (Haliaeetus leucocephalus), which are confined to North America, are one of the “sea eagles” (or “fish eagles”). They form a species pair with the Eurasian white-tailed eagle (Haliaeetus albicilla) and are part of a subfamily with seven other members distributed worldwide. As their vernacular name suggests, these birds feed largely on fish, although the bald eagle is among the least specialized; in some regions only a little than half its diet may be fish. In contrast, our other eagle, the golden, will eat almost anything, but focuses on terrestrial animals.
Molecular genetic evidence (“the molecular clock”) suggests that sea eagles evolved over 30 million years ago in the vicinity of what is now the Himalaya, but was then the broad shallow Tethys Sea, which once extended through what is now the Middle East. (The Mediterranean is a small remnant, but closing arm.) These eagles are still most common near fertile coastal areas, where abundant resources support greater numbers. In particular, the salmon runs of the Pacific Northwest, western Canada and Alaska provide large numbers of large fish in concentrated, vulnerable populations.
I spent the late summer and early fall of 1983 in Alaska. I thought I might live up there and was exploring different parts of the state by hitchhiking around. My hippie predilections were clear and several people suggested I visit the southern tip of the Kenai Peninsula, a relatively progressive part of a mostly conservative state. South of Soldotna, the already thin human population, thinned some more. One of the few signs of civilization was the power line that ran alongside the two-lane road. As a reflexive birdwatcher, I kept an eye on the lines, hoping to see raptors and other birds sitting on them.
When I saw a bald eagle, I pointed it out to my ride. Alaskans are blasé about the grandeur of their state. “Oh, that’s nothing,” he said. “Wait till you get further south. When you get down close to Homer, you might see a hundred at once. We call them ‘Homer pigeons’ because they just sit up there on the wires like pigeons do everywhere else.”
On a coast where every little creek and river had a salmon run, I did indeed see hundreds of bald eagles. Big, fierce animals are rare, except some times, some places, when and where there is an enormous amount to eat.
Bill Chaisson, who has been a birdwatcher since age 11, is a former editor of the Eagle Times. He now works for the Town of Wilmot and lives in Sutton.