Wild Side: Bald eagles

Sightings on the rise on the Vineyard.

An immature bald eagle, in the sort of dull, mottled plumage that we see most often on the Vineyard. — Ken Magnuson

As recently as the publication of Vineyard Birds II in 2007, the bald eagle was considered a scarce vagrant on the Island, recorded at a rate of roughly one per year. Since then, this bird has turned up with steadily increasing frequency here, and sightings of multiple birds have likewise grown more common. At present, I’d describe the bald eagle as an uncommon but regularly migrant, expected in small numbers every year. Most sightings reflect wandering birds in winter; but summer records are also growing routine, probably reflecting the wanderings of immature birds that have not yet found a mate or established a territory.

This winter has fit neatly into the pattern. Eagles have been reported steadily throughout the season, and as many as half-dozen individuals may have been present at once. While these highly mobile birds may be spotted anywhere, it’s clear that the south shore and the great ponds offer their preferred habitat.

Found across most of the continental United States and Canada, the bald eagle has always varied widely in its regional abundance. Alaska has always been a stronghold of the species; during the 20th century, tens of thousands were killed there for their presumed competition with the salmon fishery, without making any apparent dent in the eagle’s numbers. And a non-migratory population in Florida was considered by one 1937 account to comprise the majority of bald eagles in the lower 48 states. While bald eagles have always occurred in interior river drainages in the U.S., a shortage of nesting sites has always limited this species across the plains states, and in general, the bald eagle has always been a predominately coastal bird.

Famously, numbers of this huge raptor began to plummet around 1960, partly as a result of habitat loss and persecution but mainly because of the catastrophic impact insecticides like DDT had on eagle reproduction. By the early 1970s, extirpation of the bald eagle in at least the contiguous 48 states was a real possibility, and the embarrassing symbolism of killing off our national bird helped catalyze the passage of the Endangered Species Act in 1973.

As the amount of DDT in the food chain declined following the banning of that chemical, the bald eagle (as well as many other predatory birds) rebounded energetically. Historical nesting areas were rapidly recolonized. And this bird, a rarity in Massachusetts just a few decades ago, is once more a familiar part of the Bay State avifauna. A small breeding population occurs in the protected watershed of the Quabbin Reservoir in central Mass., and wandering, migrating, or wintering birds can turn up anywhere within the Commonwealth.

Vast birds with broad, plank-like wings and heavy bills — wingspan may exceed seven feet and a big female may weight more than a dozen pounds – bald eagles soar with unspeakable grace and propel themselves with slow, powerful wingbeats. At long range, eagles can be distinguished from turkey vultures — the only species expected here that rivals a bald eagle in size — by their flat wings; soaring on slightly raised wings, turkey vultures show a distinct “V” shape.

You’d think that a mature bald eagle, a huge bird with deep brown plumage and pure white head and tail feathers, would be a slam dunk in terms of identification. But at long enough range it’s possible to mistake a soaring great black-backed gull for an eagle (I’ve done it). And the sad fact is that fully adult birds are rather scarce on the Vineyard: the vast majority of eagles that turn up here are in one of the highly variable immature plumages. Brown mottled with paler feathers, immature bald eagles are easy to dismiss as hawks if you’re not paying attention, and hawks are often reported as eagles by inexperienced observers. It’s also common for immature bald eagles, lacking the iconic “bald” head, to be mistaken for golden eagles, which are vanishingly rare on the Vineyard.

Accounts of the bald eagle emphasize the versatility of this species. A hungry eagle can reportedly run down a duck or brant in level flight. Eagles can snag shallow-swimming fish with the dexterity of an osprey. Mammals as large as lambs and fawns can be taken by a bald eagle, and at the other extreme, these raptors have the dexterity to snag small rodents or even small songbirds. In addition to doing their own shopping, bald eagles are notorious for stealing the prey of other raptors, and for exploiting carrion, especially fish kills, as scavengers. One of my favorite historical observations was of an eagle nest that held 14 muskrat traps – minus the muskrats, which had been fed to the bird’s young!

Breeding eagles on the Vineyard appear to be unlikely; we have a shortage of the tall trees that are preferred by the eagles of our region for their massive nests, which may be used across decades and grow to more than a ton in weight.. But look for this species to continue growing in regularity and numbers here. When you spot one, remember the history of this species, which reflects both the unintended consequences of human “progress” and the ability of nature to rebound in response to thoughtful conservation.