Nature Notes: Ospreys and Eagles

by Rob Bierregaard

Benjamin Franklin once famously wrote that the Turkey would be a more respectable choice “as the representative of our country” than the Bald Eagle, a bird he described as being “of bad moral character.” Without digressing into the details of the quote’s origins, it makes a great lead-in to the fraught relationship between two iconic birds of prey: the Eagle and the Osprey.

However regal their stature, Bald Eagles have a few behaviors one could hardly call “majestic.” Eagles eat carrion, and they love to steal hard-earned fish from male Ospreys return- ing to their nests to feed their young. They have even been reported strafing Double-crested Cormorant colonies, setting the cormorants into a such a panic that they regurgitate their food. After the cormorants scatter, the eagles return to the scene of the crime to eat the fish that the panicked cormorants coughed up. Yum.

Bald Eagles and Ospreys do not “play well together.” Besides the kleptoparasitism—a fancy term for stealing food from another species—Bald Eagles will prey on young Ospreys in their nests; a nest full of young Ospreys might be described as an hors d’oeuvre platter for passing Bald Eagles. If the Osprey parents are near the nest, they may be able to defend their young, but if both are off hunting, their young are left defenseless—and Bald Eagles are not above killing adult Ospreys on occasion.

In the good old days (pre-1950), before we dumped DDT onto the salt marshes of Long Island and southern New England, Bald Eagles and Ospreys were common residents in the area who, over many millennia, had come to an ecological détente. Bald Eagles were widespread, while Ospreys, which numbered about 1,000 pairs between New York City and Boston, were mostly concentrated in three large colonies: one at the mouth of the Connecticut River, another on Gardiner’s Island, and a third in the Narragansett Bay.

These dense clusters—300 pairs on Gardiner’s Island, an area just over five square miles—relied on huge shoals of Atlantic menhaden, the silvery filter-feeding fish that migrated each spring from the Chesapeake Bay into Long Island Sound and Narragansett Bay.

The colonial nesting that we witness in Ospreys is extremely rare in birds of prey, most of which are territorial and don’t tolerate nesting pairs of their own species in their home ranges. Because (and this is especially true for those in coastal regions) the Osprey’s prey are spread out over large areas—male Ospreys may hunt as far as 20 miles from their nests—it’s simply not possible to defend feeding territories. So these birds may nest in colonies to obtain information from their neighbors. Male Ospreys preparing for a foraging trip will watch other males coming into the colony with fish. If they see a male with a fish like a menhaden, which travels in large schools, they’ll head in the direction from which the successful menhaden catcher came. If, by contrast, they see an Osprey coming in with a species that doesn’t school, such as a flounder, they’ll ignore that bit of information.

Bald Eagle on Osprey Cam Nest, October 2022.

Paul Spitzer, who spent decades studying the southern New England and Long Island Ospreys from the nadir of their population decline in the mid 1960s, suggested another explanation for colonial nesting: Spitzer speculated that Ospreys were concentrated in colonies in part as a defense against Bald Eagles. No Bald Eagle could get anywhere close to the clusters of Osprey nests within these colonies; with 30 or 40 nests all within sight of each other, there were always a few male Ospreys hanging around after delivering food. As soon as one spotted an incoming eagle, the alarm calls began, and a squadron of angry Ospreys assembled to drive the intruder away.

Along came dichloro-diphenyl-trichloroethane (DDT). For two decades—the 50s and 60s—while the insecticide reduced the number of mosquitoes, we inadvertently diminished the numbers of several sensitive species of birds of prey. DDT worked its way up the food chain, multiplying in concentration, until it reached such levels that Ospreys and Bald Eagles, as well as many other raptor species, were unable to produce enough young to compensate for normal adult mortality. Population numbers plummeted. The 1,000 breeding pairs of Ospreys between New York City and Boston crashed to only 100 pairs. Bald Eagles practically disappeared from the landscape, and Peregrine Falcons no longer bred east of the Mississippi.

The population crashes of these apex predators was the canary in the proverbial coal mine. Eventually, scientists figured out that DDT was the culprit, and its use was banned in the U.S.

More quickly than many expected, DDT levels declined to the point where raptors were able to breed successfully, and their numbers began to increase. Ospreys took about a decade to rebound. In some areas, their recovery was given a boost by the introduction of young into areas where the species was no longer breeding.

Because they breed at younger ages than Bald Eagles, Osprey numbers increased faster than those of Eagles, and the species expanded its range. Man-made nesting structures—either nesting platforms designed for Ospreys or those the birds coopted such as cell towers, light poles, and even chimneys—helped the population grow rapidly, exceeding pre-DDT levels not long after the turn of the century.

So, Ospreys in southern New England and on Long Island found themselves in an environment replete with nesting platforms—and lacking in Bald Eagles. Ospreys spread across the region, no longer concentrated in the three original colonies, moving north through Connecticut and both westward in northern Massachusetts and eastward, onto Cape Cod. The population on Martha’s Vineyard grew from two pairs in 1969 to over 110 pairs in 2023. As many as 500 pairs now nest on Cape Cod.

Eagle fleeing with fish stolen from Osprey. Photo by Andy Morffew, Creative Commons

As Osprey populations have grown and expanded, Bald Eagles, too, have been increasing across much of their range and are becoming a presence in the Osprey-dominated landscape. Besides preying on young and occasionally adult Ospreys, Bald Eagles have been recorded usurping Osprey nests.

Because they are not strictly tied to a diet of fish, Bald Eagles don’t have to migrate to the Caribbean and South America, as most North American Ospreys do during the northern winter. Instead, Bald Eagles rely on carrion and prey on waterfowl during the winter. Because of this, they can start nesting as most Ospreys are just beginning their long trek back to their breeding grounds.

During the winter, Osprey nest platforms are great perches for Bald Eagles, so it’s not unusual to see wintering eagles resting on Osprey platforms—even providing a great show on Fishers Island’s Osprey Cam nest in October 2022.

Being considerably larger than Ospreys, Bald Eagles need a substantially larger nest; as a rule, most Osprey platforms are too small to support a Bald Eagle nest, but where Ospreys have gone “old school” and nested in dead trees, the situation is ideal for Bald Eagles to usurp an Osprey nest. On Martha’s Vineyard, Bald Eagles have tried to commandeer Osprey tree nests at least four times over the past five or six years. In no instance were the eagles successful in seizing an established Osprey tree nest. We’re not sure why the eagles were unsuccessful in three of their tries, but we have a pretty good idea after Gus Ben David, the unofficial Osprey “godfather” of Martha’s Vineyard—who has shepherded the island’s Osprey population from two pairs in 1969 to over 110 in the 2020s— witnessed a spectacular battle over a contested nest.

The nest in dispute was in a dead tree in a vast savannah-like expanse along the Vineyard’s south shore. Ospreys, who had used the nest for two years, had amassed a substantial nest. In the winter of 2022, while the Ospreys were down south somewhere, a pair of Bald Eagles took over the nest, upscaled it to Bald Eagle dimensions, and laid eggs. In late March, the Ospreys returned to find their nest occupied.

Gus got a call from the local caretaker and went out to watch the action. In the protracted battle, the Ospreys repeatedly flew high above the nest and dove at the female eagle in the nest. As the Ospreys drew near, the eagle would flip onto her back and present her talons to the swooping Ospreys. The relentless attack, how- ever, proved too much for the eagles, who abandoned the nest. Gus suspected that the eagles’ eggs were broken in the melee.

Later that spring, I visited the site of the battle, where I saw the female Osprey incubating her own eggs, looking strangely small in the now huge nest.

What does the future hold for these two species?

Bald Eagles are unlikely to nest near concentrations of Ospreys, but where there are large trees and only scattered Ospreys, we can expect to see Bald Eagles gaining a foothold in new areas. As this happens, we can expect more aggression and conflict between the two species. The Eagles’ encroachment will likely put pressure on isolated Osprey pairs, while those nesting in proximity to other Ospreys are not likely to suffer. In time, the two species will approach a new equilibrium. Osprey numbers may go down slightly, but they are too well established across the landscape to suffer any existential threat from increasing Bald Eagle numbers.

One new wrinkle in the story may come from a relatively new and microscopic player: highly pathogenic avian influenza, or HPAI. Transmitted primarily by waterfowl, this virulent disease has infected over 82 million birds, including huge numbers in the poultry industry, since it was first detected in 2016. Because Bald Eagles prey heavily on waterfowl, they are susceptible to infection. The dramatic decline in their numbers in the Great Lakes region is likely the result of HPAI infection. Ospreys, with their fish-only diet, seem little affected by the disease.

How this may play out in southern New England is hard to predict. It may well slow the increase in Bald Eagle numbers in the region, and thus reduce pressure on pioneering Osprey pairs nesting far from others, but it shouldn’t be much of a mover to the Osprey Nation.

Whatever happens, with so many eyes and nest cams now focused on Ospreys, we’re sure to have a front row seat as these two mighty birds of prey sort out their relationship.

Ornithologist Rob Bierregaard first became involved with the HLFM in 2013 when he was invited to the Island to tag a male Osprey with a transmitter. He returned in 2014 to tag another. Since then he has written articles and given virtual and live talks for the Museum on natural history subjects.