Unanswered Questions About Vacated Osprey Nests

Essayist Margaret Renkl writes in a recent column for The New York Times (July 7, 2025) that the Northern House Wren is "a tiny, feathered terrorist" because of its wanton destruction of the Carolina Chickadee nests (including eggs and young) that graced her backyard.  Nevertheless, that explosion of blood lust failed to dampen her enthusiasm about this spring which was replete with young birds leaving their nests and making their way in the world.  Amid her pleasure in the season, she grappled with myriad questions about the ways of the birds she observed (for instance, "Why did the baby cardinals leave the nest too early, still half bald and flightless?"), questions that brooked no easy, simple answers.  It was this clutch of questions that prompted Renkl to posit that, whereas once she had little tolerance for unanswered questions or ones not answered quickly (e.g., the results of some medical test), "I am older now, and these days not knowing often feels like a gift."

At this point in the essay, she shifts from unanswered questions involving nature on a small scale or her own personal well being, to the more fundamental question of what will happen in the long term environmentally to the planet, with those in charge evincing little regard for the consequences of their actions.  It's in the not knowing the outcome that she finds hope; the dire expectations haven't been realized yet, so, in the space between the now and the future, action might be taken.  "I am grateful for the way that not knowing allows room for a future that is different from the one I fear."

I'm not grappling in this post with the long term environmental consequences of how people currently in power are intent on sacrificing the future for some short term payoff in the present.  Rather, I want to share a couple of Renkl-like questions that have left me stumped.  Over the past three years, I have been monitoring a small number of Ospreys each summer on the North Fork of Long Island.  It wasn't until July of this year that I returned to my summer cottage and found that, of seven nests that were active in July and August of 2023 and 2024, three are vacant.  By "active," I mean these were nests where, as best I could determine, short of climbing up to peer into the cavity, female birds were incubating eggs and the Osprey couple was subsequently feeding and tending to young.  Nests I deemed "vacant" this year were ones without any Ospreys in or on them.  Significantly, none appears to be damaged.  I admit that I do not know if these nests were the scene of some nesting activity earlier in the season, and were subsequently abandoned before I started observing at the beginning of July.

In a post in 2023 I described an active nest very near my summer cottage that, despite the commotion visited on the it and its occupants by the passage of daily commuter trains, boasted healthy parents and robust, demanding young.  It was also active in 2024.

Now it stands empty.

My first question was:  What happened to the pairs that once raised young in this nest and the other two?  My second was an attempt to place the experience of my nests into context:  What's the expected year-to-year vacancy rate for Osprey nests?

I am unable to answer the first question with any certainty.  Though Renkl suggests she has a willingness not to have an immediate answer to her nature questions, she did pursue some with a bit of determination.  That's the case here.  I do want to know the fate of my pairs, though, as I explain below, not having an answer leaves some room for hope.

The starting point, I guess, is that, as with nearly every kind of bird, the nest during the breeding season is the focal point of the Osprey's activity.  The breeding pair becomes singularly attached to its nest, not only dedicating great time and energy to its construction, but, often, returning year after year to the same nest, repairing and adding to it.  (See, for example, Alan F. Poole, Ospreys:  The Revival of a Global Raptor, 2019, p. 80.)  Which makes it puzzling why so many of the nests I'd observed over the past three years now are vacant, some even with vegetation now growing in them.

The factors that can influence why any single nest might be occupied one year and not the next are many.  The most dire, of course, is that one or both of the birds that dedicated upwards of six months to it, perished on the several thousand mile long migration the Ospreys took to and from their wintering ground in the southern hemisphere.  Alan Poole in his 1989 work on ospreys describes various ways of estimating the raptor's mortality rate.  (Ospreys:  A Natural and Unnatural History, 1989.)  For large banded populations this may be a relatively straight forward calculation.  He writes that preliminary data for two such populations offer estimated annual mortality rates of between 10 and 17 percent.  (1989, p. 142)  As I noted in my previous post on Ospreys, mortality in the earliest years of the bird's life is quite high:  "Fewer than half of the birds that fledged will survive to breed."  (For similar data, see, Hinterland Who's Who:  Osprey, Minister of the Environment, Canada, 1993, p. 4.)  The most dangerous periods in the bird's life are during the long migrations to and from the wintering grounds, and the wintering period itself.  It's also the case that mortality rates are relatively low in early adulthood through late adulthood.  (Federico De Pascalis, et al., Shift in Proximate Causes of Mortality for Six Large Migratory Raptors Over a Century, Biological Conservation, volume 251, 2020.)  The percentage of absent breeding pairs in the seven nests is higher (at 43 percent) than the estimated mortality rates among adults just cited.  This does make me question whether mortality, alone or at all, might account for these vacant nests.

Of particular relevance to my concern is Poole's observation:

Because most Ospreys are faithful to breeding sites, one can reasonably assume that any established breeder failing to return to its nesting territory (or nearby) at the start of the season has died.  (1989, p. 142)

Though the import of his comment is profoundly negative (no bird returning to the site equals a dead bird), he does hint at a more benign outcome for my birds.  One or more of these osprey pairs that, in past years, used the now vacated nests might have found some other nearby location more to its liking.

I then considered what might influence that decision.  Is the relatively substantial human activity in the area around some of the nests I've been tracking discouraging Ospreys from returning?  Likely not.  Ospreys adapt readily to urbanized locations.  A very interesting study on the factors that influence the success (not the presence) of Osprey nests in an urbanized Florida location found that it was principally the timing of the nesting activity which affected success rates (earlier nests fared better than later), not urbanization.  Results from this study are affected by the fact Florida hosts migratory and non-migratory Osprey populations; the latter may nest earlier than the former.  (Elizabeth A. Forys, et al., Predictors of Osprey Nest Success in a Highly Urbanized Environment, Journal of Raptor Research , Volume 55, Number 4, 2021.)  The authors found no significant effect on nest success from the location factors they measured, including the nature of the land surrounding the nests - forests, grassy tracts, parks and ball fields, and "urban cover."  They write:

In conclusion, this study provides further evidence that Ospreys can be productive in highly urban environments and this might be particularly true for areas like Pinellas County that are surrounded by water.  (p. 493)

That conclusion rings true with my local populations.  There is substantial human activity in this environment (e.g., repeated passage of commuter trains) which has not deterred Ospreys from nesting and nesting successfully in the area in the past, and there's been no obvious change this year in that activity.  I would note that, after the 2023, breeding season, the local power company added a platform to the utility pole seen in the photographs above.  That change to the pole in one year didn't preclude nesting in the following.  Might this modification, nevertheless, have rendered the site sufficiently less attractive so the pair this year looked elsewhere?  Perhaps.

It is possible that some threat to an Osprey nest is now present in the neighborhood, say a Great Horned Owl, though there's no evidence of that, and it's not clear why all three of the vacated nests were affected.  As already noted, none of the nests appeared damaged, ruling out that potential deterrent to reoccupying a nest.

So, this first question remains unanswered.

As for the second, I tried to determine if the vacancy rate of my nests is unusual.  It certainly appeared so to this uninformed observer.  My journey through the research has not been very productive.  Much of the research on Osprey nesting is focused on the success of the nesting activity (as is the one just cited for Florida).  In contrast, if one asks the question regarding year-to-year use of a nest, there appears to be little data readily at hand.  I've tried to tease out the answer to that question from data provided in various published sources, data not focused on the vacancy issue.  I turned particularly to state projects monitoring Osprey nests, but, so far (the search continues), I have found only one with reported figures even tangentially speaking to my concern.

The Connecticut Audubon Society, in its 2024 report on the observations recorded by its Osprey Nation (a network of citizen scientists dedicated to monitoring and reporting on the state's Osprey population), noted that, in that year, there were "105 vacant platforms or comparable sites previously used."  (Osprey Nation 2024 Season Report, November 8, 2024, p. 6.)  If I read that description correctly, that's potentially the numerator I need - nests used previously, now vacant. But, what's the denominator?

Based on the data reported by the Society, the aggregate total number of nests observed in 2024 was 945.  This includes 726 active nests, 31 nests that were removed or somehow destroyed, 83 abandoned nests, and the 105 vacant nests.  That's not the appropriate denominator since the count of active nests includes ones newly observed that year, and some of the removed, destroyed, or abandoned nests might not have been used in prior years.  ("Active" is defined as nests in which observers saw birds in the incubation posture or young not ready to fly.)

Recognizing that it's not wholly defensible, I turned to the prior year's count of active nests - 688 - and added it to the 105 vacant nests from 2024, to calculate a potential denominator of 793.  That generated a vacancy rate in Connecticut in 2024 of approximately 13 percent.

If this is at all close to a reasonable year-to-year vacancy rate in my locality, then the 43 percent (3 of 7 nests) vacancy rate of my nests is dramatically higher.  I recognize, though, that it's folly to draw any conclusions from data based on such an absurdly small sample.  So, I will leave off, for the moment, the search for an answer to that and my initial question.

To end on a bit more of an upbeat note:  even with nearby vacant nests, I have still been enjoying the piercing calls of Ospreys soaring high overhead while I work in the yard of my summer cottage or walk around the neighborhood, down to the beach, and along the road that abuts the marshy, reedy extension of the bay.  There are Ospreys here and in number, some with nests that have escaped my detection.  I have seen the raptors with plump, glistening fish in their talons, taking a meal to mates and fledglings in nests somewhere nearby.  I hope my previous tenants are among them.

Another bit of good news.  At some distance from my cottage, in the heart of the town of Riverhead, I found an Osprey nest new to me.  What a beautiful sight, even with the unintended product placement.

Ospreys On My Mind

This has been a summer of ospreys.  I am surrounded  by these glorious raptors.  Less than a tenth of a mile from my Long Island summer cottage, a massive osprey nest sits atop a utility pole (a structure replete with electrical lines).

It holds a brood of two, perhaps three young ospreys.  Their mother spends much of her time with them in the nest, guarding, watching, calling for food, and, at times, distributing pieces of the fish caught by their father.  He, in turn, when not out in search of fish, roosts nearby.

Joining the usual sounds of summer at the cottage are crystalline, high pitched, ringing chirrups (well, that’s what they sound like to my ear – descriptions in the literature vary quite widely), often starting slowly and well spaced, then rising in frequency and intensity.  These are primarily the begging calls of the female osprey, pleading with her mate to bring the food needed by their voracious offspring.  Ornithologist Alan F. Poole, in his recent, essential book, Ospreys:  The Revival of a Global Raptor, posits that there are three kinds of osprey calls with many variations:  alarm calls responding to a threatening interloper, guard calls signaling to other ospreys that a strange osprey is approaching, and begging calls voiced by mothers and young.  (Poole, p. 24-25; full citations are provided in the Sources section at the end of this post.)  The begging calls I’ve been hearing are apparently necessary to prompt the adult male to fulfill his biological role of providing for his family.  As with all such relationships, some males are more responsive than others.

Though I know it’s really not, my cottage feels for all the world as if it is at the epicenter of the osprey revival:  within roughly three to four miles of it are three other nests occupied by ospreys.  What an amazing turnaround from a half century ago, when the outlook for these and other beautiful avian raptors was bleak, their numbers devastated by the damage done by DDT and its breakdown products.

A nascent environmental movement in the mid 20th century, sparked by publication in 1962 of Rachel Carson’s Silent Spring, targeted the uncontrolled, widespread use of this and other insecticides.  The story of how the damage from DDT was stopped and reversed centers, in part, on Charles F. Wurster, an organic chemist, who was instrumental in the successful campaign against DDT which involved the filing of a series of lawsuits and much public testimony that galvanized public opinion against the insecticide.  A great deal of credit for the osprey revival that surrounds me must be given to Wurster who, it is sad to note, passed away in early July this year at age 92.  (Diamond, 2023)  He was among those marshalling and presenting evidence in the 1960s and 1970s that the insecticide was spreading uncontrollably worldwide, well beyond the areas in which it was directly applied.  They documented how its breakdown components seriously harmed wildlife, including avian raptors such as osprey who, at the top of the food chain, built up high levels of the insecticide in their bodies by consuming contaminated animals lower in the food chain, a process known as “biological concentration.”  These high levels of the insecticide led the birds to lay eggs with thin, fragile shells which broke under the pressure of the brooding mate.  (See, for example, Ackerman, 1996/2019, p. 46.)  The threat to the survival of these iconic birds (including the bald eagle) was great.  In 1967, Wurster was one of the founders of the Environmental Defense Fund which was instrumental in securing a nationwide ban on the use of the insecticide in 1972.

Poole notes that, although in North America the application of DDT decimated osprey populations (some specific populations declining by as much as 90 percent), once the ban took hold, osprey populations began a striking rebound.  Indeed, in some areas, the successful recovery has led to slowing population growth as available nesting sites are occupied, leading to movement of some birds into new, neighboring areas.  He reports that in Southern New England and Long Island (where my cottage is), there are at present approximately 1,500 breeding pairs of ospreys, primarily near saltwater.  (Poole, p. 43-44)

Ospreys’ scientific name is Pandion haliaetus.  The birds breeding in North America are P. haliaetus carolinensis, one of several subspecies.  They have a wingspan of about 63” and an overall body length of 23”.  Surprisingly, given how commanding they appear, they weigh only some 3.5 pounds.  (Sibley, 2003)  Ospreys’ diet consists nearly completely of fish.  As a result, they are often referred to as fish hawks, though they are not truly hawks.  P. haliaetus is the only member of the family Pandionidae which, in turn, is a member of the Accipitriformes order, a taxon that does include hawks, eagles, vultures, kites.  Ospreys share a number of attributes with hawks and eagles, such as acute eyesight, robust talons, hooked beaks, and reversed sexual dimorphism (females are larger than males).  But their piscatory diet has led to the evolution of a constellation of characteristics distinguishing them from all other birds of prey.  They are singularly constructed to hunt and capture fish.  Their fish-hunting armory includes long, nearly featherless legs well structured to reach into the water for their prey, particularly sharp, deeply curved talons (one of which on each foot can rotate so the bird can hold a struggling fish with two talons on each side), and longer and narrower wings bent at the wrist facilitating hovering and rising from the water after a dive.  (Poole, p. 11-12)  Pictured below is an osprey watching over a marsh near my cottage (this bird is not likely one associated with my nest).

The “M” silhouette of the wings of an osprey in flight and its high speed dive into the water in pursuit of a fish are among the most prominent and visible hallmarks of the bird, along, of course, with their piercing calls and massive nests.  Seen below are two ospreys in flight, quite possibly the pair nesting near me.  Their distinctive wing profile is clear.  Given the tattered wings, the bird in the lower right is a female because only they molt during the nesting season.  Males require a full complement of feathers for their constant hunting.

Nearly all of the osprey populations breeding in the northern hemisphere migrate in the fall to over-wintering areas in the southern hemisphere.  Adults reverse the journey in the spring, many returning to the same mates and same nesting area of the prior spring and summer.  Indeed, they may return to the same nests where they raised broods previously.   Their migrations are staggering feats of physical endurance and navigation, covering thousands of miles, many of those miles over open ocean waters or deserts.  Ospreys migrating from the Northern Hemisphere travel some 2,500 to 3,500 miles on average over the course of 4 to 5 weeks.  Poole offers a detailed look at the migration patterns of these birds.  (Poole, p. 106 et seq.)

When the young in the nest near me fledge they will undertake the same migration as their parents in the fall, but will remain in over-wintering areas for an additional year.  When they migrate back to the north, they will often return to the areas in which they fledged 18 months earlier.  Their early lives are full of perils, particularly those associated with these migrations.  Fewer than half of the birds that fledge will survive to breed.  (Carpenteri, p. 75)  

Ospreys are singularly adaptable, cosmopolitan birds, found worldwide (on all continents, save Antarctica).  They are clearly very tolerant of humans and human activity.  The various descriptions of where ospreys build their nest testifies to that.  In her lyrical description of nature along the Delaware side of the Delaware Bay (Birds by the Shore), science writer Jennifer Ackerman devotes a chapter to ospreys and recounts:

I have the good fortune to live within a three-mile radius of five active osprey nests.  One sits atop a platform on the double cross-arms of an old utility pole in the marsh at the center of town, hard by a railroad and King’s Highway.  The highway carries the crush of traffic disgorged from the Cape May-Lewes ferry, a steady stream of tourists hell-bent for a seaward peep.  (Ackerman, p. 39)

The nest down the road from me on Long Island also abuts clamorous human activity.  Pictured below is the nest in its full context, situated right next to a railroad bridge over a road, and near a street level railroad crossing (not shown).  All of this means the nest’s occupants endure the dinging railroad crossing bell, the screeching train wheels, and piercing whistle blows as a Long Island Rail Road commuter train makes eight daily trips east and west.

The conditioning of these young birds to this environment certainly expands the range of nesting places they will accept when they are adults.  With the ongoing destruction of the environment near the water along much of the east coast of North America, and with it the reduction in available natural osprey nesting sites, the adaptability of ospreys has stood them in good stead.  They take readily to artificial nesting sites, to wit, the crossbar at the top of a utility pole.  A broadscale effort to construct safe, artificial nesting sites is underway in many areas, and Poole reports that perhaps as many as 60% of osprey now nest on artificial sites.  (Poole, p. 43)

With their dramatic hunting dives, their calls, their open and exposed nests, ospreys cannot be ignored.  They predate the appearance of our species.  The extant osprey species has been around for perhaps fewer than two million years, while two paleontologically accepted, now extinct species of Pandion were present much earlier:  P. homalopteron from the mid-Miocene (some 16 to 13 millions ago) and P. lovensis from the late-Miocene (about 9 million years ago).  (Warter; Becker; Florida Museum of Natural History)

I think it significant that two accomplished nature writers have been moved to suggest that there is an ancestral memory connecting us to ospreys.  Jennifer Ackerman muses that, from the first appearance of humans on water’s edge, ospreys were there, and so we may inherently associate the bird with the seashore.

Perhaps the osprey exists on a mental map of an earlier world passed down from our ancestors, and the birds in its landscape enters us like the parental.  Perhaps it is also the other way around:  Perhaps it contains us as part of its element, having seen us through the ages, through our infancy and whole tumult of civilized man.  (Ackerman, 51-52)

In a strikingly similar vein, Alan Poole has pondered the “parallel lives of humans and Ospreys over the course of our evolution.”  (Poole, p. 15)  At each stage of our movement across the world, as we explored seashores and pursued fish, ospreys were probably part of our lives.  He concludes,

And it’s a good bet that in these early human societies Ospreys were part of nighttime conversations around campfires, woven into myth and culture, much as Ospreys enter the lives and conversations of people who live around them today.  (p. 15)

Yes, when we encounter these raptors, there may well be a natural psychological affinity.  Walt Whitman experienced this, I think, when, one day in June, 1878, he sat on a river bank, watching a bird roosting on dead tree on the opposite shore.  It was, he wrote,

a haughty, white-bodied dark-wing’d hawk – I suppose a hawk from his bill and general look – only he had a clear, loud, quite musical, sort of bell-like call, which he repeated again and again at intervals.

Clearly, this was an osprey, perhaps marking Whitman’s presence with his calls.  The bird then flew repeatedly over the water and left Whitman with an indelible memory: 

Once he came quite close over my head; I saw plainly his hook’d bill and hard restless eyes.  (Whitman, p. 111-112)

Sources

Jennifer Ackerman, Birds by the Shore:  Observing the Natural Life of the Atlantic Coast (previously published as Notes from the Shore), 1995 and 2019.

Jonathan J. Becker, Pandion Lovensis, A New Species of Osprey from the Late Miocene of Florida, Proceedings of the Biological Society of Washington, Volume 98, 1985.

Stephen D. Carpenteri, The Fish Hawk Osprey, 1997.

Dan Diamond, Charles Wurster, Scientist Who Battled to Ban Pesticide DDT, Dies at 92, The Washington Post, July 25, 2023.

Florida Museum of Natural History, Pandion lovensis, Zachary Seth Randall, original author.

Alan F. Poole, Ospreys:  The Revival of a Global Predator, 2019.

David Allen Sibley, The Sibley Field Guide to Birds of Eastern North America, 2003

Stuart L. Warter, A New Osprey from the Miocene of California (Falconiformes:  Pandionidae), Collected Papers in Avian Paleontology Honoring the 90th Birthday of Alexander Wetmore, Smithsonian Contributions to Paleobiology, Number 27, 1976.

Walt Whitman, Specimen Days, Dover Edition published in 1995 reproducing the 1883 publication.