Sunday, March 14, 2021

All about osprey: Widespread, prehistoric, fish eaters

OspreyThe osprey is second only to the peregrine falcon, as the most widely distributed bird of prey in the world, found on every continent but Antarctica, while picking up regional names like fish hawk, fish eagle and seahawk. There are probably half a million osprey globally, and osprey are one of the clearest indications of the health of any shallow fresh, brackish or saltwater habitat.

Like eagles, osprey are generally monogamous and tend to use the same nests year after year, so a successful osprey family indicates lots of fish, and since osprey are not as rigidly territorial as some other predators, the more osprey nests a habitat supports, the more likely the general health of the ecosystem is good. 

Osprey, like loons, require open water to make a living. Adirondack osprey migrate up to 3,500 miles, to Florida, Central America and South America, often covering 150 miles per day, returning to their nests in the spring, as lake and pond ice melts. Mortality occurs most frequently during migration, as osprey encounter hazards ranging from starvation and rough weather to oil spills or electrocution from live wires. The male arrives around mid to late March, about a week before the female, and he starts gathering sticks and softer material, such as aquatic plants, for the nest. Upon her arrival, the female gets to work preparing the nest for the two to four eggs she will lay in April over a few days. 

Nests can be on rocky outcrops, the tops of utility poles or on artificial platforms constructed by people to that end. Electric poles are convenient but carry electrocution and fire dangers for osprey and potential outages for consumers, so utilities often insulate live wires and construct elevated platforms for osprey nests, to minimize the threat of fire. Some nests have been in use through several generations for many decades, for example the collapsed top of a white pine which leans precariously over the West Branch of the Ausable River, at the Adirondack Wildlife Refuge, which has been used every year for the twenty years we have been here.

While the last surviving human being, homo sapiens, emerged only 200,000 years ago, fossil remains of osprey go back about 13 million years. Osprey are piscivorous, meaning 98% of what they eat are fish, and natural selection and evolution has turned their body structure into an amalgamation of specialized features to make fish catching and eating the way of life for the osprey, to such an extent that the osprey has its own taxonomic genus, Pandion and family, Pandionidae.

ospreyOsprey are large raptors, comparable in size to the larger falcons and buteo hawks, about two feet from bill to tip of tail, with a wingspan averaging fifty to seventy inches. Osprey are thinner than these raptors, with lower wing loading, meaning they usually hunt while in flight, and as with other raptors, females are slightly larger than males. Osprey grab fish which average five to eleven ounces, and nine to fourteen inches in length, but they are powerful raptors, sometimes picking up and flying away with fish close to their own weight. I have seen our local male carrying two fish at one time! Osprey can live up to thirty years in the wild, but seven to ten years is more typical.

As the ice melts and aquatic insects hatch, their activities draw fish towards the surface. Osprey vision is adapted to spotting fish in shallow water from 30 to 130 feet above the water, from which they can plunge from a hover, diving up to 3 feet deep, with their talons extended. As an osprey dives it can adjust the angle of its plunge to account for the distorting light refraction of the fish. The nostrils close to prevent the osprey from inhaling water. 

Osprey are successful in about one out of every four dives, may hunt up to 12 miles away from their nest, while the average hunt lasts about twelve minutes. Our wild ospreys seem to do most of their hunting within view of the nest, and one of the joys of Spring is their constant soothing calls, almost like a whistle, nicely disguising the deadly intent of their activities. I have watched the male startle fly fisherman, standing in their waders, by grabbing fish less than 20 feet from the fisherman.

With owls, osprey share a reversible outer toe on each foot, which allows them to grab a fish with two toes on each flank. The toes are all the same size, featuring sharp spicules for controlling a struggling fish. While most raptors have grooved talons, the osprey has rounded meat hook talons which have backward facing scales, making struggle counterproductive for the fish. Grabbed fish are often repositioned by the osprey, so that they face in the osprey’s direction of flight, improving aerodynamics.

osprey chicksAt this early stage in spring, while Mom is incubating the eggs, which hatch sequentially in 35 to 45 days, the male osprey grabs, for example, a trout or northern pike, flies to a feeding branch, and proceeds to eat the head of the fish. It then proceeds to deliver the body of the fish to Mom. Dad will relieve Mom on the nest, while Mom takes her fish to a nearby feeding branch. As the chicks grow, Mom will help out with the fishing, with both osprey delivering up to six pounds of fish daily to the nest. Osprey work hard, providing food for their growing brood, while defending the nest from crows, ravens, great horned owls and climbing racoons. Kleptoparasitic eagles may harass an osprey, or even another eagle, in the hopes the original predator will drop and abandon the fish they are carrying.

Osprey look almost comically messy after eating, so they go through an elaborate cleansing routine, using their talons to clean the bill, and preen their feathers, while oiling the feathers to make them shed water easier after a dive, using lubricant from their uropygial glands. This may be followed by flying low over the water, dragging and rinsing bill and feet.

Young osprey fledge in about ten weeks, and their ability to provide food for themselves, greatly affects the timing of the autumn migration, which is typically much more spread out than the Spring arrival. Mom osprey is the first to leave the nest, living on her own for weeks, feeding herself and recovering some of the weight she lost while raising her brood, preparing for her solitary migration. The fledges become more dependent on Dad, as they learn how to become  adult ospreys, supporting themselves.

There is no evidence that first year ospreys migrate with either of their parents, and it appears that they are on their own during and after that first migration, staying south for a full year, and we don’t yet understand how they know where to migrate to, or how to get back up north. Are they following the sun, the stars, the coastline, perhaps detecting earth’s magnetic fields? At this point there is probably more data than solid theory. Osprey reach sexual maturity in their third or fourth year, but availability of nesting sites and potential mates may affect the timing.

osprey in flightDDT was first used during World War 2 to control mosquitoes, and was introduced into agriculture after the war to control insect pests, at a time where anything which helped food production was rushed into use, without a thorough examination of its effects on the overall food chain. With the publication of Rachel Carson’s “Silent Spring” in 1962, and her warning about the unchecked use of pesticides on song birds, it was discovered that DDT leached into the soil and water of the ecosystem, being ingested by every critter in the food chain, whose diet, or the diet of animals they consumed included insects, and in a process called biomagnification, the higher in the food chain the consumer was, the more devastating the impact of DDT.

Bald eagle and osprey populations, already under pressure by hunting and habitat destruction, crashed, as birds experienced the thinning of egg shells through an alteration in their calcium metabolism, which would cause eggs to collapse under the weight of the parent. The Endangered Species Acts of 1969 to 1972 protected habitat, as well as individual animals, finally resulting in a ban on the use of DDT, and a more careful approach to the use of chemicals which have effects far beyond their intended target.

Photos from top: An osprey in rehab, eating, by the late Deb MacKenzie; osprey nest by Anne Fraser; chicks are public domain by USGS; and osprey in flight by Anne Fraser. All courtesy of Adirondack Wildlife Refuge.

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Steve and Wendy Hall run the Adirondack Wildlife Refuge and Rehab Center in Wilmington. They've been rehabbing and releasing wild animals for over 45 years, specialize in predators, keep wolves as the cornerstone of their educational program, and have lived in the Adirondacks for the past 20 years. The Adirondack Wildlife Refuge became a non-profit about 10 years ago.Visit to learn more.


9 Responses

  1. Scott MacMillin says:

    A fascinating article that answers a lot of my questions about our pair of Osprey in Lake Pleasant. This pair if very visible to anyone driving along Rt 8 where they perch on the pole platform over the Lake Sacandaga outlet.

  2. Boreas says:

    Great article!

  3. Steve B. says:

    Good article. These birds have seen a resurgence on the south shore of Long Island in the 20 years I’ve lived here. It’s pretty much a certainty to spot them and many of these birds populate this shore area. I’m a 1/4 mile from a tidal inlet and have seen them soaring over my house, which I had never seen a decade ago. Pretty cool !.

    • AG says:

      Yes I’ve seen them on Long Island too. Testament to the cleaning of waters in the NYC area – I’ve seen them in even The Bronx and Queens

  4. Vanessa says:

    Thanks Steve this is great. Birds of prey remind me so much of dinosaurs. That first pic is fantastic! Doesn’t he or she look the picture of an ancient creature? ❤️

  5. As recently as 40 years ago there were active programs to boost the osprey populations on the Chesapeake Bay. Today The Chesapeake Bay supports the largest breeding population of osprey in the world comprising an estimated 20,000 of the worldwide breeding population of 100,000. It’s essentially impossible to canoe or kayak anywhere on the Bay or its tributaries without seeing a number of them. Many shoreline residents have installed platforms for them to nest on. Bald eagles have also become increasingly common

  6. Jackie Woodcock says:

    I learn something new everytime I read one of your articles! Thank you for sharing all the amazing things you know about wildlife.

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