Ospreys are large birds of prey (55 to 58 cm long), with a wingspan from 145 to 170 cm. Their long wings have are bent at the carpal ("wrist") joints. They are bright white underneath, with dark brown patches at the carpal joints and a speckled dark brown necklace. They have a dark stripe through each eye, and a dark brown back. Ospreys have light blue-gray feet, yellow eyes and a black beak. Juvenile ospreys look a lot like adults, but their dark feathers have light colored tips that make them look speckled on their backs and wings. They also have orange eyes. Ospreys begin to look like adults when they are about 18 months old. (Poole, 1989; Poole, 1994; Snyder and Snyder, 1991)
Female ospreys are usually bigger than males. They have a bigger wingspan (5 to 10% longer than males) and are heavier. In North America, male ospreys weigh 1200 to 1600 g and females weigh 1600 to 2000 g. Female also usually have darker feathers than males. (Poole, 1994)
Ospreys in different regions of the world look a little different from one another. For example, ospreys that live in tropical regions are smaller than ospreys that breed farther north or south from the equator. There are four subspecies of ospreys. Each of these lives in a different region of the world and looks a little different. (Poole, 1994)
Ospreys have several adaptations for hunting fish. They have long legs for reaching into the water and dense, oily plumage that keeps them from getting waterlogged. They also have special valves that keep water out of their nostrils when they dive for fish. Osprey feet are specially adapted for holding on to slippery fish. They have spiny footpads called spicules, long, sharp claws, and a toe that can be turned backward to keep fish from escaping.. (Poole, 1994; Snyder and Snyder, 1991)
Ospreys have a worldwide distribution. They live on every continent except Antarctica. Ospreys do not breed in South America or Indo-Malasia, but they sometimes live there in the winter. Large populations of ospreys are found in Scandinavia and the Chesapeake Bay region of the United States. There are four subspecies of ospreys, which are found in different regions of the world. (Poole, 1989; Poole, 1994; Poole, et al., 2002; Steidl, 1991)
Ospreys have a wide distribution because they are able to live almost anywhere that has safe nest sites and shallow water with lots of fish. Nests are usually found within 3 to 5 km of a water body such as a salt marsh, mangrove (Rhizophora) swamp, cypress (Taxodium) swamp, lake, bog, reservoir or river. (Poole, 1989; Poole, 1994)
Ospreys need structures that can support their big nests, and that are safe from climbing predators, like raccoons. In order to be safe from predators, ospreys usually build their nests in places that are difficult for predators to reach, like on the side of a cliff, or over water or on a small island. Over-water nests are built on structures like buoys, channel markers, dead trees and special platforms that people build for ospreys. Ospreys also nest on other man-made structures that are very high, like power poles, radio and TV towers, buildings and even billboards. (Ewins, 1996; Henny and Kaiser, 1996; Poole, 1989; Poole, 1994; Poole, et al., 2002)
Some ospreys live in the same place all year. These ospreys are called “residents” or “sedentary.” Resident ospreys usually start breeding between December and March. Other ospreys migrate between their breeding grounds and wintering grounds. These ospreys breed in areas where it is too cold to spend the winter. Migratory ospreys breed later during the year than residents. They start breeding in April or May. (Poole, 1989)
In migratory osprey populations, the male usually arrives at the nest site a few days before the female. When the male has chosen a nest site, he often performs a flashy aerial flight display near the nest site. Males probably perform this display to attract females and to scare off other males. The male and female build the nest together, using sticks for the outside and softer materials like seaweed, kelp, grasses or cardboard for the lining. They often also include trash that they find laying around, including fishing line, plastic bags and soda cans and bottles. Osprey pairs use the same nest year after year, so returning pairs do not need to build a nest. However, each spring they must spend some time repairing their nest and adding materials before they can lay eggs. (Poole, 1989; Poole, 1994)
Once a pair has built their nest, the male begins to deliver fish to the female. The male brings food to the female so that she will mate with him. The male continues to hunt all of the fish for the female and the chicks until the chicks have fledged. Once the chicks start flying, the female begins hunting too. (Poole, 1989; Poole, 1994)
Ospreys are normally monogamous. One male mates with one female, and mating pairs stay together until one of them dies. However, males occasionally have more than one mate. This is called polygyny. When this happens, it is usually in areas where nests are very close together, so that the male can defend and bring food to both nests. (Poole, 1994)
Ospreys are able to breed when they are three years old. However, in areas where there are not enough nest sites, ospreys may not breed until they are five years old. Once they have begun breeding, ospreys breed once per year. (Poole, 1989; Poole, 1994)
Non-migratory ospreys breed in the winter and spring and lay their eggs between December and March. Migratory ospreys breed in the spring and summer and lay their eggs in April and May. The female lays one egg every day or every other day until she has laid two to four eggs. The male and female both incubate the eggs, which hatch after about 40 days. Because the eggs are laid one or two days apart, they also hatch one or two days apart. Chicks that hatch first are bigger than the other chicks and are usually dominant over the smaller chicks. If the parents cannot provide enough food for all of the chicks in the nest, the smallest chicks do not get enough food and sometimes die. This is called brood reduction. (Ehrlich, et al., 1988; Poole, 1989; Poole, 1994; Snyder and Snyder, 1991)
When osprey chicks hatch, they are covered in white down with brown streaks on the face, back, and wings. When the chicks are 10 days old, the white down is replaced by charcoal-colored down. Chicks begin to grow feathers when they are two weeks old. Osprey chicks begin to fly when they are 48 to 76 days old. Once they can fly, osprey chicks begin to hunt for themselves, though they usually also take food from their parents until they can catch enough fish to feed themselves. (Poole, 1989; Poole, 1994)
Male and female ospreys both care for their young. They feed the chicks and protect them from predators and cool, wet weather. The male provides all of the fish for the chicks before they can fly. This means that males have to catch up 10 fish each day for the female and the chicks. The parents tear the fish into small pieces for the chicks to eat. Osprey parents hunt fish for their chicks until the chicks can hunt enough fish to feed themselves. Chicks can usually feed themselves two to eight weeks after they begin flying. (Poole, 1989; Poole, 1994)
During the first few weeks after hatching, osprey chicks cannot control their body temperature very well. The female parent broods the chicks frequently for the first two weeks and during very hot or cool weather until they are about four weeks old. Both parents spend a lot of time protecting the nest from other ospreys and potential predators. (Poole, 1989; Poole, 1994; Poole, et al., 2002)
Ospreys are a relatively long-lived bird species. The oldest known osprey in North America was a 25-year old male. The oldest known female was 23 years old. However, very few individuals live to this age. Chance of survival from one year to the next varies between populations, but is estimated to be approximately 60% for young ospreys (less than 2 years old) and 80 to 90% for adult ospreys. (Poole, et al., 2002)
Ospreys can be migratory or sedentary (non-migratory). Non-migratory populations breed and winter in the same location. Migratory osprey populations generally breed north of the non-migratory populations and winter south of them. (Fernandez and Fernandez, 1977; Poole, 1989; Poole, 1994)
Ospreys can build their nests close together in colonies or far from other pairs. Most raptors don’t live in colonies because they defend a hunting territory around their nest. Ospreys have to hunt over such a large area to find enough fish that they can’t defend a hunting territory. Sometimes, they even hunt together because they can catch more fish that way. Instead of defending a territory, ospreys just defend their nest from other ospreys and from predators. (Poole, 1989; Poole, 1994; Poole, et al., 2002)
Breeding ospreys are known to travel as far as 14 km from their nest during hunting forays. Non-breeding individuals are known to travel as far as 10 km between their daytime feeding grounds and their roosts. (Poole, et al., 2002)
Ospreys use up to five different calls to communicate with each other. They also use visual displays, like special flight displays or specific body positions to communicate. Often calls and visual displays are used together. Calls are used to for begging, alarm, courtship, and nest defense. Alarm calls are often given when a potential predator or disturbance such as a boat or human approaches the nest. Ospreys giving alarm calls usually stand very erect on the nest, or dive at the disturbance. (Poole, 1989; Poole, 1994; Poole, et al., 2002)
Ospreys are unusual among raptors for being piscivores. Their diet consists almost entirely of fish (≥99% of prey items). They are opportunistic, and will eat whatever fish species they can catch – either in shallow waters, or near the surface of deeper waters. North American ospreys are known to eat more than 80 different species of fish. (Poole, 1989; Poole, et al., 2002)
Ospreys hunt for fish while flapping and gliding 10 to 40 meters above the water. When an osprey spots a fish, it hovers briefly, then dives toward the water. Just before hitting the water, the osprey swings its legs forward and bends its wings back, plunging feet-first into the water. The osprey uses strong wing beats to lift itself and the fish out of the water. Once airborne again, the osprey rearranges the fish in its feet, carrying it with one foot in front of the other so that the fish is facing forward. This position probably makes the fish easier to fly with. The osprey then takes the fish to a perch to eat. (Poole, 1989; Poole, 1994; Poole, et al., 2002)
Ospreys generally eat fish starting at the head and working toward the tail. A male who is also hunting food for a mate and chicks during the breeding season will usually eat at least part of the fish before delivering the rest to the female. Ospreys do not usually need to drink water. They get enough water from the fish that they eat. (Poole, 1989; Poole, 1994; Poole, et al., 2002)
Ospreys successfully catch fish on 24 to 74% of their dives. Ospreys are usually more successful at mid-tide and when the weather is calm. (Poole, et al., 2002)
Though ospreys eat mostly fish, they have occasionally been seen eating other things, including birds, snakes, voles, squirrels, muskrats (Ondatra zibethicus), salamanders, conchs, and even a small alligator (Alligator mississippiensis). Reports of ospreys feeding on carrion are rare. However, they have been observed eating dead white-tailed deer (Odocoileus virginianus) and opossum (Didelphis virginiana). (Poole, et al., 2002)
Ospreys are vulnerable to predation from aerial predators, such as owls and eagles . In North America, Bald eagles and great horned owls are known predators of osprey nestlings and (occasionally) adults. The speckled appearance of osprey chicks camouflages them in the nest and may be an adaptation to minimize predation by diurnal avian predators like the bald eagle. (Poole, 1989; Poole, et al., 2002)
Raccoons, snakes and other climbing animals are probably predators of osprey eggs and nestlings. This is probably why ospreys in many areas build their nests over water. Crocodiles may also prey on wintering ospreys. Nile crocodiles sometimes kill ospreys bathing and roosting near water in Africa. (Poole, 1989; Poole, et al., 2002)
While ospreys provide food for some species, they are probably not the main food source for any species. Because ospreys prey on fish, they probably have a small effect on local fish populations. Like most predators, ospreys are host to many different species of parasites, including feather mites. (Poole, et al., 2002)
Ospreys nests are used by many other species of birds. Smaller cavity-nesting species, such as common grackles, tree swallows, barn swallows, European starlings and house sparrows build nests inside osprey nests. Other larger species take over osprey nests for their own use in the spring before the resident ospreys return. In North America, these species include great blue herons, Canada geese, bald eagles, Red-tailed hawks, Great horned owls, herring gulls and common ravens. (Poole, et al., 2002)
Ospreys in some areas, especially northern forested regions, may have historically depended on beavers to create habitat for them. Beavers create osprey habitat by building dams, which create shallow ponds for fishing and dead trees for building nests. (Poole, et al., 2002)
There are no known negative impacts of ospreys on humans. In the past, some fishermen have believed that ospreys competed with them for fish. However, studies have shown that ospreys take a much smaller amount of fish than the fishermen, and do not compete with fishermen for fish. (Poole, 1989)
Ospreys help attract ecotourism to areas. They may also be a valuable indicator species for monitoring the health of large rivers, bays and estuaries. If water is polluted, the pollutants will be found in the fish that the ospreys eat. Since ospreys are sensitive to many of these pollutants, the health of the ospreys can tell us about the health of the aquatic ecosystem where they live. (Poole, 1989; Poole, et al., 2002)
Ospreys are not listed under the Endangered Species Act. However, this species is listed as threatened, endangered or a species of special concern in several U.S. states, including Michigan. Ospreys are also protected under the U.S. Migratory Bird Act and CITES Appendix II. They are not listed on the IUCN Red List.
In the late 19th and early 20th centuries, the main threats to osprey populations were egg collectors and shootings. Shooting an egg-collection became less common by about 1950, though ospreys are still shot occasionally. Osprey populations in many areas were accidentally poisoned by the pesticide DDT (dichloro-diphenyl-trichloroethane), and many populations shrunk from the 1950’s through the 1970’s. DDT was banned in the U.S. around 1970, but is still used in some countries where ospreys spend the winter. Populations of ospreys grew again after DDT was banned. Ospreys are now almost as abundant as they have ever been. Ospreys have been helped by people constructing artificial nest structures for them, hacking projects and new habitat that has been created by man-made reservoirs. (LaPierre, 1991; Poole, 1989; Poole, et al., 2002)
Bones belonging to earlier Pandion species from the mid- to late-Miocene (approx. 13 million years ago) were found in California and Florida. These prehistoric osprey species were slightly less robust than modern ospreys, but otherwise very similar. (Poole, et al., 2002)
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