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Introduction
Material for this section was developed from the following:
The Osprey is 52-60cm long with a 152-167cm wingspan. It has white underparts and long, narrow wings with four "finger" feathers at the end of each, which give it a very distinctive appearance. The Osprey call is a series of sharp, whistles, cheep, cheep, or yewk, yewk.
Established Osprey pairs tend to return to their old nest sites, while new pairs, or pairs which have lost their nests, must seek out new sites – a process which can take several weeks and involve several abandoned nests. Unfortunately, due to the fairly large sticks and nest sizes, few natural sites will hold a nest. Ospreys prefer nests with the following characterisitics: near food and therefore near water (a maximum of 3 to 5 km away), an open area suitable for landing (Ospreys tend to manoeuvre poorly in tight quarters), protected from predators (this is typically accomplished by being surrounded by water), and near other Ospreys (because this typically indicates to the Osprey that an area is suitable). The Ospreys will usually build large, stable nests of sticks and other material. During nest building, pairs can make over a hundred trips a day for nest material. Prior to laying eggs, the nest may be lined with flatter and softer material to support the eggs, such as grass and seaweed. Unfortunately, besides seeking cardboard, they may also retrieve plastic bags, old fishing line and other material, which can smother or ensnare the young at birth. When ready to breed, the male osprey will make a sharp eeeeet-eeeeet-eeeeeet sound. The male will take to a slow, high and undulating flight, sometimes after a successful hunt with a fish in its talons, or nesting material, or nothing at all. This display will also sometimes be made due to nearby perceived threats: predators or humans. Once settled in a nest site, females will make what are known as begging calls to nearby males if they are hungry or unpaired. Once paired, the male will feed its mate and guard her prior to egg laying by following her in flight, sometimes even swooping low over her back and touching her with talons. Once paired, Ospreys rarely “divorce” unless they fail to breed or their nest is destroyed. Ospreys also tend to pair with birds of their own age. After breeding, the female Osprey will lay its eggs, typically 10-30 days after arriving in breeding territory. The eggs are about the same size and shape as those of large domestic hens. The eggs laid first are the most likely to succeed, as later eggs are given less energy and attention. Chicks from late hatching eggs can be much lighter than their older nestmates. You can tell that an Osprey has laid its eggs when it sits in the centre of the nest and rocks back and forth prior to settling down. Hatching begins when the Osprey chicks break through the egg shell with their bill. They will fully hatch within one or two days of the first hatching activity. The parents will feed the helpless chicks, which are initially kept warm by their thick down plumage. The plumage fades away about 10-12 days later as the chicks enter a “reptilian stage” with black scaly skin. Feathers slowly fill in the gaps and replace the down, resulting in a feathered and relatively active bird by the time the Ospreys are 30 days old. A dominant chick will, if hungry, dominate over the remaining chicks, taking the available food and forcing the remaining chicks to starve to death or be knocked out of the nest. Alternatively, if old enough, an unwelcome chick may be able to move to another nest and dominate the chicks in that location. The parents will not interfere with this process. Before fledging (taking flight, which usually occurs about 50 to 60 days from hatching), the young Ospreys will begin exercising their wings and developing the muscles required to help them during their first tentative flights. Eventually they will begin to hop, and continue doing so until one day their hop lifts them over the edge of the nest and into the air. Young Ospreys must then undertake a critical period of building strength and competence required for hunting and migration. Early hunts are typically unsuccessful and the parents provide much needed fish. Once independent and feeding themselves, the Osprey young will drift away from their birth nests and begin life as an independent young Osprey. Ospreys primarily catch and eat live fish only. There have been rare circumstances where Ospreys have been seen eating mollusks, snakes, aquatic mammals, voles, squirrels or even other birds, however live fish make up 99% of the Osprey diet. Ospreys are opportunists, concentrating on catching the fish that is most available. They typically like to eat the same fish throughout the breeding season. They will usually concentrate on fish weighing 150 to 300 grams and range in length from 25 to 35 centimeters, although larger and smaller fish are sometimes taken as well. A single osprey can only eat about 300 grams at one feeding.
Ospreys dive feet first. They are buoyant and, despite long legs, they can penetrate only about a meter below the water surface. This means that the birds catch only surface fish or those in shallow flats and shorelines. Most Ospreys hunt while flying, rather than observing from elevated locations, despite the fact that it burns 10 times the energy. The Osprey tends to hunt from five to 40 meters above the water. When the Osprey sees a fish, it will dive by tucking back its wings and falling steeply while maneuvering with its wing and tail to keep on track towards its target. After gripping a fish, it may rest briefly on the water before taking off with fast and high wing strokes. During flight, the Osprey will tend to rearrange the prey by placing one foot in front of the other so the fish’s head points forward and cuts wind resistance. The efficiency of the Osprey dives for prey tends to range from 30% to 90%. Ospreys in Georgian Bay migrate long distances, as opposed to subtropical Ospreys, which only move locally after breeding. Most information on migration has been gathered from the banding of birds. In late summer, adult Ospreys in northern regions will head for tropical areas, followed a few weeks later by Osprey chicks, which migrate alone. Ospreys are often seen heading south through Florida, indicating that they use it as their shortcut across the Gulf of Mexico. Groups of up to 800 Ospreys have been seen flying south along the eastern coast of the US. Overwater crossings are difficult due to a lack of updrafts. Typically, hawks and eagles will circumvent water on their migrations, but not Ospreys. Ospreys have been seen crossing the Mediterranean into Africa, and crossing the Gulf of Mexico to South America (without taking a break in the Caribbean). Ospreys do not have significant fat reserves for energy, so their migrations must usually be interrupted with occasional feeding. Despite this fact, Ospreys have been known to cross the 2000 km Sahara desert in a continuous flight of 40 to 66 hours. Migration from Georgian Bay begins in the last two weeks of August. Typically, non-breeders will migrate several weeks earlier, while the remaining Ospreys wait for their fledglings to become sufficiently strong for the journey. Ospreys do not enjoy migration. Usually the process is driven by a lack of fish, which begin to avoid the shallows and go deeper in the lake during the colder seasons. The young Ospreys typically have to learn this lesson for the first years of migration. Eventually, this natural process starts to trigger an internal clock in the adult Osprey, so that it can leave before the fish become scarce. Ospreys will start wintering in late November with negligible movement from their wintering site. An Osprey will usually return to the same site or area for wintering year after year. Most sites are located near the equator, although some Ospreys are known to have travelled as far as Chile for wintering. Groups of Ospreys will typically live close together during the winter. In spring, the migration north is two to three times as fast due to the pressure to arrive and breed. Early nesting Ospreys will have the most surviving young. Two-year olds do not breed and therefore tend to leave slightly later than the adult Ospreys. One-year olds often do not even migrate. Before the presence of industrial humankind, Ospreys were widespread and abundant, well adapted to their natural predators. However, the presence of guns, chain saws, toxic chemicals, synthetic fish nets and other modern products have brought potential for greater ecological harm. All-in-all, Ospreys have survived recent ecosystem changes fairly well. There are few successful natural predators to the adult Osprey. Perhaps the most successful predators are owls, which have the power and talons capable of subduing an Osprey. Osprey eggs and chicks are obviously more vulnerable and racoons have been known to prey on Osprey nests quite frequently. However, Ospreys tend to be quite adept at recognizing nest sites which are at threat from climbing predators. Island nesting and shoreline nests therefore tend to be the most popular. From the shooting and trapping perspective, besides predation from hunters, Ospreys are sometimes pursued by fisherman or fish farmers, especially in developing countries. North Americans have not hunted the Osprey to the same degree as European hunters prior to World War II. In many regions of Osprey habitation around the world, forests
have been heavily developed for timber and often tall trees and
forests bordering lakes have been cut, destroying existing and potential
nest sites, while also causing silt to cloud waters where Ospreys
fish. In more developed locations, excess sewage and fertilizers
can cause nutrient overloading and result in loss of vegetation
and fish deaths. Also, beavers tended to provide a good number of
nest sites by flooding river areas creating shallow fishing areas
with isolated nesting areas. After the fur trappers reduced beaver
populations, these sites became considerably more difficult to find.
Since Ospreys have faced population decimation in a number of areas, care has been taken to manage the issues facing the Ospreys and they have responded quite well. Ospreys need management to preserve their numbers and ensure that they avoid improper nest sites, such as those on utility poles. By building artificial nest sites for Ospreys, people are able to disperse Ospreys to new regions of great promise. The re-establishment of wild populations of Ospreys leads to improved distribution, density and breeding success of the species. Building nest sites is important due to the scarcity of stable, predator-free natural sites which accommodate the Osprey’s bulky nests. The decline in beaver populations and the shallow water reservoirs they create has resulted in a drop in Osprey nesting sites over recent decades. Today, hydrologists have taken over where the beaver left off by creating artificial reservoirs and wildlife floodings. People have helped contribute to the Osprey nesting issue by building nesting platforms in live trees or on poles near the water. These quality sites help to keep Ospreys at established locations and to attract new pairs of Ospreys. Nest platforms for Osprey population management have a long history and go back to New England when farmers would put wagon wheels near barnyards so that Ospreys would nest and keep “chicken hawks” away. In North America, many people use discarded utility poles as platform supports. Overall, platforms have accomplished three key management objectives:
Building platforms also allows people to possibly attract birds to new sites away from problem nest sites such as utility poles, channel markers and buoys. To successfully populate an area, Ospreys also need accessible food sources. This is a more difficult element to control and will most likely depend on the proximity of nest sites to shallow waters containing fish populations.
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