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Natural History
General Description:
The osprey, sometimes known as the fish hawk, is a large raptor that
typically weighs between 1200-1900 grams (2.64 - 4.18 lb.). As with most
raptors, the female is generally 1/3 larger than the male. When observed
in flight, it appears dark brown from above and white from below. Light
brown breast banding, present in both sexes, is usually more prominent in
females and can resemble the appearance of a "necklace." The beak is black,
in contrast to the white head which is masked by dark brown, from the blue
colored cere, around the light yellow eye and down the nape. (Eye color
in juveniles ranges from red to orange.) The pale blue feet are covered
with strong scales. The tail is brown and is crossed by darker brown bands.
The wings are long, and the wingspan averages between 3 and 4 feet. When
perched, the wing tips extend beyond the tail.
In the hand, three unique traits can be observed. 1) An osprey's nostril is long and slit-like and can be closed during underwater dives for fish. 2) Sharp spicules on the underside of the toes aid in grasping slippery prey. 3) The talons are nearly circular in cross section, as compared to most other raptors, which have a flat edge on the backside. The foot structure exhibits a zygodactic condition in which the fourth (outer) toe can face either anteriorly or posteriorly. Thus, there can be two toes forward and two backward or three toes forward and one backward. This is thought to be an adaptation for holding on to a wiggling fish. When alarmed, the hackles (the feathers on the back of the head) will be raised, and it will make a "kip, kip" alarm call.
Diet and Hunting Strategies:
Ospreys' diet is comprised almost entirely of fish. Studies suggest that nearly 98% is fish, while the rest is comprised of frogs, turtles, and small mammals. The types of fish that are hunted depend solely on what is locally available. Osprey will hunt in both fresh and salt water. The size of the prey has been reported to average 25% of the bird's body weight.
The osprey employs several methods of hunting depending on prevailing conditions. An individual may hunt from a perch or the air. It will survey the area, using its visual acuity to scan the water below. Once a particular fish is selected, the bird will dive nearly vertically and plunge into the water. It will grab the fish underwater with its talons and immediately fly off to a nearby perch to eat. Osprey feathers are coated with oil during preening, so the water rolls right off and the bird can take off with ease. If it sees a weak fish floating near the surface, it may hunt by approaching from a long gentle glide, plucking the fish effortlessly from the water. Both osprey and bald eagles commonly use this drier, more graceful strategy. Ospreys' hunting success rate is quite high and has been reported to range from 50% to 80% for adults. Of course, the youngsters are going to have a tougher time and thus a lower success rate.
Geographical Distribution:
Ospreys' distribution is worldwide. They occur on every continent except Antarctica. North American populations are found throughout Canada, the Great Lakes region, the entire east and west coasts, and the Islands of the Caribbean. There are two recognized sub-species in North America. Pandion haliaetus carolinensis is found on the mainland, and Pandion haliaetus ridgwayi is found throughout the Caribbean. Osprey will nest near large bodies of water and are usually seen away from lakes, bays, river systems, and costal areas only during migration. Birds that spend the breeding season in the far north will migrate down into the temperate zone (South Florida, the Gulf Coast, Mexico, and Central and South America) during the winter months. Their flights will extend over long distances of water. For example, a bird in South Florida may fly to Cuba for the winter.
Breeding and Reproduction:
The osprey forms a pair bond that lasts throughout the breeding season. Long term pair bonding is common but is due to strong fidelity for the nest site. Birds that migrate far from the breeding grounds will usually return to the very same nest the following season. If one of the birds does not return, it will readily be replaced in time to produce and raise young that season. Young birds will begin breeding during their second or third year of life, depending on the competition for food and nest sites. It is believed that they will return to next close to where they themselves fledged. A young pair will begin the breeding season by looking for a suitable next site. The three most basic criteria are 1) an abundant supply of fish, 2) water that is clear enough to see prey easily, and 3) a platform to support a nest that is near the fishing site. Large trees (both live and dead), rock ledges, and even man-made structures are used as platforms. Osprey will even nest on utility poles, old duck blinds, buoys, and channel markers. Nests can become quite large if added onto each year. Therefore it is only logical that the nest site be large enough for such a structure. The birds seem to be relatively tolerant of nearby human activity and will build nests along roadsides and areas of heavy boat traffic. Since they do not aggressively defend territories where food is plentiful, it is common to see nests on adjacent channel markers that are less than a mile apart.
Courtship lasts between two and three weeks. During that time the male will bring fish or nesting material to the female, and perform aerial displays and dives. On average, 2-3 eggs are laid at intervals of 1-3 days. Incubation begins after the first egg is laid and lasts usually 35-40 days. Both sexes participate in incubation duties. Asynchronous hatching occurs relative to the order of laying. This delay in hatching results in substantial size differences in the developing nestlings. The young grow very rapidly and are ready to fledge when about 50 days old. They continue to stay near the parents for another 50 days while they learn the hunting and flying skills that they will use for the rest of their lives. The family will go their separate ways just prior to the fall migration. Nesting success is quite high, but the young face bigger problems when they leave the nest. Major causes of post-fledgling mortality are electrocution, emaciation, gunshot wounds, and poisoning.
Taxonomic and Ecological Information:
The osprey is one of the most unique birds of prey in the world. It doesn't neatly match any of the grouping characteristics used to place other raptors into a family classification. Taxonomists used to include the osprey in the family accipitridae. That classification was never a widely popular one, even though the bird was in its own subfamily, pandioninae. Recently, upon the recommendation of the American Ornithologists' Union, taxonomists created the new family pandionidae of which the osprey is now the only member. The reclassification is important because it makes the osprey's evolutionary lineage independent from all other members of the accipitridae family. Figure 1. below illustrates the osprey's position in both family classification schemes.
Figure 1. Classification systems of the order falconiformes. The older system (A) places osprey in the accipitridae family, and the newer one (B) places it in the pandionidae family.
The osprey is indeed a specialist. It dines on fish 98% of the time, its hunting strategy of diving feet first into the water is unique, it has distinct body features that are exclusive to the species, and it has a general lifestyle not seen in any other raptor. This bird is also a good example of an R-selected species. It will put a lot of energy and effort into raising a few young with very high nesting success. Contrast that to a K-selected species, such as the Barn Owl, which raises as many as 8 or 9 young per clutch in the hope that most will survive. K-selected species sacrifice nesting success for high numbers of young to saturate an area to its carrying capacity, whereas R-selected species sacrifice the consequences of post-fledging mortality for high nesting success. The life span of ospreys has not been extensively studied, but a few individuals have been reported to have lived 25 years.
Management Information:
The importance of providing artificial platforms for nest sites cannot be understated. Providing stable nest sites in a safe location can dramatically increase nesting success. Platforms can be erected in areas of known osprey nesting. The birds will gladly accept the structure as their own. If birds choose these sites over power poles, it decreases the likelihood of electrocution and can save power companies money. Nests often become so large that they must be taken down to avoid the risk of fallen power lines.
Another benefit to consider is the number of birds that can be produced
as a result of the platform. If a pair nest successfully, raising on average
three young per year for 15 years, 45 young will be produced. Those young
will then reproduce, and the population derived from the original nest site
will increase exponentially. As the local population grows, competition
for nest sites will also increase. The availability of enough platforms
then becomes more important.
Of course, all the nest sites in the world mean nothing if the surrounding habitat is destroyed or polluted. Large trees along shorelines serve as places to perch. Clean water supports strong populations of fish. Absence of pesticides is critical. The general rule when managing for a predatory species is to focus efforts on managing the habitat first, so that natural populations will be attracted to the area. Osprey, like all raptors, are good indicators of environmental health. If healthy raptor populations exist, that means all environmental conditions on which they depend are healthy also.
Threats and Status:
Human interference is probably the number one threat to osprey populations. Being fish eaters, the osprey was one of the many species that was affected by the use of the pesticide DDT. As with the bald eagle and peregrine falcon, egg shells of the osprey were thin and weak, and reproduction was essentially extinguished. A ban on the use of this chemical in the United States has led to a recent resurgence in osprey populations. Although the threat is gone in the U.S. and Canada, DDT is still used in other developing countries, and residues continue to be present in osprey tissues today.
Other threats to the bird include powerlines; gunshots; other forms of poisoning; polluted lakes, rivers, and streams; and loss of forested habitat along major waterways. Breeding populations in South Florida, the Chesapeake Bay watershed, Canada, Alaska, and Baja California appear stable today. The species has never been formally listed as endangered or threatened by the U.S. Department of the Interior but was designated an "ecologically sensitive" species by the U.S. Forest Service. The bird is protected under the Migratory Bird Treaty Act and the Bald Eagle Act, which provides protection for all birds of prey.
References:
Johnsgard, Paul A. 1990. Hawks, Eagles, and Falcons of North of America.
Smithsonian Institution Press.
Clark, William S. and Wheeler, Brian K.
1987. A Field Guide to Hawks North America. Houghton Mifflin Company, Boston,
MA