Monotypic Species - The Osprey

The osprey has physical characteristics and behavioural patterns that set it apart from other raptors. Because of this, some authorities have assigned it its own genus (Pandion) and family (Pandionicae). It is thus a monotypic species like the giraffe, moose and African painted dog, all of which are the only representatives of their genera.

Some of the problems that have beset taxonomists is that the osprey differs from other diurnal birds of prey by having rounded rather than grooved talons, and toes of equal length. Only ospreys and owls (in the raptor family) have a reversible outer toe which allows them to grasp with two toes pointing forward and two back.

The genus name comes from the mythical Greek king, Pandion, grandfather of Theseus. In one of the Greek legends, Theseus is transformed into an eagle.

Osprey with FishCredit: Wikimedia

The osprey is also known as the sea hark, fish hawk or fish eagle. As these common names suggest, this large bird of prey has a diet of fish, although it is not a sea eagle.

There are four subspecies of the osprey which are generally recognised but there are very few differences between any of them. Pandion haliaetus ridgwayi and P.h.cristatus frequent the Caribbean islands and Australia/Tasmania respectively and are non-migratory. The former commemorates Robert Ridgway, the American ornithologist while the latter is the smallest of the subspecies.

The Eurasian subspecies is the nominate species, P.h.haliaetus and the North American species is P.h.carolinensis. This is a larger, darker-bodied bird.

Range
The osprey is found in all temperate and tropical regions except Antarctica.

Habitat
The osprey is found anywhere where a body of water will provide the food it needs. It does not breed in South America but is a migrant there and it is absent from Antarctica.

Description
The osprey may be over 60 cm in length and have a 180 cm wingspan. Weights vary from an average of 1.7 kg for P.h.carolinensis to 1.25 kg for P.h.cristatus.

Osprey NestCredit: Wikimedia

The upper surfaces are a deep glossy brown and the underparts white. The breast may be streaked with brown. The white head has a dark mask across the eyes which have golden to brown irises. The nictating membrane is pale blue.

There is a blue cere above the black beak. The feet are white with black talons which are strongly curved. The tail is short but the wings are long and narrow with four long, finger-like feathers and a shorter fifth feather.

Males have a slimmer body, narrower wings, a less distinct band across the breast (if there is one at all) and paler underwing coverts. The differences are quite easy to pick between a breeding pair but sexing a single bird is more difficult.

When in flight, it has a gull-like appearance because of the arched wings and drooping ‘hands’.

Behaviour
Osprey often recycle their nests each year, doing a bit of renovation before laying their eggs. Where there are high densities of ospreys and insufficient nest sites the osprey will delay breeding. Some localities provide artificial platforms for the osprey and these are utilised by the birds. One organisation in New Jersey, Citizens United to Protect the Maurice River and Its Tributaries Inc has designed a platform and offers the plans and materials list to anyone who would like to supply ospreys with a breeding platform.

The website of the group is http://www.cumauriceriver.org/pages/osprey.html and there is a link to download the plans.

Migration
European ospreys winter in Africa. Those from America and Canada either stay in southern USA (California and Florida) or migrate to South America. They cover between 260 to 280 km per day and will sometimes travel at night, especially over water.

Nutrition
Ninety-nine percent of the diet is fish. Almost any type of freshwater fish is taken with a length range of 25 to 35 cm and weight range of 50 to 2000 grams (average 150 to 300 grams). The osprey usually forages by day, soaring and circling over a body of water. They often hover momentarily before diving down feet first and snatching up their prey. The feet have reversible outer toes and backward-facing scales on the talons. These act as barbs to hold the prey.

OspreyCredit: Wikimedia

Their vision is adapted to spotting fish in the water. They plunge in feet first and may submerge to a depth of a metre. They have also been seen snatching birds in flight. Ospreys may also eat rodents, small reptiles, amphibians, rabbits and other birds.

Reproduction
The osprey makes a large nest from a heap of sticks, driftwood and seaweed. Mating is usually for life. Two to four whitish eggs with bold splotches are laid within a month and incubated for about five weeks. The eggs measure 6.2 x 4.5 cm and weigh about 65 grams.

Newly hatched chicks weigh only 50 to 60 grams but fledge in 8 to 10 weeks. If food is scarce, the first chick to hatch is the most likely to survive. The typical lifespan is 7 to 10 years.

Threats
The main threats prior to the mid 20th century were egg collectors, hunting and birds of prey. During the 1950 and 60s there was a significant drop in populations in many areas. This was believed to be due in part to reduced reproductive levels due to the effects of insecticides such as DDT. The pesticide interfered with calcium metabolism causing thin-shelled (thus easily broken) and/or infertile eggs. Once DDT was banned and persecution was reduced, the numbers of osprey improved.

The major threats to the osprey in North America are bubo owls and bald eagles. The latter also practise kleptoparasitism, stealing the osprey’s catch for itself.

Conservation
The osprey has a large global population and is listed by the IUCN as ‘of least concern’. However the osprey has been absent for decades from some former territories in South Australia. Other nesting sites in South Australia (Eyre Peninsula and Kangaroo Island) are at risk due to unmanaged coastal recreational activities and encroaching development.

Ospreys are held in at least nine zoos and institutions around the world.

The osprey is the provincial bird of Sodermanland, Sweden and Nova Scotia, Canada.