Australasian Osprey

Appearance

"Pandion cristatus", comparable in size as a medium-sized raptor or large hawk, occurs near the coast and large water bodies and is highly adapted to hunt and capture marine animals. The plumage is dark brown on the upper surfaces, and pale at the head and lower parts. The throat is white, a black line extends from this through the eye to a dark patch around the ear, demarcating this from the pale colour at the head. When not in flight, the feathers at the crown form a small crest. The breast is dappled with brownish patches, lacing below the neck, this is darker and more strongly banded in the female. The female of the species also tends to be larger. The wing measurements of the females range from 410 to 478 millimetres, males are 391 to 470 mm. The weight of the female is 1.2 to 1.6 kilograms, males are 0.9 to 1.2 kg. The size range of the black bill is 29 to 35 millimetres, and the upper mandible is acutely hooked. The legs and feet of the species are strong and a light grey or white colour. The pattern of the scales at the tarsus is reticulate. The irides are yellow. The juveniles resemble adults, although the iris colour is a darker orange-yellow and plumage is distinguishable in multiple and subtle ways.

In flight, the eastern osprey has bowed wings with narrow primary feathers that are angled and separate, giving a fingered appearance to the observer below. The eyes are set toward the front of the head, which along with the neck is proportionally small. The species swivels and cranes its neck while observing its surroundings in a curious and characteristic manner. The osprey resembles the white-bellied sea eagle "Haliaeetus leucogaster", which has similar habitat and range, although the adult size is only that of the larger species' juvenile; the wings of an osprey are sharply angled rather than the up-swept outline of the soaring eagle.

Ospreys differ in several respects from other diurnal birds of prey, toes are of equal length, its tarsi are reticulate, and its talons are rounded, rather than grooved. The eastern and western osprey and owls are the only hunters whose outer toe is reversible, allowing them to grasp their prey with two toes in front and two behind. This is particularly helpful when they grasp slippery fish.

Behavior

The species is solitary or paired with a breeding partner, the association of several individuals in family groups are unusual records. They are a diurnal species, ranging out from a nest or perch over water bodies during daylight, but have been recorded hunting at night. Their flight may be high, soaring over and surveying the water, or quartering closer to the surface; some flight is seemingly unrelated to hunting.

Reproduction

Rocky outcrops just offshore are used in Rottnest Island off the coast of Western Australia, where there are 14 or so similar nesting sites of which five to seven are used in any one year. Many are renovated each season, and some have been used for 70 years. The nest is a large heap of sticks, driftwood, turf or seaweed that is usually built in the fork of a dead tree or limb, cliff faces are also utilised. trees, rocky outcrops, utility poles, artificial platforms or offshore islets. Continually occupied nest structures may reach up to two metres in height. Nests can be as wide as 2 meters and weighing about 135 kg.

Generally, eastern ospreys reach sexual maturity and begin breeding around the age of three to four years.

Eastern ospreys usually mate for life, although polyandry has been recorded in several instances. The breeding period varies according to local seasons: this begins between September and October in southern Australia, April to July in northern Australia and June–August in southern Queensland. In spring the pair begins a five-month period of partnership to raise their young. The clutch size is usually two to three eggs, sometimes up to four, and are able to brood twice in a season. These are laid within a month, and relies on the size of the nest to conserve heat. The egg shell is white or buff with bold splotches and spots of reddish-brown, sometimes so dark as to be black; purple or grey blotches may appear beneath the surface of the shell. The egg measurements are about 62 x 45 millimetres and weigh about 65 grams. The eggs are incubated for about 35–43 days to hatching.

The newly hatched chicks weigh 50 to 60 grams and fledge in 8 to 10 weeks. A study on Kangaroo Island had an average time between hatching and fledging of 69 days. The same study found an average of 0.66 young fledged per year per occupied territory, and 0.92 young fledged per year per active nest. Some 22% of surviving young either remained on the island or returned at maturity to join the breeding population. When food is scarce, the first chicks to hatch are most likely to survive. The typical lifespan is 7–10 years, though rarely individuals can grow to as old as 20–25 years.

A nest examined in 1902 found fish skeletons at the edge and a seaside plant 'pig-face' in full growth.

Food

The diet is largely local species of fish, although a favoured target in Australia is known to be mullet when available. Occasional records are given for other marine life—sea snakes, molluscs and crustaceans—and for terrestrial species of reptiles, insects, birds and mammals. They are known to capture sea birds in flight.

Ospreys have vision that is well adapted to detecting underwater objects from the air. Prey is first sighted when the eastern osprey is 10 to 40 metres above the water, after which the bird hovers momentarily then plunges feet first with wings raised; a large splash is made as it enters the water. The descent toward their prey may be done in stages, and they may immerse themselves up to a depth of 1 metre. After seizing their target they use heavy wing beats to rise from the water's surface, resuming a regular action with the fish being carried head first toward the shore. The prey is "slung torpedo-fashion" with a leading foot behind the head and the other clasping behind, this aligning habit distinguishes "Pandion" from indifferent clutching of prey by fishing eagles. Their large prey is not swallowed immediately, instead being butchered at a perch or nest site outside the breeding season.

The genus "Pandion" has several adaptations that suit its piscivorous lifestyle, these include reversible outer toes, sharp spicules on the underside of the toes, closable nostrils to keep out water during dives, and backwards-facing scales on the talons which act as barbs to help hold its catch.
The osprey has dense plumage which is oily and prevents its feathers from getting waterlogged.