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Eastern Ospreys

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.

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