Although immature ospreys may return to the UK, pair up and even build trial nests when they are two years old, they normally breed for the first time in their third to fifth year.
Ospreys are believed to be largely monogamous, and strongly faithful both to nest and mate.
The nest, called an eyrie, is generally built on the top of a large tree, usually a conifer, but deciduous trees are also used. In parts of their range, ospreys may nest on cliff ledges, coastal rocks, buoys and electricity pylons. Man-made structures are used more regularly in North America than in Europe, although a small number of the Scottish ospreys nest on electricity pylons.
These long-lived birds are mainly site faithful and some nests have been in use for some 20 years, with the birds adding to it each year. The nest is a large structure made of branches and twigs, lined with small twigs, moss, bark and grass. It takes both birds 14-21 days to complete a new nest, which at completion can be 120-150 cm across and 50-60 cm deep. As more material is added in later years, the nest can grow to a depth of 150-200 cm.
If ospreys fail to breed successfully, they often start to build a new nest known as a ‘frustration eyrie’, which they may use for nesting the following year. Ospreys use specially made nesting platforms readily, and many of these sites are in regular use in Scotland.
In the second half of April, the female lays two or three eggs at 1-3 day intervals and incubates them for 37 days per egg. Even though chicks hatch a few days apart, aggression and dominance by the older chick is rare. This asynchronic hatching is typical for most birds of prey.
If food is short, at least the oldest chick will get enough and survive. Nest failures are most commonly caused by adverse weather conditions, food shortage, inexperience of birds nesting for the first time, and occasionally by egg collectors robbing the nest.
Like most other birds of prey, ospreys divide the nesting duties clearly between the pair. The female does most of the incubating, brooding and direct feeding of the young. She guards them throughout the nestling period, and will share the hunting at later stages when the chicks are larger. The male, on the other hand, is the major provider of fish for the female and young.
After fledging at c. 53 days, both parents provide food for the young, which stay close to the nest for a further two months. Many juvenile birds die before they reach maturity at three years old. Those that reach breeding age can expect to live on average about eight years. The oldest known wild osprey was 32 years old.