In Scotland, an example of successful re-introduction
Most people have heard of the golden eagle, and would probably regard it as the king of Britain’s birds of prey, but less well-known is the even larger white-tailed eagle (Haliaeetus albicilla). With a length of up to 90 centimetres (35 inches) and a wingspan of 240 centimetres (94 inches) it is even larger than the bald eagle of North America. Female birds are larger than males.
The white-tailed eagle, which is also known as the sea eagle, was once fairly common in Scotland, Ireland and some parts of England, but became extinct due to persecution during the 18th and 19th centuries. The last record of breeding was on the Isle of Skye in 1916.
However, the white-tailed eagle has now been re-introduced to some remote parts of northern Scotland. The programme began in 1975, with birds being brought from Scandinavia. The first successful breeding was recorded in 1985, and further introductions in the 1990s have ensured that there is now a self-sustaining population.
In Scotland, it is noticeable that white-tailed eagles and golden eagles tend to occupy different areas. This is partly because golden eagles tend to nest on rocky ledges, such as on coastal cliffs and inland precipices, whereas white-tailed eagles are usually tree nesters. Although the two species of eagle have similar food sources, the white-tailed eagle can survive on less food than the golden eagle, which is compensation for the fact that golden eagles are better fliers and will usually win a direct competition over a carcass.
The future of the white-tailed eagle would seem to be assured in Scotland. The birds are long-lived, with 21 years being an average lifespan, so a pair of birds has many chances to raise broods. Needless to say, the white-tailed eagle is fully protected, not only by law but also by the remoteness of its habitat from centres of human population.
As noted above, the white-tailed eagle is particularly large. It has a large powerful bill which enables it to feed from large carrion, such as deer. The plumage is brown apart from a lighter coloured head (although not as light as the bald eagle) and the short white tail that is evident in flight.
The flight can look clumsy, but the huge wingspan allows white-tailed eagles to hang on the wind for long periods of time.
White-tailed eagles feed mainly on fish, smaller birds and carrion. They usually fish by taking their prey from close to the surface in their talons, but can also dive to seize fish from further down. They perform a useful function by feeding on the carcasses of dead deer and sheep. They will also take rabbits and hares, or steal food from other birds.
White-tailed eagles mate for life, unless one partner dies and the survivor will then find a new mate. The rarely seen courtship ritual consists of the birds locking their talons in mid-air and cartwheeling down until they nearly hit the ground or water, before parting and flying up again.
The preferred nest sites in Scotland are tall trees in remote wooded areas, such as on islands in freshwater lochs. They therefore do not compete in this respect with golden eagles, which prefer rocky cliffs for nesting. The nest comprises a structure of sticks and small branches, which is used year after year, often being added to so that it can reach an enormous size, of up to six feet in diameter and four feet in height. However, the nesting hollow itself is relatively small and lined with softer material.
The clutch is usually of two eggs, although it might only be one or sometimes three. If the food supply is good two chicks will be raised, but otherwise a weaker chick may be allowed to die and serve as food for the other. When two chicks survive they are usually very tolerant of each other. When weather conditions are poor at the wrong time of year chick mortality can be high, with pneumonia being a particular problem among the Scottish population of white-tailed eagles.
Incubation of eggs is performed by both parents, with hatching taking place after up to 45 days. The chicks are fed at the nest, firstly by the male parent and later by both adults, for up to 75 days, at which point they are ready to fledge. The young birds will stay in the vicinity of the nest for some time longer, learning the skills of flying and hunting for food by watching their parents.