The osprey, a charismatic piscivorous raptor, has returned to the top of the food web in the Caledonian Forest thanks to the efforts of conservationists
Special thanks to Gordon McRuvie for sponsoring the production of this Species Profile.
The osprey is the most-widely distributed raptor, or bird of prey, in the world, occurring on every continent except Antarctica at various stages in its migratory life cycle. It is generally found in coastal areas or near bodies of freshwater, and four different subspecies are recognised. The Eurasian subspecies (Pandion haliaetus haliaetus) breeds in Europe, North Africa and Asia, from Scotland across Scandinavia and Russia to Japan, and overwinters in Africa, India and Southeast Asia. The American subspecies (P. h. carolinensis) breeds in North America, from Alaska across Canada to Newfoundland, around the Great Lakes and along both coasts of the USA. Overwintering takes place mainly in Central and South America, as far south as Chile and Argentina. The other two subspecies breed in the Caribbean (P. h. ridgwayi) and Australia, New Guinea and nearby islands (P. h. cristatus) respectively, but they do not migrate.
In medieval times, the osprey was widely-distributed throughout the UK, but suffered extensive persecution and then in the 19th centuries it was exterminated in England and was greatly reduced in numbers and range in Scotland. The last recorded pair nested at Loch Loyne in 1916, after which the species was presumed absent until naturally-recolonising birds nested successfully at Loch Garten in Strathspey in 1954. The population has increased steadily since then as a result of the work of conservationists and currently stands at about 190 breeding pairs.
Ospreys can be seen in most parts of the UK during their spring and autumn migrations, but the core breeding population is in the Scottish Highlands, in areas with Scots pine forests, rivers and freshwater lochs. At some sites, such as Glen Affric, their return has been encouraged by the construction of artificial nests. Dispersing birds have begun breeding in the Borders and in Dumfries and Galloway, while others have spread to the Lake District in England. In recent years ospreys have been reintroduced to the East Midlands in England, and in 2004 birds dispersing from there bred successfully in Wales.
Because of its vast range worldwide, the osprey is not considered to be at risk at a global level, although its numbers have been reduced as a result of the effects of pesticides such as DDT, habitat loss and persecution. In the UK the osprey was red-listed as a species of high conservation concern for many years, but has recently been downgraded to the amber list, of medium conservation concern, because of the expansion of the breeding population.
The osprey is the only member of its family, the Pandionidae, and is a large bird of prey, 55-60 cm. in length and with a wingspan of 145-170 cm. Females are usually 5-10% larger than males, and weigh on average 1.6 kg as against 1.4 kg. for males. Both sexes have similar colouration, with the plumage being dark brown on the back, tail and upper wings. The head and body are white, with a dark eye stripe, a speckled crown and mottled light brown feathers on the upper breast. The flight feathers are greyish-brown and barred on the underside, and the primary feathers have long dark brown tips to them. The tail is similarly barred, and is darker brown at the tips. Other features include eyes with bright yellow irises, a black, sharply-curved beak and pale blue-grey feet. Vocalisations consist mainly of whistled notes that vary in pitch and intensity, depending on whether they are display calls or alarm calls etc.
The osprey has a number of specialised morphological adaptations for its fish-eating lifestyle, including nasal valves which close when it dives underwater. It has relatively long legs for a raptor and equal-lengthed toes, the outer of which is reversible, so that slippery fish can be grasped tightly with two toes in front and two behind. The talons are long and deeply-curved, and the undersides of the feet are covered in spiny bumps called spicules that help to hold the prey.
The diet consists almost entirely of fish, and the osprey is unusual amongst raptors in being so exclusively piscivorous. Both freshwater and marine fish are taken, and in the Highlands the main prey includes trout (Salmo trutta), pike (Esox lucius) and flounder (Platichthys flesus). Hunting usually begins with a hovering flight over water, or occasionally from a perch overlooking water. When a fish is targeted, the osprey dives down with its wings swept back and just before impact it brings its talons forward to plunge feet-first into the water and grasp the fish under the surface.
Success rates in catching fish vary from about 20% to over 50% of attempts, depending on the ability of the individual osprey, and a bird can sometimes be completely submerged as it takes a fish. Flapping its wings strongly, the osprey rises out of the water and once airborne it carries the fish with one foot in front of the other, so that the head is facing forward - this is presumed to be a more aerodynamic position, which makes it easier for the osprey to fly with its prey. The fish is taken to a perch, often near the nest, where it is generally eaten headfirst.
Breeding begins when ospreys are three to five years old, and in Scotland, birds return at the end of March or early April. Courtship consists of aerial displays by the male bird near a nest site, which serve to attract potential mates and discourage rivals. Nests are known as eyries and are constructed out of sticks in the top of a tall tree, most often a Scots pine (Pinus sylvestris). Elsewhere in their range ospreys occasionally nest on cliffs. Ospreys pair for life and nests are used year after year, with large ones being up to a metre in width and height.
Mating takes place at the nest, and in late April or early May the female lays her clutch of eggs - usually three, but sometimes two or four. The eggs are a pale colour with brown blotches, and are 62 x 46 mm. in size. Incubation is carried out by both birds, but only the male hunts at this time, sharing the fish with the female when she is sitting on the eggs. The chicks hatch after 35 days and the female broods them continuously, with the male bringing fish for her and the young. Unlike some other raptors, aggression amongst the chicks is rare and usually they all survive. As the chicks grow, their down is replaced by feathers, and they fledge at 53 days of age. For about two weeks they return to the nest to be fed by the male, while they strengthen their wings and learn to fish for themselves.
Migration southwards begins in late August, and the ospreys' route from Scotland takes them over Spain and Portugal, through North Africa, to the West African countries of Senegal and The Gambia. Arriving in October, they spend the winters in the coastal areas and mangrove forests, where fish are plentiful. Ospreys have occasionally been recorded as living for 25 years, but for most individuals the average life expectancy is about 5 years.
As a raptor, the osprey helps to regulate the numbers of its prey species. This is more significant in rivers and small bodies of freshwater than it is in coastal or marine waters, because of the more limited populations in the former. Ospreys are an unwilling source of fish for some other birds, such as crows (Corvus spp.) and gulls (Larus spp.), that will mob and harry a bird in flight, sometimes forcing it to drop the fish it is carrying. Other birds, such as Canada geese (Branta canadensis), have been known to usurp osprey nests by arriving earlier and occupying them, with the ospreys unable to evict the squatters, despite repeated dive-bombing. Several species of small cavity-nesting birds, including starlings (Sturnus vulgaris) and sparrows (Passer domesticus) build nests within osprey nests.
In Scotland, ospreys do not have any predators apart from humans, but in North America bald eagles (Haliaeetus leucocephalus) and great horned owls (Bubo virginianus) take chicks from the nest, and raccoons (Procyon lotor) and snakes are suspected of preying on eggs and nestlings. In common with other raptors, ospreys are host to many different parasites such as flukes, that are consumed in their prey, and to scavenging mites, such as feather mites (Bonnetella fusca), that feed on feather fragments, skin debris etc.
It has been suggested that in the past ospreys were dependent, in some parts of the boreal forest zone, on beavers (Castor spp.) for the creation of good habitat. The shallow ponds created by beaver dams would provide good fishing opportunities and trees that drowned in the flooded areas would provide good nesting sites.
Alan Watson Featherstone
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