Black-Throated Diver of the UK and Ireland

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The Black-Throated Diver (gavia arctica) is an elegant, medium-sized diving bird that may be seen along the coasts of Britian and Ireland in the winter. Small number of these divers breed in Scotland but do not breed in Ireland. It is also known as the Black-Throated Loon and the Arctic Loon in North America.

The Black-Throated Diver is distinguished by its white flank, a slightly rounded head, a thin, straight bill and narrow body. It is the least common of the three species of divers that inhabit Britain and Ireland. Adults measure 60 to 70 cm (or 23.6 to 27.6 inches) long and weigh between 2 and 3 kilograms (or 4.4 and 6.6 pounds). The wingspan is 1.1 to 1.3 meters (or 3.6 to 4.3 feet).

Males and females are alike. In the summer, the head and the hindneck is pale grey with a black patch on the foreneck bordered on each side by black and white stripes. The mantle is chequered with black and white. The plumage is duller in the winter with a lighter crown and a grey-brown hindneck. The upperparts are black. There is a white patch on the flank which is distinctive. The slender, straight bill is grey or whitish with a black tip and is dagger-shaped. Immature birds resemble the adults in winter but have a brown hindneck and whitish scaling on the upperparts.

The Black-Throated Diver belongs to the Gaviidae family of birds that includes its cousins the Red-Throated Diver and the Great Northern Diver. It is larger than the Red-Throated Diver and smaller than the Great Northern Diver. There is often confusion between Red-Throated Diver and Black-Throated Diver in the winter.

Adults start breeding when they are 2 or 3 years of age. They are long-lived birds but their average lifespan is generally unknown. The oldest recorded bird was 28 years of age.

Black-Throated Divers, who are solitary birds, inhabit salt-water areas in winter that include sheltered coasts, shallow bays, large estuaries, large coastal lakes and rarely inland (reservoirs). Inshore waters along the coast are preferred and are not often found offshore. They fly from the coast to large, deep freshwater lakes and lochs with small islands in the breeding season. These divers are not often found near salt-water environments in the breeding season. Small numbers breed in north and north-west Scotland. Young birds remain in coastal areas until they are able to breed.

Their diet is mainly fish (sandeels, herring, sprats, minnows, flatfish) and crustacesans although amphibians and sometimes insects will also be eaten. Fish are hunted by diving from the surface of the water. They forage for food either singly or in small groups who swim close together and low in the water.

It is a streamlined and skilful diver with waterproof plumage. The large, webbed feet are set far back from its body which allows it to propel itself from the surface and under the water. The wings are sometimes used to swim beneath the sea etc. They are able to dive to depths of 3 to 6 meters (9.8 to 19.7 feet) and remain underwater for 2 minutes although 50 seconds is average. It usually reappears quite far away from where it dived.

Black-Throated Divers are rare, resident birds that may be seen along the coast of Britain in winter, particularly the north-west of Scotland and Cornwall. It is absent from the Orkney and the Shetland Islands and rarely seen inland in winter. It is a scarce winter visitor in both Northern and the Republic of Ireland. It is thought some resident birds fly from Scotland to winter in Northern Ireland. Wintering birds from Scandinavia can be seen between October and April. Small numbers of Black-Throated Divers may be seen flying eastwards through the English Channel in April and May. Sandy bays and inlets along the coast are preferred by wintering birds.

The flight of these birds is strong and direct and can fly long distances. The wings are short in comparison to their bodies and must run over the surface of the water to gain enough speed to become airborne.

Black-Throated Divers only breed in north-west Scotland and do not breed in England, Wales or Ireland. Breeding pairs are monogamous and usually mate for life. They stay together in the winter and return to their breeding grounds in April or May. Males and females make long, wailing and yodelling calls in the spring and summer but are silent in winter. A territory is established where nesting and foraging takes place.

The breeding habitat is restricted to large, deep, freshwater lochs with small islands and little or no disturbance. The nests is built are on islands because it is safe from predators. Specially designed rafts floating in lochs are also used. Foraging takes place in shallow water, usually near the nest. Chicks are fed mainly fish (Wiley online Library).

Nests are built near the waterline because Black-Throated Divers cannot walk very well on land. The nest is a mound of vegetation. Rafts were placed on lochs in Scotland by the RSPB and used by nesting pairs. It was found that nests on rafts produced a high survival rate for chicks (Taylor and Francis). The reason for the increase is because nests made near shores are more susceptible to flooding and changing water levels than nests built on rafts.

One or two eggs are laid in May or June which are olive-brown in colour spotted with black. Eggs hatch after 29 days and chicks are fledged after 27 to 32 days. Chicks can swim almost immediately after hatching. Both parents look after the young who sleep on their parent backs. The young remain on the surface of the water while their parents dive for food.

Adults and young birds leave their breeding grounds for the coast between August and September. A post-breeding moult occurs when these birds are flightless. Moulting occurs on the coast (sheltered bays etc) in Scotland.

The global population is not considered Vulnerable by conservationist groups due to its extremely wide range (Eurasia, Arctic, North America). The population in Europe is described as declining (Birdlife International)

The Black-Throated Diver is classified as of Least Concern by IUCN.

The breeding population has recently increased in Scotland. "Since the introduction of the raft programme in the late 1980s, the Black-throated diver breeding population has rebounded from a low of 180 pairs to 240 pairs in 2012" (Wildlife Extra). The Black-Throated Diver is an amber listed species of bird in both Britain and Ireland, where it is only a winter visitor.

The Black-Throated Diver was included on the list of birds that are "the most threatened and requiring conservation action under the UK Biodiversity Action Plan (UK BAP)" in 2012 (Defra).

The European population of Black-Throated Divers was reported to have halved between 1970 and 2007 ( Telegraph).

Anglers in boats and tourism could affect the breeding population as they are very susceptible to disturbance when nesting. Egg collectors and an increase in natural predators (Hooded Crows, foxes) could also cause declines ( Wiley online Library).

Reasons given for the decline are a susceptibility to human disturbance when nesting, drowning in fishing nets, disturbance by shipping vessels (causing a fragmentation and loss of suitable feeding and resting habitats (Helcom - page 3) illegal egg collecting, wind farms, pollution from oil and lower fish stocks because of commercial fishing.