Black-throated diver

The black-throated diver (Gavia arctica) is less commonly seen in the UK than its red-throated cousin, but is commoner in mainland Europe. It breeds in northwest Scotland and winters around much of the British coast. As might be gathered from its Latin name, its summer breeding grounds extend far to the north, including Greenland, although the November migration extends as far south as the Mediterranean.


Appearance

The black-throated diver is larger than the red-throated diver, and bulkier, but smaller than the great northern diver. It measures around 55-70 centimetres in length (up to 27 inches). It has a more slender bill and thinner neck than the great northern.

The summer plumage of the black-throated diver is similar to that of the great northern, with clearly-defined black and white markings on the back and lighter underneath. However, the head is much lighter in colour, except for the distinctive black throat and black and white stripes up the sides of the neck. The black throat is topped by a short “barcode”. Males and females have similar plumage.

In winter, the three species of diver are much harder to tell apart in terms of plumage. They all have brown heads and backs, although the amount of brown on the heads and necks varies between them, with the black-throated diver having brown plumage to below the eyeline and on the neck, to the half-way point from back to front. The throat and chest are white.


Behaviour and feeding

The black-throated diver is silent in flight but has a loud, rhythmic wailing song.

The black-throated diver, as might be expected, is at home on the water, with its food consisting mainly of fish, for which it dives from the surface. It will take other food, including crustaceans, molluscs and worms, and sometimes frogs. As with many water-based birds, evolution has pushed their legs far back on their bodies, which is fine for swimming and diving but not so good when the bird is on land, where walking is a struggle.


Breeding

Black-throated divers nest on small islands in fresh-water lochs in Scotland, the nest comprising nothing more than a shallow depression in the grass. They are easily disturbed, so, given their slow progress on land, the nests are often perched next to the water so that they can escape quickly if danger threatens.

The clutch, laid in April or May, may be one, two or three eggs, which the parent birds take turns to incubate for around 30 days. When they hatch, the chicks join their parents on the water and the family group stays together for another two months. Larger groups of divers will later make their way to the coast, even before the young are able to fly.

The black-throated diver has amber status in terms of conservation. This is due in part to its limited breeding environment, its slow rate of reproduction (only one small brood per year) and its susceptibility to marine pollution.

© John Welford