Cape drake

Red-throated Diver

The Red-throated Diver (Gavia stellata), known in North America as the Red-throated Loon, is the smallest and most widely distributed member of the loon or diver family.

Taxonomy and etymology

First described by Danish naturalist Erik Pontoppidan in 1763, the Red-throated Diver is a monotypic species, with no distinctive subspecies despite its large Holarctic range. Its relationship to the four other diver species is complex; though all belong to the genus Gavia, it differs more than any of the others in terms of morphology, behaviour, ecology and breeding biology.

The genus name Gavia comes from the Latin for "sea mew", as used by ancient Roman naturalist Pliny the Elder. The specific epithet stellata is Latin for "set with stars" or "starry", and refers to the bird's speckled back in its non-breeding plumage.

Description

The Red-throated Diver is the smallest and lightest of the world's diver species, ranging from 55–67 cm (24–27 in) in length with a 91–110 cm (35.8–43.3 in) wingspan, and averaging in mass. Like all divers, it is long-bodied and short-necked, with its legs set far back on its body. Compact and slender, it is the only diver with an all-dark back in breeding plumage. The sexes are similar, though males tend to be slightly larger and heavier than females. In breeding plumage, the adult has a grey head and neck, red throat patch, white underparts and dark mantle. The non-breeding plumage is drabber with the chin, foreneck and much of the face white, and considerable white speckling on the mantle. Its bill is thin, straight and sharp, and the bird often holds it at an uptilted angle. Though the colour of the bill changes from black in summer to pale grey in winter, the timing of the colour change does not necessarily correspond to that of the bird's overall plumage change. As an adaptation for diving its nostrils are narrow and elongated. Its iris is reddish.

In flight, the Red-throated Diver has a distinctive profile; its small feet do not project far past the end of its body, its head and neck droop below the horizontal (giving the flying bird a distinctly hunchbacked shape) and its thin wings are angled back. It has a quicker, deeper wingbeat than do other divers.

Voice

The Red-throated Diver has a drawn-out wailing cry, which has been transcribed as aarOOoa, aarOOoa.

Habitat and distribution

The Red-throated Diver breeds in northern Eurasia and North America (generally north of 50 north latitude), and winters at sea, along inshore waters, and on large lakes as far south as the Mediterranean. It breeds on small freshwater ponds in tundra regions, but feeds largely in the sea even when breeding.

Behaviour

Because its feet are located so far back on its body—attaching very close to the posterior of the bird—the Red-throated Diver is not capable of walking on land; however, it can use its feet to shove itself forward on its breast. It is able to take off directly from land, the only species of diver that can.

The Red-throated Diver is a diurnal migrant, traveling singly or in loose groups, often high above the water. In eastern North America (and possibly elsewhere), it tends to migrate near the coast rather than farther offshore. It is a strong flier, and has been clocked at speeds between 75 and 78 kilometres per hour (47–49 mph). Like all members of its family, the Red-throated Diver goes through a simultaneous wing moult, losing all its flight feathers at once and becoming flightless for a period of 3–4 weeks. However, unlike other divers—which undergo this moult in late winter—the Red-throated Diver loses its ability to fly sometime between early August and November.

Food and feeding

Like all members of its family, the Red-throated Diver is primarily a fish-eater, though it sometimes feeds on molluscs, crustaceans, frogs, aquatic invertebrates, insects, fish spawn or even plant material. It seizes rather than spears its prey, which is generally captured underwater. Though it normally dives and swims using only its feet for propulsion, it may use its wings as well if it needs to turn or accelerate quickly. Pursuit dives range between 2–9 m (6.5–30 ft) in depth, with an average underwater time of about a minute. The fish diet of the Red-throated Diver has led to several of its folknames, including "sprat borer" and "spratoon".

Breeding

The Red-throated Diver is a monogamous species which forms long-term pair bonds. Both sexes build the nest, which is a shallow scrape (or occasionally a platform of mud and vegetation) lined with vegetation and sometimes a few feathers, and placed within a half-metre (18 in) of the edge of a small pond. The female lays 1–3 eggs (two is the norm) which are incubated for 24–29 days, primarily by the female. The eggs, which are greenish or olive-brownish spotted with black, measure 75 x 46 millimetres (3.0 X 1.8 in) and have a mass of , of which 8 percent is shell. Incubation is begun as soon as the first egg is laid, so they hatch asynchronously. The young birds are precocial upon hatching: downy and mobile with open eyes; both parents feed them (small aquatic invertebrates initially, then small fish) for 38–48 days. Parents will perform distraction displays to lure predators away from the nest and young. Authorities disagree as to whether adults carry young on their backs while swimming with some maintaining that they do and others the opposite.

Conservation status and threats

Though the Red-throated Diver is not a globally threatened species, as it has a large global population and a significant global range, there are populations which appear to be declining. Numbers counted in U.S. Fish and Wildlife Service surveys in Alaska show a 53 percent population decline between 1971 and 1993, for example, and counts have dropped in continental Europe as well. In Scotland, on the other hand, the population has increased by some 16 percent over the past 12 years, according to surveys done by the Royal Society for the Protection of Birds and Scottish Natural Heritage.

The Red-throated Diver is one of the species to which the Agreement on the Conservation of African-Eurasian Migratory Waterbirds (AEWA) applies; in the New World, it is protected by the Migratory Bird Treaty Act of 1918. Oil spills, habitat degradation, and fishing nets are among the main threats this species faces. In addition, high levels of mercury in the environment have led to reproductive failures in some areas, including parts of Sweden.

In human culture

As recently as the 1800s, the Red-throated Diver was thought to be a foreteller of storms; according to the conventional wisdom of the time, birds flying inland or giving short cries predicted good weather, while those flying out to sea or giving long, wailing cries predicted rain. In the Orkney and Shetland islands of Scotland, the species is still known as the "rain goose" in deference to its supposed weather-predicting capabilities.

Many other folk names exist as well, among them cape drake, cape race, little loon, pegging-owl loon, pepper-shinned loon, scape-grace, and sprat loon.