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Red-throated Diver
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The Red-throated Diver (Gavia stellata), known in North America as the Red-throated Loon, is a migratory aquatic bird that is found in the temperate regions of the northern hemisphere. It is the smallest and most widely distributed member of the loon or diver family.
Around in length, the Red-throated Diver is a nondescript bird in winter, greyish above fading to white below.
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Encyclopedia
The Red-throated Diver (Gavia stellata), known in North America as the Red-throated Loon, is a migratory aquatic bird that is found in the temperate regions of the northern hemisphere. It is the smallest and most widely distributed member of the loon or diver family.
Around in length, the Red-throated Diver is a nondescript bird in winter, greyish above fading to white below. During the breeding season, it acquires the distinctive reddish throat which gives rise to its common name. Fish form the bulk of the diet, with invertebrates and plants sometimes eaten as well. A monogamous species, the Red-throated Diver forms long-term pair bonds.
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. Pontoppidan initially placed the species in the now-defunct genus Colymbus, which contained grebes as well as divers. By 1788, however, German naturalist Johann Reinhold Forster realized that grebes and divers were different enough to warrant separate genera, and moved the Red-throated Diver (along with all other diver species) to its present genus. Its relationship to the four other divers is complex; though all belong to the same genus, it differs more than any of the others in terms of morphology, behaviour, ecology and breeding biology. It is thought to have evolved in the Palearctic, and then to have expanded into the Nearctic.
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. "Diver" refers to the family's underwater method of hunting for prey, while "red-throated" is a straightforward reference to the bird's most distinctive breeding plumage feature. The word "loon" is thought to have derived from the Swedish lom, the Old Norse or Icelandic lómr, or the Old Dutch loen, all of which mean "lame" or "clumsy", and is a probable reference to the difficulty that all divers have in moving about on land.
Description
The Red-throated Diver is the smallest and lightest of the world's diver species, ranging from in length with a wingspan, and averaging in weight. Like all divers, it is long-bodied and short-necked, with its legs set far back on its body. The sexes are similar, although males tend to be slightly larger and heavier than females. In breeding plumage, the adult has a grey head and neck (with narrow black and white stripes on the back of the neck), a triangular red throat patch, white underparts and a dark mantle. It is the only diver with an all-dark back in breeding plumage. 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. Its nostrils are narrow slits located near the base of the bill. Its iris is reddish.
When it first emerges from its egg, the young Red-throated Diver is covered with fine soft down feathers. Primarily dark brown to dark grey above, it is slightly paler on the sides of its head and neck, as well as on its throat, chest, and flanks, with a pale grey lower breast and belly. Within weeks, this first down is replaced by a second, paler set of down feathers, which are in turn replaced by developing juvenile feathers.
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 adult Red-throated Diver has a number of vocalisations, which are used in different circumstances. In flight, when passing conspecifics or circling its own pond, it gives a series of rapid yet rhythmic goose-like cackles, at roughly five calls per second. Its warning call, if disturbed by humans or onshore predators, is a short croaking bark. A low-pitched moaning call, used primarily as a contact call between mates and between parents and young, but also during copulation, is made with the bill closed. The species also has a short wailing call, which descends slightly in pitch and lasts about a second; due to strong harmonics surrounding the primary pitch, this meowing call is more musical than its other calls. Another call—a harsh, pulsed cooing that rises and falls in pitch, and is typically repeated up to 10 times in a row—is used in territorial encounters and pair-bonding, and by parent birds encouraging their young to move on land between bodies of water. Known as the "long call", it is often given in duet, which is unusual among the divers; the female's contribution is longer and softer than her mate's.
Young have a shrill closed-bill call, which they use in begging and to contact their parents. They also have a long call used in response to (and similar to that of) the long call of adults.
Habitat and distribution
The Red-throated Diver breeds primarily in the Arctic regions of northern Eurasia and North America, and winters in temperate coastal waters. In North America, it winters regularly along both coasts, ranging as far south as the Baja California Peninsula and the Gulf of California in northwestern Mexico; it has been recorded as a vagrant in the interior Mexican state of Hidalgo.
Some of its folknames in northeastern North America—including cape race, cape brace, cape drake and cape racer as well as corruptions such as scapegrace—originated from its abundance around Cape Race, Newfoundland.
Behaviour
Because its feet are located so far back on its body, the Red-throated Diver is not capable of walking on land; however, it can use its feet to shove itself forward on its breast. Young use this method of covering ground when moving from their breeding pools to larger bodies of water, including rivers and the sea. It is the only species of diver able to take off directly from land.
The Red-throated Diver is a diurnal migrant, which travels 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 from 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".
For the first few days after hatching, young Red-throated Divers are fed aquatic insects and small crustaceans by both parents. After 3–4 days, the parents switch to fish small enough for the young birds to swallow whole. By four weeks of age, the young can eat the same food—of the same size—as their parents do. Young birds may be fed for some time after fledging; adults have been seen feeding fish to juveniles at sea and on inland lakes in the United Kingdom, hundreds of kilometers from any breeding areas.
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 two eggs (though clutches of 1–3 have been recorded); they 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. Ornithologists 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 population and a significant 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 increased by some 16 percent between 1994 and 2006, according to surveys done by the Royal Society for the Protection of Birds and Scottish Natural Heritage. In 2002, Wetlands International estimated a global population of 490,000 to 1,500,000 individuals; global population trends haven't been quantified.
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 Americas, 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. On the breeding grounds, Arctic and Red Foxes are major predators of eggs, while Great Skuas, Arctic Skuas and various species of Larus gulls (including Great Black-backed Gulls and Glaucous Gulls) are predators of both eggs and young.
In human culture
Used as a food source since prehistoric times, the Red-throated Diver is still hunted by indigenous peoples in some parts of the world today. Eggs as well as birds are taken, sometimes in significant numbers; during one study on northern Canada's Igloolik Island, 73% of all Red-throated Diver eggs laid within the 10 km2 (3.9 mi2) study site over two breeding seasons were collected by indigenous inhabitants of the island. The species was also central to the creation mythologies of indigenous groups throughout the Holarctic. According to the myth—which varies only slightly between versions, despite the sometimes-vast distances between the groups who believed it—the diver was asked by a great shaman to bring up earth from the bottom of the sea. That earth was then used to build the world's dry land.
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.
Bhutan and Japan have issued stamps featuring the Red-throated Diver.
Sources
External links
- on Philadelphia's Academy of Natural Sciences's Visual Resources for Ornithology website
- on Handbook of Birds of the World's Internet Bird Collection website
- on xeno-canto.org's website
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