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Gavia stellatared-throated diver(Also: red-throated loon)

By Alicia Ivory

Geographic Range

Holarctic, breeding far into the high Arctic, and winters mainly on the northern coasts of the Atlantic and Pacific. Also in the Great Lakes, and the Black, Caspian, and Mediterranian Seas.

Habitat

It breeds mostly on fresh water, typically in fairly open moorland, and may occupy stretches of water of almost any size. It is often found to be nesting by small pools. It winters on inshore waters along sheltered coasts, occasionally inland.

  • Aquatic Biomes
  • lakes and ponds
  • rivers and streams
  • coastal

Physical Description

The red-throated loon is the smallest, slightest of the divers. It stands at 53-69 cm., and its wingspan ranges from 106-116 cm. During the breeding season, the upper body is a solid dark brown. The head and upper neck is grayish, with a large, glossy colored patch on the foreneck. It is white underneath and the tail is dark. In the winter, the face and foreneck are pure white, and the upper part is dark brownish and finely spotted with white. Males average slightly larger than females, and have a heavier head and bill. Its neck is thick, and the nostrils are narrow and elongated, as an adaptation to diving. The iris is reddish, especially in adults during the breeding season. The body is designed for swimming, with short, strong legs set far back on the body. The legs are perfect for moving through water, although this design makes walking on land difficult. The three front toes are webbed, and these loons have short, well-defined tails. They can vary their buoyancy in order to remain underwater, with the whole body submerged and only the eyes and bill visible above the surface. Adult loons shed their flight feathers simultaneously at the end of the breeding season and are thus unable to fly for several weeks. The body feathers are molted only in early spring and early autumn. (del Hoyo, Elliot, and Sargatal, 1992)

  • Average mass
    1816 g
    64.00 oz
    AnAge

Reproduction

Red-throated loons breed on freshwater lakes of the subarctic and boreal zones, with a strong preference for undisturbed sites. They readily settle on stretches of still water ranging in size from small pools to large, deep lakes, and sometimes even nest on sheltered coasts.

Loons are monogamous, forming long-term pair bonds. Pairs established from the previous season probably remain together throughout the winter, and start nesting early on after a minimal amount of display. Even newly formed pairs have simple courtship displays. Copulation takes place on dry land and is repeated frequently. It may begin on their day of arrival at the nest and continue until all eggs have been laid. The male selects the nest site.

Since loons have difficulty in walking, the site is always close to water. The nest is simply a heap of plant matter. Several pairs may build nests semi-colonially, especially when there are few tracts of suitable water within reach of their feeding areas. Thus they are tolerant of other pairs close by and only defend the area immediately surrounding the nest. However, if they are not breeding colonially, they may aggressively defend up to several hectares, including several non-nesting ponds.

Breeding starts in May in the south of the range, and in the north, timing depends on when spring thaw occurs. 1-3 eggs may be laid, but there are almost always 2. Incubation is 27 days and is performed by both partners, with the female spending more time on the nest than the male. Incubation starts when the first egg is laid. The resulting differences in age and size of the chicks means that when food is scarce, the older, larger chick gets more, and the youngest frequently starves to death within its first few days.

The chicks have dark brown down, and are paler below. By 2-3 weeks, they spend most of the time swimming, though they still rely on their parents for food until they are fully grown. Fledgling takes place at around 7 weeks. They are sexually mature at 2-3 years, and are known to have lived 23 years in the wild.

Nest failures due to predation are probably much more important than those due to human disturbance, because their range in North America, at least, does not overlap much with where humans live. (Eberl and Picman, 1993)

  • Range eggs per season
    1 to 3
  • Average eggs per season
    2
    AnAge
  • Average time to hatching
    27 days
  • Average time to hatching
    28 days
    AnAge
  • Average time to independence
    7 weeks
  • Range age at sexual or reproductive maturity (female)
    2 to 3 years
  • Range age at sexual or reproductive maturity (male)
    2 to 3 years

Lifespan/Longevity

Behavior

Though loons are generally extremely awkward on land, red-throated loons have been known to travel long distances on land. When seriously disturbed, they may even move to a new pool with their chicks. It is the lightest and most agile species of the genus and it has the largest wing-beat amplitude, and only the red-throated loon can take off from the ground, or alight directly on it. After breeding, these loons move to coastal waters, and sometimes gather in large flocks in particularly rich feeding areas. At such localities, the birds roost and feed communally. Aggressive behavior may be observed here, but it doesn't develop. They spend long hours caring for their plumage, and their elaborate bathing practices involve rigorous wing shaking, rolling, diving and somersaulting. Roosting takes place mainly on water, but can occasionally occur on land during the breeding season. The loon's characteristic call is extremely loud and can be heard far away. It is used to proclaim the occupation of a territory. It sounds like a long, low-pitched whistle with some very clear notes interspersed. It is made by both mates at once. When disturbed or threatened, the red-throated loon produces a raven-like croaking call of warning. It also uses a short, frequently repeated, gooselike cackle, which it gives when flying over its own or neighboring territory. Red-throated loons have a variety of ritualized behaviors, including a series of stereotyped swimming ceremonies, which are performed by both partners.

Communication and Perception

Food Habits

Red-throated loons prefer to forage in marine waters and never forage in their nesting pond, unlike other loons. (Eberl and Picman, 1993)

The red-throated loon obtains most of its food underwater, in dives that have been recorded at 2-9 meters, and average 1 minute. Prey is located visually, so these loons favor clear waters for foraging, and they do not fish at night. The prey consists of small or medium sized fish, including cod, herring, sprat, sculpins, and occasionally crustaceans, mollusks, frogs, fish spawn and insects. Food is usually swallowed before the loon surfaces. Their esophagus is relatively elastic, but a few have suffocated after swallowing too large a fish. When they find a suitable prey species in abundance, they will fully exploit it. (del Hoyo, Elliot and Sargatal, 1992)

  • Animal Foods
  • amphibians
  • fish
  • insects
  • mollusks
  • aquatic crustaceans

Economic Importance for Humans: Positive

Red-throated diver skins are sometimes used to make ceremonial dresses.

Conservation Status

Red-throated loons are fairly sensitive to human disturbance and will desert the breeding lake if there is too much human activity. Direct human disturbance causes most breeding failures. The red-throated loon is also affected in places by changes in water level. It may suffer seriously from acidification of breeding waters and heavy metal pollution. It is highly vulnerable to oil spills, especially near rich fishing grounds where large congregations of birds may form in winter. Diver skins are sometimes used commercially.

Contributors

Alicia Ivory (author), University of Michigan-Ann Arbor.

Glossary

Arctic Ocean

the body of water between Europe, Asia, and North America which occurs mostly north of the Arctic circle.

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

Palearctic

living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

World Map

acoustic

uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

coastal

the nearshore aquatic habitats near a coast, or shoreline.

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

fertilization

union of egg and spermatozoan

internal fertilization

fertilization takes place within the female's body

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

piscivore

an animal that mainly eats fish

seasonal breeding

breeding is confined to a particular season

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

tactile

uses touch to communicate

visual

uses sight to communicate

References

del Hoyo, J., Elliot, A., and Sargatal, J. Handbook of the Birds of the World, Vol. 1. Lynx Edicions, Barcelona, 1992.

Eberl, C., J. Picman. 1993. Effect of nest-site location on reproductive success of red-throated loons (Gavia stellata). Auk, 110: 436-444.

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