Common Loon

Description

The adult common loon can range from 66 to 91 cm (26 to 36 in) in length with a 127 to 147 cm (4 ft 2 in to 4 ft 10 in) wingspan, slightly smaller than the similar yellow-billed loon. On average, it is about 81 cm (32 in) long and has a wingspan of 136 cm (54 in). Its weight can vary anywhere from 2.2 to 7.6 kg (4.9 to 16.8 lb). Sizes vary regionally, especially by body mass, with the smallest bodied loons on average from lower-central Canada and the Great Lakes, while westerly birds are similar or mildly larger, and loons breeding further east can appear to be significantly larger. Furthermore, males average up to nearly 27% more massive than females in some populations. Breeding loons in Maine averaged 4.65 kg (10.3 lb) in females and 5.97 kg (13.2 lb) in males, essentially the same weight as the yellow-billed loon, although the yellow-billed is still larger than in linear dimensions (especially bill length) than the Maine loons. In Ontario, 20 females averaged 4.5 kg (9.9 lb) and 20 males averaged 5.46 kg (12.0 lb). In contrast, in the Gulf of Alaska, adults of both sexes reportedly averaged 4.13 kg (9.1 lb). Adult breeding plumage consists of a broad black head and neck with greenish, purplish, or bluish sheen. It has a black bill sometimes with a pale tip, and red eyes. The neck is encircled with a characteristic black ring and has two white necklaces of eight to ten short streaks on the upper foreneck, and a noticeable collar of white, parallel lines forming a large oval on the neck-side. The central lower foreneck is pure white, and the lower neck-sides have longitudinal white lines becoming rows of small spots and black lines becoming very narrow. The upperparts are blackish or blackish grey, and each feather has small white spots on it. The upperwing is blackish and with small white spots on the non-primary coverts, whereas the underwing is paler with white coverts except the long black shaft-streaks on the axillaries. The underparts are pure white, but have some black on the undertail coverts and vent. It has a checkered black-and-white mantle and a blackish tail. The legs are pale grey on the inner half and blackish on the outer half, and the webs between the toes are flesh colored. Adult non-breeding plumage is brownish with a dark neck and head marked with dark grey-brown. The eyes are surrounded with white, and the eyelids are pale. The bill is mostly pale grey, with a dark culmen and tip, but in early spring the tip may turn whitish. The underparts, lower face, chin, and throat are also whitish. The foreneck is whitish, usually forming a wedge-shaped notch in dark neck-sides, and may sometimes reveal a shadowy trace of the neck ring or a pale collar. It has dark brownish grey upperparts with an unclear pattern of squares on the shoulders and some wing coverts spotted with white, which are usually concealed while swimming. The male and the female have similar appearances, although they exhibit sexual dimorphism in their physical dimensions with the male larger and significantly heavier than the female. The heavy dagger-like bill is evenly tapered and greyish, sometimes having a black tip. The bill colour and angle distinguish this species from the yellow-billed loon. The neck is short and thick. The common loon swims very low in the water, with sometimes only its head held above and horizontal to the surface. It must run across the water surface to get in flight. During flight, its head is slightly lower than its body, with its feet trailing behind. It has a skeletal structure made up of a number of solid bones (this is usual for the Gaviiformes and penguins but unlike most flying birds which have bones with extensive pneumatization, hollow and filled with air to make the skeleton lighter), which adds weight but helps in diving. A juvenile often has a dark, brownish-grey nape that may look darker than the pale-edged black feathers. It has a dark grey to black head, neck, and upperparts, with white throat, cheeks, and underparts. During the first winter, the bill shape of the young may not be as fully developed as that of the adult, and during the second winter, it much resembles the breeding adult, but with wing coverts lacking white spots. The common loon is distinguished from the black-throated loon (G. arctica) and the red-throated loon (G. stellata) mainly by its larger size. It usually has a steeper forehead and a bulging forecrown, somewhat similar to the black-throated loon. Its bill is heavier and the back is paler than its hindneck. It is more difficult to separate from the yellow-billed loon, but its breeding plumage has more white markings on the neck and the squares on its shoulders are usually smaller; the non-breeding plumage has darker neck-sides contrasting more sharply with pale areas and bill colour.

Range & habitat

Common loons are mainly Nearctic, and breed from 48° N to the Arctic Circle, locally south to 40° N and north to 78° N. During their breeding season in spring and summer, most common loons live on lakes and other waterways in the northern United States and Canada, as well as in southern parts of Greenland, in Iceland, in Svalbard, in Jan Mayen, and in Bear Island in Norway; and in Alaska, to the west, and very rarely in Scotland, to the east. Their summer habitat ranges from wooded lakes to tundra ponds. The lakes must be large enough for flight take-off, and provide a large population of small fish. Deep lakes with warm surface waters, relatively low biological productivity and low turbidity where their fish prey are easy to see are habitats where breeding loons are more successful in raising young. For protection from predators, common loons favour lakes with islands and coves. They are rare visitors to the Arctic coast. They are known to exhibit high breeding site fidelity. Some common loons remain in Iceland year-round, although most migrate. In North America, they winter mainly along north Atlantic and north-east Pacific coasts, many stopping off on the Great Lakes during their migration. They migrate in the day, starting about two hours after sunrise and flying at altitudes of 1500 to 2700 m above sea level, above the convective and turbulent layer of air. In winter they can be seen on North America coasts as far south as Baja California, Sonora, northern Sinaloa, southern Texas, and rarely northern Tamaulipas. In the east, several thousand winter along western European coasts, probably originating from Iceland, Greenland, and Canada. Their range extends into northwestern Europe from Finland to Portugal and southern and northwestern Spain (Galicia and Asturias), as well as the western Mediterranean off Catalonia, and off Morocco in Africa, although only a few hundred travel as far south as Iberia. Although wintering site fidelity is not well known, annually, adults are observed to return to the same wintering locations in the Pacific Ocean (Morro Bay), the Gulf of Mexico (Barataria Bay), the Atlantic Ocean (Maryland and Massachusetts), and the reservoir Lake Pateros. They usually winter along coasts and on inland lakes, bays, inlets, and streams, with birds migrating to the nearest body of water that will not freeze over in the winter: western Canadian loons go to the Pacific, Great Lakes loons to the Gulf of Mexico region, eastern Canadian loons to the Atlantic, and some loons to large inland lakes and reservoirs. They appear in most of the inland waters of the United States. The South Carolina coast, the Gulf coast adjacent to the Florida panhandle, and the Atlantic seaboard from Massachusetts to Maine have some of the highest concentrations of common loons. Occasional vagrants are recorded inland in Mexico, in San Luis Potosí and Coahuila, as well as in Chiapas and Oaxaca in the south. They are accidental in northern Japan and the Commander Islands in northwestern Pacific, and Cuba in the West Indies.

Behaviour

The common loon is an expert fisher, catching its prey underwater by diving as deep as 60 m (200 ft). With its large webbed feet, the common loon is an efficient underwater pursuit predator and adroit diver. It needs a long run-up distance to gain momentum for flight take-off and is ungainly on land, sliding on its belly and pushing itself forward with its legs. Its clumsiness on land is due to the legs being positioned at the rear of its body; the pelvic muscles are well developed, ideal for swimming but not well-suited for walking. When it lands on water, it skims along on its belly to slow down, rather than braking with its feet, as they are set too far back. The common loon swims and dives well, and flies competently for hundreds of kilometres in migration. It flies with its neck outstretched, usually calling a particular tremolo that can be used to identify a flying loon. Its flying speed is as much as 120 km/h (75 mph) during migration. Particularly during the breeding season, common loons frequently engage in territorial disputes against other water birds, including ducks and geese, and will attack or drive off competitors and intruders to their territory. Common loons sometimes form social gatherings of several adults in late summer. Studies in North America suggest that these events are attended largely by non-breeding individuals and relate to territory assessment and social familiarity rather than foraging or migration.

Adapted from Common loon on Wikipedia, used under CC BY-SA 4.0.