Gaviidae – Divers or Loons

The Gaviidae commonly called divers (UK/Ireland) or loons (North America) are a group of aquatic birds found in many parts of North America and northern Eurasia. All living species of are members of the genus Gavia, family Gaviidae and order Gaviiformes.

Generally all the species are the size of a large duck or small goose, which they resemble superficially when swimming. Like ducks and geese but unlike coots (which are Rallidae) and grebes (Podicipedidae), their toes are connected by webbing. They may be confused with cormorants (Phalacrocoracidae), which are relatives of divers and like them are heavy set birds whose bellies – unlike those of ducks and geese – are submerged when swimming. Flying divers resemble a plump goose with a seagull’s wings, relatively small in proportion to the bulky body. The bird holds its head pointing slightly upwards during swimming, but less so than cormorants do. In flight the head droops more than in similar aquatic birds.

Males and females have identical plumage. All species are largely patterned black-and-white in summer, with grey on the head and neck in some species, and one has a red-throat patch when in breeding plumage. All have a white belly. This resembles many sea-ducks (Merginae) – notably the smaller goldeneyes (Bucephala) – but is distinct from most cormorants which rarely have white feathers, and if so usually as large rounded patches rather than delicate patterns. All species of divers have a spear-shaped bill.

In winter plumage is dark grey above, with some indistinct lighter mottling on the wings, and a white chin, throat and underside. The species can then be distinguished by certain features, such as size and colour of head, neck, back and bill, but often reliable identification of wintering divers is difficult even for experts – particularly as the smaller immature birds look similar to winter-plumage adults, making size an unreliable means of identification.

Gaviiformes are among the few groups of birds in which the young moult into a second coat of down feathers after shedding the first one, rather than growing juvenile feathers with downy tips that wear off as is typical in many birds. This trait is also found in tubenoses (Procellariiformes) and penguins (Sphenisciformes), both relatives of divers.

They are excellent swimmers, using their feet to propel themselves above and under water while their wings provide assistance. Because their feet are far back on the body, loons are poorly adapted to moving on land, and usually avoid going onto land, except when nesting. They are all decent fliers, though the larger species have some difficulty taking off and thus must swim into the wind to pick up enough velocity to become airborne.

They find their prey by sight. They eat mainly fish, supplemented with amphibians, crustaceans and similar mid-sized aquatic fauna. Specifically, they have been noted to feed on crayfish, frogs, snails, salamanders and leeches. They prefer clear lakes because they can more easily see their prey through the water. They use their pointed bill to stab or grasp prey. They eat vertebrate prey headfirst to facilitate swallowing, and swallow all their prey whole. To help digestion, they swallow small pebbles from the bottoms of lakes. While they spend most of their time at sea, they nest during the summer on freshwater lakes or ponds. Smaller bodies of water will usually only have one pair. Larger lakes may have more than one pair, with each pair occupying a bay or section of the lake. Red-throated divers, however, may nest colonially, several pairs close together, in small Arctic tarns and feed at sea or in larger lakes, ferrying the food in for the young.