Loon

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

Description

The loon, the size of a large duck or smallgoose, resembles these birds in shape when swimming. Like ducks and geese but unlike coots (which are Rallidae) andgrebes (Podicipedidae), the loon's toes are connected by webbing. The bird may be confused with cormorants(Phalacrocoracidae), not too distant relatives of divers and like them are heavy set birds whose bellies – unlike those of ducks and geese – are submerged when swimming. Flying loons 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.   
                

Male and female loons have identical plumage. Plumage is largely patterned black-and-white in summer, with grey on the head and neck in some species. 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.

Males are larger on average, but relative size is only apparent when the male and female are together.

In winter plumage is dark gray 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 color 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 ofdown 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) andpenguins (Sphenisciformes), both relatives of the loons.

Behaviour and ecology

Loons 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.

All loons are 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. Only the Red-throated Diver (G. stellata) can take off from land. Once airborne, their considerable stamina allows them to migrate long distances southwards in winter, where they reside in coastal waters. North European loons migrate primarily via the South Baltic and directly over land to the Black Sea or Mediterranean. Loons can live as long as 30 years and can hold their breath for as long as 90 seconds while underwater.

Despite the roughly equal participation of the sexes in nest building and incubation, analysis has shown clearly that males alone select the location of the nest. This pattern has the important consequence that male loons, but not females, establish significant site-familiarity with their territories that allows them to produce more chicks there over time. Sex-biased site-familiarity might explain, in part, why resident males fight so hard to defend their territories.

Most clutches consist of two eggs, which are laid in May or June, depending upon latitude. Loon chicks are precocial, able to swim and dive right away, but will often ride on their parents' back during their first 2 weeks to rest, conserve heat, and avoid predators.

Chicks are fed mainly by their parents for about six weeks but gradually begin to feed themselves over time. By 11 or 12 weeks, chicks gather almost all of their own food and have begun to fly.

Biologists, especially from Chapman University, have extensively studied the mating behavior of the Common Loon (G. immer). Contrary to popular belief, pairs seldom mate for life. Indeed, a typical adult loon is likely to have several mates during its lifetime because of territorial takeover. Each breeding pair must frequently defend its territory against "floaters" (territory-less adults) trying to evict at least one owner and seize the breeding site. Territories that have produced chicks in the past year are especially prone to takeovers, because nonbreeding loons use chicks as cues to indicate high-quality territories. One-third of all territorial evictions among males result in the death of the owner; in contrast, female loons usually survive. Birds that are displaced from a territory but survive usually try to remate and (re)claim a breeding territory later in life.