The loons (e.g. North America) or divers (e.g. UK/Ireland) are a group of aquatic birds found in many parts of North America and northern Eurasia. All living species of loons are members of one genus, Gavia, family, Gaviidae, and order Gaviiformes all of their own.
The loons are the size of a large duck or small goose, which they somewhat resemble in shape when swimming. Their plumage is largely black-and-white, with grey on the head and neck in some species, and a white belly, and all species have a spear-shaped bill.
The European name "diver" comes from the bird's habit of catching fish by swimming calmly along the surface and then abruptly plunging into the water. The North American name loon comes from the bird's haunting, yodelling cry.
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 get airborne. Only the Red-throated Diver 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. Loons can live as long as 30 years.
To help digestion, loons swallow small pebbles from the bottoms of lakes. Similar to grit eaten by chickens, these gastroliths may assist the loon's gizzard in crushing the hard parts of the loon's food such as the exoskeletons of crustaceans and the bones of frogs and salamanders. The gastroliths may also be involved in stomach cleaning as an aid to regurgitation of indigestible food parts.
Loons may inadvertently ingest small lead pellets, released by anglers and hunters, which will slowly lead to the loon's death by lead poisoning. Jurisdictions that have banned the use of lead shot and sinkers include Maine, New Hampshire, Vermont, some areas of Massachusetts, Yellowstone National Park, Great Britain, Canada, and Denmark.
Loons build their nests close to the water, preferring sites that are completely surrounded by water. They may use the same site from year to year. Loons will use a variety of materials found nearby to build their nests including pine tree needles, leaves, grass, moss, and sometimes clumps of mud. Both the male and female help with nest building and incubation, which usually lasts 26-31 days. If the eggs are lost, the pair may re-nest, often in the same general location.
Usually one or two eggs are laid in June. Loon chicks are precocial, able to swim right away, but are often seen riding on their parents' back. This behavior allows the chicks to rest, conserve heat, and avoid predators such as large carnivorous fish, snapping turtles, gulls, eagles, and crows. After a day or two, chicks cease returning to the nest but remain in their parent's company.
Chicks remain with and are fed exclusively by their parents for about eight weeks. After eight weeks, chicks will begin to dive for some of their own food. By 11 or 12 weeks of age, chicks are able to gather almost all of their own food and may be able to fly.
A pair may mate for life, although banding studies have shown that loons will sometimes switch mates after a failed nesting attempt and even between nesting attempts in the same season. Male loons appear more faithful to breeding territories than to mates.
All living species are classed in the genus Gavia.
More recently, it has become clear that the Anseriformes (waterfowl) and the Galliformes are the most ancient groups of modern birds, while loons belong to a more modern radiation. What is also generally accepted as a fact is that loons and grebes are not closely related at all, but rather one of the most stunning examples of convergence in the known birds. The Sibley-Ahlquist taxonomy still allied the loons with the grebes in its paraphyletic "Ciconiiformes", and it is almost certain that the relationships of loons lie with some of the orders united therein. Alternatively, loons have tentatively been considered to share a rather close relationship with waders, penguins or procellariiform seabirds.
The conflicting molecular data is not much resolved by the fossil record. Modern loons are only known with certainty since the Eocene, but by that time almost all modern bird orders are known or strongly suspected to have existed anyway. The Late Eocene to Early Miocene genus Colymboides was widespread in Western and Central Europe; it is usually placed in the Gaviidae already, but may actually be more primitive; it is quite distinct from modern loons and could well be paraphyletic. From the genus Gavia, about a dozen fossil species have been discovered to date, which are known from the Early Miocene onwards and had a more southerly distribution, like today's California, Florida and Italy:
"Gavia" portisi from the Late Pliocene of Orciano Pisano (Italy) is known from a cervical vertebra that may or may not have been from a loon. If so, it was from a bird slightly smaller than Common Loon. Older authors were quite sure the bone was indeed from a Gavia and even considered G. concinna a possibly junior synonym of it. This is now regarded as rather unlikely for reasons of biogeography. Interestingly, an Early Pliocene loon skull form Empoli (Italy) was referred to G. concinna. The vertebra may now be lost, making "G." portisi a nomen dubium.
In addition, there are some much older forms that are sometimes assigned to the Gaviiformes. From the Late Cretaceous, the genera Lonchodytes (Lance Formation, Wyoming) and Neogaeornis (Quinriquina Formation, Chile) have been described; the latter might have been a primitive loon, but possibly a hesperornithiform, and both have sometimes been allied with the orders which are considered related to loons. Doubtfully valid and surrounced by considerable dispute is the supposed Late Cretaceous loon Polarornis (Seymour Island, Antarctica). Eupterornis from the Paleocene of France has some features reminiscent of loons, but others seem more similar to Charadriiformes such as gulls (Laridae). A piece of a carpometacarpus supposedly from Oligocene rocks near Lusk, Wyoming was described as Gaviella pusilla, but this also shows some similarities to the plotopterids. Parascaniornis, sometimes allied to the loons, has more recently determined to be a junior synonym of the hesperornithiform Baptornis.