Loon

Divers/Loons
	Great Northern Diver or Common Loon; (Gavia immer).
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Aves
Order:	Gaviiformes; Wetmore & Miller, 1926
Family:	Gaviidae; J.A. Allen, 1897
Genus:	Gavia; Forster, 1788
Species
	Gavia adamsii; Gavia arctica; Gavia immer; Gavia pacifica; Gavia stellata;

The loons (N.Am.) or divers (UK/Ireland) are a group of aquatic birds found in many parts of North America and northern Europe. A loon is the size of a large duck or small goose, which it somewhat resembles in shape when swimming.

The 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. All living species of loons are members of one genus (Gavia^[1] in a family (Gaviidae), and order (Gaviiformes) all of their own.

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, yodeling cry.

Ecology

Loons are excellent swimmers, using their feet to propel themselves above and under water and the wings for assistance. Because their feet are far back on the body, loons are badly adapted to moving on land. They usually avoid going onto land, except for nesting.

All loons are decent fliers though the larger species have some difficulty taking off, needing a run 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.

Diet

Loons eat fish, amphibians, and crustaceans, which they hunt under water, finding their prey by sight. This includes crayfish, frogs, snails, salamanders and leeches. They prefer clear lakes because it is easier to see prey. The loon's pointy bill is used to stab or grasp prey. Vertebrate prey is eaten head first to facilitate swallowing.

To help digestion, loons swallow small pebbles from the bottom of a lake. Similar to grit eaten by chicken, these gastroliths may assist the loon's gizzard in crushing the hard parts of the loon's food (the exoskeletons of crustaceans and the bones of frogs and salamanders), as prey is swallowed whole, or the gastroliths may 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.^[2]

Reproduction

During the summer, loons nest on fresh water lakes and/or large ponds. Smaller bodies of water (up to 0.5 km²) will usually only have one pair. Larger lakes will have more than one pair, with each pair occupying a bay or section of the lake.

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 — pine tree needles, leaves, grass, moss, and sometimes clumps of mud. Both the male and female help with nest building and with 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 do not return to the nest anymore.

Chicks are fed exclusively by their parents for the first few weeks of life, and up until eight weeks of age, stay with their parents most of the time. 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.^[3] Male loons appear more faithful to breeding territories than to mates.^[4]

Systematics and evolution

Species

All living divers are classed in the genus Gavia.

Red-throated Diver or Red-throated Loon, Gavia stellata.
Black-throated Diver or Arctic Loon, Gavia Arctica.
Pacific Diver or Pacific Loon, Gavia pacifica - sometimes included in G. Arctica
Great Northern Diver or Common Loon, Gavia immer.
White-billed Diver or Yellow-billed Loon, Gavia adamsii

Relationships and evolution

The loons were formerly often considered to be the most ancient of the northern hemisphere bird families; this idea grew basically out of the perceived similarity of shape and (probably) habits between loons and the entirely unrelated extinct Cretaceous order Hesperornithiformes. However, the two groups are merely the product of convergent evolution and adapted in a similar way to a similar ecological niche^[5].

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.^[6]

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. The following have been scientifically described:

Gavia egeriana (Early Miocene of Czechoslovakia)
Gavia brodkorbi (Late Miocene of Orange County, USA)
Gavia concinna Wetmore, 1940 (Early - Middle Pliocene of W and SE USA)^[7]
Gavia howardae Brodkorb, 1953 (Middle Pliocene of San Diego, USA)
Gavia palaeodytes Wetmore, 1943 (Middle Pliocene of Pierce, USA)
Gavia portisi (Regalia, 1902) (Late Pliocene of Italy)^[8]
Gavia fortis
Gavia moldavica
Gavia paradoxa
Gavia schultzi

Undescribed fossils are known from the eastern USA (Middle Miocene), Maryland (possibly Calvert Formation which is also Middle Miocene in age)^[9], and the Yorktown Formation (Early Pliocene at Lee Creek Mine, South Carolina where 3 unknown species are represented.

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^[10] 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^[11]. Parascaniornis, sometimes allied to the loons, has more recently determined to be a junior synonym of the hesperornithiform Baptornis.

Loons in popular culture

The Common Loon is the provincial bird of Ontario and is depicted on the Canadian one-dollar coin, which has come to be known affectionately as the "loonie".^[12] It is also the official state bird of Minnesota,^[13] and Mercer, Wisconsin is identified as the "Loon Capital of the World."^[14]

References

^ Etymology: gavia, Latin for smew - a quite unrelated species that also happens to be a black-and-white marine fishing bird. It is not likely that the Ancient Romans had much knowledge of loons, as these are limited to more northern latitudes.
^ If your jurisdiction is not listed here, this does not mean that lead shot and weights are legal. For a review of the impact of lead shot and alternatives, see Minnesota PCA (2007). For a review of gastrolith function, see Wings (2007).
^ McCormick, Damon (June 2005). "Common Loons at Seney NWR, June 2005" (PDF). United States Fish and Wildlife Service. Retrieved 2007-05-12.
^ "Animal Field Guide: Common Loon". Retrieved 2007-05-12.
^ This was proposed in the early 20th century already (Stolpe 1935).
^ Olson (1985), Mayr (2004)
^ May be a synonym of G. portisi (Wetmore 1953).
^ May include G. concinna (Wetmore 1953).
^ Wetmore (1941), Olson (1985).
^ See for example the discussion in Mayr (2004).
^ Discussed in Olson (1985).
^ Stewart, Barry D (2004), Across The Land: A Canadian Journey Of Discovery, Trafford Publishing, p. 143, ISBN 1412022762
^ Heinrichs, Ann (2003), Minnesota, Compass Point Books, p. 44, ISBN 0756503159
^ Moran, Mark; Sceurman, Mark; Godfrey, Linda S; Hendricks, Richard D (2005), Weird Wisconsin: Your Travel Guide to Wisconsin's Local Legends and Best Kept Secrets, Sterling Publishing, p. 78, ISBN 0760759448

Brodkorb, Pierce (1953): A Review of the Pliocene Loons. Condor 55(4): 211-214. PDF fulltext
Mayr (2004): A partial skeleton of a new fossil loon (Aves, Gaviiformes) from the early Oligocene of Germany with preserved stomach content. J. Ornithol. 145: 281-286 doi:10.1007/s10336-004-0050-9 PDF fulltext
Minnesota Pollution Control Agency (2007): Let's Get the Lead Out! Non-lead alternatives for fishing tackle. Version of June, 2007. Retrieved 2007-JUL-23.
Olson, Storrs L. (1985): Section X.I. Gaviiformes. In: Farner, D.S.; King, J.R. & Parkes, Kenneth C. (eds.): Avian Biology 8: 202-214. Academic Press, New York.
Stolpe, M. (1935): Colymbus, Hesperornis, Podiceps:, ein Vergleich ihrer hinteren Extremität. J. Ornithol. 80(1): 161-247. [Article in German] doi:10.1007/BF01908745
Wetmore, Alexander (1941): An Unknown Loon from the Miocene Fossil Beds of Maryland. Auk 58(4): 567. PDF fulltext
Wings, Oliver (2007): A review of gastrolith function with implications for fossil vertebrates and a revised classification. Acta Palaeontologica Polonica 52: 1-16 PDF fulltext