Loon

Loons 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. The loon uses its pointy bill to stab or grasp prey. They eat vertebrate prey headfirst to facilitate swallowing, and swallow all their prey whole.

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, that will contribute to lead poisoning and the loon's eventual death. Jurisdictions that have banned the use of lead shot and sinkers include but are not limited to Maine, New Hampshire, Vermont, some areas of Massachusetts, Yellowstone National Park, Great Britain, Canada, Michigan, and Denmark.[9]

Loons nest during the summer on freshwater lakes and/or large ponds. Smaller bodies of water (up to 0.5 km²) 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. The red-throated loon, 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.[7][8]

Loons mate on land, often on the future nest site, and build their nests close to the water, preferring sites that are completely surrounded by water such as islands or emergent vegetation. Loons use a variety of materials to build their nests including aquatic vegetation, pine needles, leaves, grass, moss and mud. Sometimes, nest material is almost lacking. Both male and female build the nest and incubate jointly for 28 days. If the eggs are lost, the pair may re-nest, usually in a different location. Since the nest is very close to the water, rising water may induce the birds to slowly move the nest upwards, over a metre.[7][8]

• 
  Common loon on the nest
• 
  Juvenile red-throated loon
• 
  Common loon feeding its young
• 
  Immature common loon with crayfish

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.[10]

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 two 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.[7][8] In 2019, a necropsy of a bald eagle found floating on a Maine lake (beside the floating body of a loon chick) found that the eagle had been stabbed through the heart by an adult loon's beak.[11]

Biologists, especially from Chapman University, have extensively studied the mating behaviour 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 re-mate and (re)claim a breeding territory later in life.[12][13][14][15]

In 2020, a loon hatched for the first time in over a century in Southeastern Massachusetts at Fall River, the Massachusetts Division of Fisheries and Wildlife and Biodiversity Research Institute. The chicks were relocated in 2015 with the hopes of re-establishing breeding and nesting patterns.[16]

The Paleocene Waimanu (an early member of the Sphenisciformes) resembled loons in some aspects of its head and bill, but it was already flightless and used its feet for steering rather than propulsion

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. In particular Enaliornis, which was apparently an ancestral and plesiomorphic member of that order, was sometimes used to support claims of Albian (Early Cretaceous) Gaviiformes.[28][29]

More recently, it has become clear that the Anseriformes (waterfowl) and the Galliformes are the most ancient groups of modern birds. It is possible, though not at all well-supported, that these were distinct by the end of the Albian 100 million years ago (Ma). Loons belong to a more modern radiation. They were once believed to be related to grebes, which are also foot-propelled diving birds, and both species were once classified together under the order Colymbiformes. However, as long ago as the 1930s, it was determined that the two groups are not that closely related at all and are merely the product of convergent evolution and adapted in a similar way to a similar ecological niche. The similarity is so strong that even the most modern cladistic analyses of general anatomical features are easily misled into grouping loons and grebes.[28][30][31]

The Sibley-Ahlquist taxonomy still allied the loons with the grebes in its massively paraphyletic "Ciconiiformes", and it is almost certain that the relationships of loons lie with some of the orders placed therein. Namely, other recent authors have considered loons to share a rather close relationship with seabirds such as penguins (Sphenisciformes), tubenoses (Procellariiformes), waders (Charadriiformes) – and perhaps the newly discovered clade Mirandornithes which unites grebes (Podicipediformes) and their closest living relatives, the flamingos (Phoenicopteriformes). It is perhaps notable that some early penguins had skulls and beaks that were in many aspects similar to those of the living and fossil Gaviiformes.[30][32]

Red-throated loon (G. stellata), the smallest living Gavia species. Some Miocene members of this genus were smaller still.

In prehistoric times, the loons had a more southerly distribution than today, and their fossils have been found in places such as California, Florida and Italy. The conflicting molecular data regarding their relationships is not much resolved by the fossil record; though they seem to have originated at the end of the Late Cretaceous like their presumed relatives, modern loons are only known with certainty since the Eocene. By that time almost all modern bird orders are at least strongly suspected to have existed – if not known from unequivocally identified specimens – anyway.[31]

Colymboides, the oldest unequivocal gaviiform genus known as of 2009, is widely known from early Priabonian – about 37 million years ago (Ma) in the Late Eocene – to Early Miocene (late Burdigalian, less than 20 Ma) limnic and marine rocks of western Eurasia north of the Alpide belt, between the Atlantic and the former Turgai Sea. It is usually placed in the Gaviidae already, but usually[33] in a subfamily Colymboidinae, with the modern-type loons making up the Gaviinae. But the Colymboides material is generally quite distinct from modern loons, and may actually belong in a now-extinct family of primitive gaviiforms.

Furthermore, the supposed genus could well be paraphyletic, so that for example Dyspetornis – which is now contained therein – might have to be separated again. A leg of an undescribed small loon was found in the Late Oligocene deposits at Enspel (Germany); it too may or may not belong to Colymboides. Of the crown genus Gavia, nearly ten prehistoric species have been named to date, and about as many undescribed ones await further study. The genus is known from the Early Miocene onwards, and the oldest members them are rather small (some are smaller than the red-throated loon). Throughout the late Neogene, the genus by and large follows Cope's Rule.[34][35]

Fossil in Vienna

List of fossil Gavia

• Gavia sp. (Early-Middle Miocene of eastern United States)[36]
• Gavia egeriana Švec, 1982 (Early Miocene of Czechoslovakia ?and Cheswold, Delaware, United States –? Yorktown Early Pliocene of Lee Creek Mine, South Carolina, United States)[37]
• Gavia schultzi Mlíkovský, 1998 (Middle Miocene of Sankt Margarethen, Austria)[38]
• Gavia sp. (Calvert Middle Miocene ?or Pleistocene of Maryland, United States) – same as Gavia cf. immer below?[39]
• Gavia spp. (Middle Miocene of Steinheim, Germany) – three species[38]
• Gavia brodkorbi Howard, 1978 (Late Miocene of Orange County, United States)[40]
• Gavia moldavica Kessler, 1984 (Late Miocene of Chişinău, Moldova)[38]
• Gavia paradoxa Umanska, 1981 (Late Miocene of Čebotarevka, Ukraine)[38]
• Gavia concinna Wetmore, 1940 (Late Miocene/Early Pliocene of west and east United States)[41]
• Gavia fortis Olson & Rasmussen, 2001 (Yorktown Early Pliocene of Lee Creek Mine, South Carolina, United States)
• Gavia sp. (Early Pliocene of Empoli, Italy)[42]
• Gavia sp. (Early Pliocene of Kerč Peninsula, Ukraine)[38]
• Gavia palaeodytes Wetmore, 1943 (Bone Valley Early/Middle Pliocene of Pierce, Florida, United States)[43]
• Gavia howardae Brodkorb, 1953 (Yorktown Early Pliocene of Lee Creek Mine, South Carolina, United States – San Diego Middle[44] Pliocene of San Diego, California, United States)[45]
• Gavia cf. concinna (San Diego Middle/Late[44] Pliocene of San Diego, California, United States) – two species?[46]
• Gavia sp. (Early Pleistocene of Kairy, Ukraine)[38]
• Gavia cf. immer (Pleistocene of California and Florida, United States) – possibly a G. immer paleosubspecies[34]

"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 the 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 due to the quite distinct range and age. The Early Pliocene Gavia skull from Empoli (Italy) was referred to G. concinna, and thus could conceivably have been of "G." portisi if that was indeed a loon. The holotype vertebra may now be lost, which would make "G." portisi a nomen dubium.[34][47]

In addition, there are some older fossils that are sometimes assigned to the Gaviiformes. From the Late Cretaceous, the genera Lonchodytes (Lance Formation, Wyoming, United States) and Neogaeornis (Quinriquina Formation, Chile) have been described; both are usually allied with orders which are considered related to loons. In particular the latter is still sometimes explicitly proposed as a primitive loon as they both were initially, but other authors consider Neogaeornis a hesperornithiform; note however that neither Gaviiformes nor Hesperornithiformes are known from the Southern Hemisphere or anywhere near it. Lonchodytes was more certainly quite close to loons, but probably closer still to some of the loons' relatives. Of similarly doubtful validity and surrounded by considerable dispute[48] is the supposed Late Cretaceous loon Polarornis (Seymour Island, Antarctica). Eupterornis from the Paleocene of Châlons-sur-Vesle (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 handbone also shows some similarities to the plotopterids, flightless wing-propelled diving birds, and if these are apomorphic would make an unconvincing member of the Gaviidae (though it still could be a small-winged gaviiform in a yet-undescribed family "Gaviellidae"[49]): while the carpometacarpus in Gavia is somewhat convergent to that of wing-propelled diving birds, enabling the wings to be used as rudders for quick underwater turns, Colymboides still had an unspecialized plesiomorphic hand. Parascaniornis, sometimes allied to the loons by early authors, was eventually determined to be a junior synonym of the hesperornithiform Baptornis. A supposed mid-Eocene loon fossil form Geiseltal (Germany) was erroneously assigned to Gavia.[50]

• Voices of the Loon, Robert J. Lurtsema, narrator. recorded by William E. Barklow. North American Loon Fund/National Audubon Society. (1980)