They are excellent divers, and under water they propel themselves with their feet with help from their wings; some cormorant species have been found to dive as deep as 45 metres (150 ft).

They have relatively short wings due to their need for economical movement underwater, and consequently have among the highest flight costs of any flying bird.

The French explorer André Thévet commented in 1558: "the beak [is] similar to that of a cormorant or other corvid", which demonstrates that the erroneous belief that the birds were related to ravens lasted at least to the 16th century.

[citation needed] Van Tets (1976) proposed to divide the family into two genera and attach the name "cormorant" to one and "shag" to the other,[4] but this nomenclature has not been widely adopted.

Imperial shags fitted with miniaturized video recorders have been filmed diving to depths of as much as 80 metres (260 ft) to forage on the sea floor.

Alternate functions suggested for the spread-wing posture include that it aids thermoregulation[14] or digestion, balances the bird, or indicates presence of fish.

The genus Phalacrocorax, from which the family name Phalacrocoracidae is derived, is Latinised from Ancient Greek φαλακρός phalakros "bald" and κόραξ korax "raven".

Their relationships and delimitation – apart from being part of a "higher waterfowl" clade which is similar but not identical to Sibley and Ahlquist's "pan-Ciconiiformes" – remain mostly unresolved.

Notwithstanding, all evidence agrees that the cormorants and shags are closer to the darters and Sulidae (gannets and boobies), and perhaps the pelicans or even penguins, than to all other living birds.

[20] In 2014, a landmark study proposed a 7 genera treatment, which was adopted by the IUCN Red List and BirdLife International, and later by the IOC in 2021, standardizing it.

[1][21] The cormorants and the darters have a unique bone on the back of the top of the skull known as the os nuchale or occipital style which was called a xiphoid process in early literature.

Smallish to large (65–100 cm), generally black with metallic sheen (usually blue/green), in breeding plumage with bright bare facial skin in the eye region and two crests (crown and nape).

Even the technique of using the distribution and relationships of a species to figure out where it came from, biogeography, usually very informative, does not give very specific data for this probably rather ancient and widespread group.

While the Leucocarbonines are almost certainly of southern Pacific origin—possibly even the Antarctic which, at the time when cormorants evolved, was not yet ice-covered—all that can be said about the Phalacrocoracines is that they are most diverse in the regions bordering the Indian Ocean, but generally occur over a large area.

Some Late Cretaceous fossils have been proposed to belong with the Phalacrocoracidae: A scapula from the Campanian-Maastrichtian boundary, about 70 mya (million years ago), was found in the Nemegt Formation in Mongolia; it is now in the PIN collection.

[30] As the Early Oligocene "Sula" ronzoni cannot be assigned to any of the sulid families—cormorants and shags, darters, and gannets and boobies—with certainty, the best interpretation is that the Phalacrocoracidae diverged from their closest ancestors in the Early Oligocene, perhaps some 30 million years ago, and that the Cretaceous fossils represent ancestral sulids, "pelecaniforms" or "higher waterbirds"; at least the last lineage is generally believed to have been already distinct and undergoing evolutionary radiation at the end of the Cretaceous.

[30] During the late Paleogene, when the family presumably originated, much of Eurasia was covered by shallow seas, as the Indian Plate finally attached to the mainland.

Lacking a detailed study, it may well be that the first "modern" cormorants were small species from eastern, south-eastern or southern Asia, possibly living in freshwater habitat, that dispersed due to tectonic events.

A Late Oligocene fossil cormorant foot from Enspel, Germany, sometimes placed in Oligocorax, would then be referable to Nectornis if it proves not to be too distinct.

All these early European species might belong to the basal group of "microcormorants", as they conform with them in size and seem to have inhabited the same habitat: subtropical coastal or inland waters.

While this need not be more than convergence, the phylogeny of the modern (sub)genus Microcarbo – namely, whether the Western Eurasian M. pygmaeus is a basal or highly derived member of its clade – is still not well understood at all as of 2022.

While the numerous western US species are most likely prehistoric representatives of the coastal Urile or inland Nannopterum, the European fossils pose much more of a problem due to the singular common shag being intermediate in size between the other two European cormorant lineages, and as of 2022 still of mysterious ancestry; notably, a presumably lost collection of Late Miocene fossils from the Odesa region may have contained remains of all three (sub)genera inhabiting Europe today.

Similarly, the Plio-Pleistocene fossils from Florida have been allied with Nannopterum and even Urile, but may conceivably be Phalacrocorax; they are in serious need of revision since it is not even clear how many species are involved.

Archaeological evidence suggests that cormorant fishing was practised in Ancient Egypt, Peru, Korea and India, but the strongest tradition has remained in China and Japan, where it reached commercial-scale level in some areas.

[38] Cormorants feature in heraldry and medieval ornamentation, usually in their "wing-drying" pose, which was seen as representing the Christian cross, and symbolizing nobility and sacrifice.

[42] In some Scandinavian areas, they are considered good omen; in particular, in Norwegian tradition spirits of those lost at sea come to visit their loved ones disguised as cormorants.

In Homer's epic poem The Odyssey, Odysseus (Ulysses) is saved by a compassionate sea nymph who takes the form of a cormorant.

In 1853, a woman wearing a dress made of cormorant feathers was found on San Nicolas Island, off the southern coast of California.

[citation needed] A cormorant representing Blanche Ingram appears in the first of the fictional paintings by Jane in Charlotte Brontë's novel Jane Eyre: One gleam of light lifted into relief a half-submerged mast, on which sat a cormorant, dark and large, with wings flecked with foam; its beak held a gold bracelet, set with gems, that I had touched with as brilliant tints as my palette could yield, and as glittering distinctness as my pencil could impart.In the Sherlock Holmes story "The Adventure of the Veiled Lodger", Dr. Watson warns that if there are further attempts to get at and destroy his private notes regarding his time with Holmes, "the whole story concerning the politician, the lighthouse, and the trained cormorant will be given to the public.

A bird famed for flight, sea fishing and land nesting was felt to be particularly appropriate for a college that unified leadership training and development for the Army, Navy and Royal Air Force.