information:filtering
Taxonomy

The genus Phalacrocorax, from which the family name Phalacrocoracidae is derived, is Latinised from Ancient Greek φαλακρός phalakros "bald" and κόραξ korax "raven". This is often thought to refer to the creamy white patch on the cheeks of adult great cormorants, or the ornamental white head plumes prominent in Mediterranean birds of this species, but is certainly not a unifying characteristic of cormorants.

The cormorant family are a group traditionally placed within the Pelecaniformes or, in the Sibley–Ahlquist taxonomy of the 1990s, the expanded Ciconiiformes. Pelecaniformes in the traditional sense—all waterbird groups with totipalmate foot webbing—are not a monophyletic group, even after the removal of the distantly-related tropicbirds. 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.

In recent years, three preferred treatments of the cormorant family have emerged: either to leave all living cormorants in a single genus, Phalacrocorax, or to split off a few species such as the imperial shag complex (in Leucocarbo) and perhaps the flightless cormorant. Alternatively, the genus may be disassembled altogether and in the most extreme case be reduced to the great, white-breasted and Japanese cormorants. 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.

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. This bony projection provides anchorage for the muscles that increase the force with which the lower mandible is closed. This bone and the highly developed muscles over it, the M. adductor mandibulae caput nuchale, are unique to the families Phalacrocoracidae and Anhingidae.

Several evolutionary groups are still recognizable. However, combining the available evidence suggests that there has also been a great deal of convergent evolution; for example the cliff shags are a convergent paraphyletic group. The proposed division into Phalacrocorax sensu stricto (or subfamily "Phalacrocoracinae") cormorants and Leucocarbo sensu lato (or "Leucocarboninae") shags does have some degree of merit. The resolution provided by the mtDNA 12S rRNA and ATPase subunits six and eight sequence data is not sufficient to properly resolve several groups to satisfaction; in addition, many species remain unsampled, the fossil record has not been integrated in the data, and the effects of hybridisation – known in some Pacific species especially – on the DNA sequence data are unstudied.

A multigene molecular phylogenetic study published in 2014 provided a genus-level phylogeny of the family.