Aequornithes (/iːkwɔːrˈnɪθiːz/, from Latin aequor, expanse of water + Greek ornithes, birds), or core water birds,[6] are defined in the PhyloCode as "the least inclusive crown clade containing Pelecanus onocrotalus and Gavia immer".

[8][9][10][11] Aequornithes includes the clades Gaviiformes, Sphenisciformes, Procellariiformes, Ciconiiformes, Suliformes and Pelecaniformes.

It does not include several unrelated groups of aquatic birds such as flamingos and grebes (Mirandornithes), shorebirds, gulls, and auks (Charadriiformes), or the Anseriformes (waterfowl).

Based on a whole-genome analysis of the bird orders, the kagu and sunbittern (Eurypygiformes) and the three species of tropicbirds (Phaethontiformes) together styled as the Eurypygimorphae are the closest sister group of the Aequornithes,[6] in a clade later named Phaethoquornithes.

[5] Gaviiformes (loons/divers) Procellariiformes (albatrosses, shearwaters, and petrels) Sphenisciformes (penguins) Ciconiiformes (storks) Suliformes (frigatebirds, gannets, cormorants, and darters) Pelecaniformes (pelicans, herons and ibises) The clade Feraequornithes was named by Sangster & Mayr, 2021 to include the Pelecanimorphae and Procellariimorphae to the exclusion of the loons (Gaviiformes).