Traditionally, the bird order Apodiformes contained three living families: the swifts (Apodidae), the treeswifts (Hemiprocnidae), and the hummingbirds (Trochilidae). In the Sibley-Ahlquist taxonomy, this order is raised to a superorder Apodimorphae in which hummingbirds are separated as a new order, Trochiliformes. With nearly 450 species identified to date, they are the most diverse order of birds after the passerines.
The swifts are a family, Apodidae, of highly aerial birds. They are superficially similar to swallows, but are not closely related to any passerine species. Swifts are placed in the order Apodiformes with hummingbirds. The treeswifts are closely related to the true swifts, but form a separate family, the Hemiprocnidae.
Treeswifts or crested swifts are a family, the Hemiprocnidae, of aerial near passerine birds, closely related to the true swifts. The family contains a single genus, Hemiprocne, with four species. They are distributed from India and Southeast Asia through Indonesia to New Guinea and the Solomon Islands.
The crested treeswift is a species of tree swift. It was for some time considered the same species as its eastern relative, the gray-rumped treeswift, but they do not interbreed where their ranges overlap. It is distinct in flight, with long, bowed wings and a deeply forked tail that usually appears narrow and pointed.
Aegialornis is a genus of prehistoric apodiform birds. It formed a distinct family, the Aegialornithidae, and was in some ways intermediate between modern swifts and owlet-nightjars, lacking the more extreme adaptations to an aerial lifestyle that swifts show today, but already having sickle-shaped wings like them. They do not appear to be a direct ancestor of modern swifts, however, but rather a group that retained an overall basal morphology. Altogether, they were not too dissimilar from modern treeswifts.
Palaeotis is a genus of paleognath birds from the middle Eocene epoch of central Europe. One species is known, Palaeotis weigelti. The holotype specimen is a fossil tarsometatarsus and phalanx. Lambrect (1928) described it as an extinct bustard, and gave it its consequent name. After a suggestion by Storrs L. Olson, a review of the type specimen and the referral of several other fossils by Houde and Haubold (1987) concluded that Palaeotis is a palaeognath and assigned it to the same order as ostriches; the Struthioniformes. In 2021, it was considered a member of the family Paleotididae alongside Galligeranoides from the Early Eocene of France, which were found to be basal members of the Struthioniformes.
Eurotrochilus is an extinct genus of stem group hummingbirds (Trochilidae) and are the closest known relatives of the crown group Trochilidae. Despite Eurotrochilus being morphologically very similar to modern hummingbirds, they still retained several primitive features and are not closely related to any specific extant hummingbird in the crown group. There are currently two described species of Eurotrochilus: E. inexpectatus and E. noniewiczi.
Eurofluvioviridavis is a genus of extinct primitive birds from the Middle Eocene Messel Pit, Germany. It contains a single species, Eurofluvioviridavis robustipes. It is related to Avolatavis and Vastanavis, other members of the family Vastanavidae.
Hokkaidornis is an extinct genus of penguin-like plotopterid from the Late Oligocene of Hokkaido, Japan.
Bambolinetta lignitifila is a fossil species of waterfowl from the Late Miocene of Italy, now classified as the sole member of the genus Bambolinetta. First described in 1884 as a typical dabbling duck, it was not revisited until 2014, when a study showed it to be a highly unusual duck species, probably a flightless, wing-propelled diver similar to a penguin.
Halcyornithidae is an extinct family of telluravian birds thought to be related to the Psittaciformes (parrots), Passeriformes (songbirds), and to the extinct Messelasturidae. Halcyornithids have been found in various Eocene formations in Europe and North America. Widespread and diverse in the Early Eocene of North America and Europe, halcyornithids are not found in locales later than the Middle Eocene. Halcyornithids were small, arboreal birds with zygodactyl feet, with two toes facing forwards and two facing back, a trait shared with other tree-dwelling families of Eocene birds like the Zygodactylidae and the messelasturids. The skull of halcyornithids features a ridge of bone above the eye called the supraorbital process, similar to birds of prey. The relationships of the halcyornithids to other birds remain uncertain. Halcyornithids have been proposed as relatives to owls and as a lineage closer to parrots than to songbirds. Most recently, halcyornithids have been identified as the sister group of the clade including parrots and songbirds. It is also possible that Halcyornithidae is paraphyletic with respect to the Messelasturidae.
Oligocolius is an unusual genus of extinct mousebird from the early to late Oligocene epoch of Germany. Oligocolius is known from two species, the type species O. brevitarsus and O. psittacocephalon. Oligocolius is a member of the family of modern mousebirds (Coliidae) and is broadly similar to them in shape. However, unlike modern mouse birds the skull and beak of Oligocolius closely resembles those of parrots, and appears to be specialised for a distinct lifestyle not found in living mousebirds.
Phoeniconaias siamensis is an extinct species of flamingo that lived in northern Thailand during the Miocene period. Its closest living relative is the lesser flamingo.
Tonsala is an extinct genus of Plotopteridae, a family of flightless seabird similar in biology with penguins, but more closely related to modern cormorants. The genus is known from terrains dated from the Late Oligocene of the State of Washington and Japan.
Waltonavis is an extinct genus of potentially leptosomiform bird from the Early Eocene London Clay Formation of Essex, United Kingdom. The genus contains two species: W. paraleptosomus and W. danielsi, both known from partial skeletons.
Psittacomimus is an extinct genus of psittacopedid bird from the Early Eocene London Clay Formation of Essex, United Kingdom. The genus contains a single species, P. eos, known from a partial skeleton.
Pulchrapollia is an extinct genus of halcyornithid bird from the Early Eocene London Clay of Walton-on-the-Naze, United Kingdom and the Nanjemoy Formation of Virginia, United States. The genus contains three species, Pulchrapollia gracilis, Pulchrapollia tenuipes and Pulchrapollia eximia.
Serudaptus is an extinct genus of halcyornithid bird from the Middle Eocene Messel pit in Hesse, Germany. The genus contains one species, Serudaptus pohli, and is known for long, raptorial claws on its zygodactyl feet.
Parapsittacopes is an extinct genus of psittacopedid bird from the Early Eocene London Clay, United Kingdom. The genus contains one species, Parapsittacopes bergdahli.
Eofringillirostrum is an extinct species of bird known from the Early Eocene Green River Formation of the Western United States and from the Messel Pit in Germany. The genus contains two species, Eofringillirostrum boudreauxi and Eofringillirostrum parvulum. They are the earliest known fossil birds to have a finch-like beak.
