Strisores

The material evidence for this group is very equivocal; the most ancient Strisores are quite nondescript tree-dwellers but already tend towards peculiarly apomorphic feet, and no Cretaceous fossils are known. Torpor and other metabolic peculiarities are very frequently found in this group, perhaps more often than in any other bird lineage. The synapomorphies that define this clade are the ossa maxillaria separated by a large cleft, a mandible with very short pars symphysialis, and rami mandibulae very slender in their distal half.

The taxonomy of this group of birds has a long and complicated history. Jean Cabanis originally coined the name Strisores in 1847 as an order encompassing a much broader group of birds subdivided into two 'tribes': The Macrochires (hummingbirds, swifts, and nightjars, including oilbirds and potoos, but notably excluding frogmouths) and the Amphibolae (hoatzin, mousebirds, and turacos).[3] Hermann Burmeister later excluded the taxa in Cabanis' Amphibolae from Strisores, but added kingfishers and motmots.[4] Subsequent authors used either definition according to their own judgement, with Baird following Cabanis',[5] and Cooper following Burmeister's usage.[6] In 1867, Thomas Henry Huxley proposed the name Cypselomorphae for hummingbirds, swifts, and nightjars (including owlet-nightjars and potoos), however, he considered frogmouths and oilbirds unrelated due to aspects of their skull morphology.[7] In the 1880s Anton Reichenow continued to use Strisores in a similar sense as Huxley's Cypselomorphae (this time also excluding the owlet-nightjars),[8] but by the late 19th Century, Strisores had fallen into disuse,[9] and this remained the case through the 20th Century.[10][11][12][13]

By the early 21st century, analyses of anatomical morphology and molecular phylogenomics demonstrated that the order Caprimulgiformes as had been used for much of the 20th century (oilbirds, potoos, nightjars, frogmouths, and owlet-nightjars) is actually paraphyletic respective to Apodiformes (hummingbirds, swifts, and treeswifts), with apodiform birds nesting deeply within caprimulgiformes and a sister taxon to the owlet-nightjars.[14][15] The discovery has led to a challenge of reconciling a Linnean hierarchy with phylogenetic relationships while still maintaining nomenclatural stability, resulting in a complicated situation where some researchers currently use the resurrected name Strisores in a new sense,[15][16] others expand the order Caprimulgiformes to include the 'traditional' apodiform families,[17] whereas others[18] use the superordinal name Caprimulgimorphae Cracraft, 2013,[19] raising the 'traditional' caprimulgiform families to the rank of order.

Proposed phylogenetic definitions of Strisores and Caprimulgimorphae treat Strisores as the crown group and Caprimulgimorphae as the total group.[20] This allows both names to be valid, with similar but not identical meanings.

Fossil of Hassiavis laticauda, a probable daedalornithean,[20] from the Messel fossil site

Strisores has a well-represented fossil record, with fossils of most major strisorean lineages known from the Paleogene.[20]

The relationships of the Early Eocene Parvicuculus and Procuculus from the southern North Sea basin are unresolved, but they bear some similarities to cypselomorphs.

The fossil evidence is quite consistent in this group. Over some 20 million years, throughout the Eocene, the present-day diversity (as well as some entirely extinct lineages) slowly unfolds. By mid-Oligocene, some 30 million years ago, the crown lineages are present and adapting to their present-day ecological niches.

By the distribution of fossils, the Paleogene radiation seems to have originated in Asia, which at that time became a highly fragmented landscape as the Himalayas lifted up and the Turgai Strait started to disappear.

Several fossil taxa are tentatively placed here as basal or incertae sedis

• Eocypselus (Late Paleocene or Early Eocene)
• Paraprefica (Early Eocene?)
• Archaeotrogonidae (Early Eocene of England ?- Late Eocene/Early Oligocene of France)
• Hassiavis (Middle Eocene of Messel, Germany) - Archaeotrogonidae?
• Protocypselomorphus (Middle Eocene of Messel, Germany)

Strisores contains the extant families Aegothelidae, Apodidae, Caprimulgidae, Hemiprocnidae, Nyctibiidae, Podargidae, Steatornithidae, and Trochilidae. Apodidae and Hemiprocnidae are grouped together as Apodi, Apodi and Trochilidae are grouped together as Apodiformes, and Apodiformes and Aegothelidae are grouped together as Daedalornithes.

Cladogram based on Prum et al.,[16] with phylogenetic definitions following Chen et al.:[20]

Cladogram based on Reddy, S. et al. (2017):[17]