Strisores

Strisores ( stry-SOH-reez), sometimes called nightbirds, is a clade of birds that includes the living families and orders Caprimulgidae (nightjars, nighthawks and allies), Nyctibiidae (potoos), Steatornithidae (oilbirds), Podargidae (frogmouths), Apodiformes (swifts and hummingbirds), as well as the Aegotheliformes (owlet-nightjars) whose distinctness was only recently realized. The Apodiformes (which include the "Trochiliformes" of the Sibley–Ahlquist taxonomy) and the Aegotheliformes form the Daedalornithes.

Description 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 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 short pars symphysialis, and rami mandibulae slender in their distal half.

Taxonomic history 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). Hermann Burmeister later excluded the taxa in Cabanis' Amphibolae from Strisores, but added kingfishers and motmots. Subsequent authors used either definition according to their own judgement, with Baird following Cabanis', and Cooper following Burmeister's usage. 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. In the 1880s Anton Reichenow continued to use Strisores in a similar sense as Huxley's Cypselomorphae (this time also excluding the owlet-nightjars), but by the late 19th Century, Strisores had fallen into disuse, and this remained the case through the 20th Century. 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. 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, others expand the order Caprimulgiformes to include the 'traditional' apodiform families, whereas others use the superordinal name Caprimulgimorphae Cracraft, 2013, 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. This allows both names to be valid, with similar but not identical meanings.

Evolution

Strisores has a well-represented fossil record, with fossils of most major strisorean lineages known from the Paleogene. Chen et al. (2019) included 14 fossil lineages in their analysis. Nonetheless, it supports the emerging consensus phylogeny well. The genus Paraprefica, probably from the Early Eocene (though this is somewhat uncertain), seems to be a basal form that at times has been allied with the oilbird and the potoos, but cannot be assigned to either with certainty. In the consensus scenario, it would represent a record of the initial divergence of the three lineages. This agrees with fossils suggesting that the basal divergence of the owlet-nightjar and apodiform branch also occurred during that time. In addition, Eocypselus, a Late Paleocene or Early Eocene genus of North America, cannot be assigned to any one strisore lineage with certainty but appears to be some ancestral form. 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 mya, the crown lineages are present and adapting to their present-day ecological niches. These Paleogene birds strongly suggest that the two main extant lineages of strisores separated about 60–55 mya (Selandian–Thanetian), and that some time around the Lutetian–Bartonian boundary, some 40 mya, the common ancestors of Nyctibiidae, Caprimulgidae and eared nightjars diverged from those of oilbird and frogmouths. The relationships of the Early Eocene Parvicuculus and Procuculus from the southern North Sea basin are unresolved, but they bear some similarities to strisores. 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)

Systematics Strisores contains the extant orders Aegotheliformes, Apodiformes (with families Apodidae, Hemiprocnidae, and Trochilidae), Caprimulgiformes, Nyctibiiformes, Podargiformes, Steatornithiformes. Apodidae and Hemiprocnidae are grouped together as Apodi, Apodi and Trochilidae are grouped together as Apodiformes, and Apodiformes and Aegotheliformes are grouped together as Daedalornithes. The classification of the various birds that make up the order has long been controversial and difficult, particularly in the case of the nightjars and the paraphyly of the traditional Caprimulgiformes in relation to "Apodiformes", traditionally considered a separate order. The IUCN adopts the following classification of Order Caprimulgiformes, which follows recent phylogenetic studies:

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