The Apodiformes is an order, or taxonomic grouping, of birds which traditionally contained three living families—the Apodidae (swifts), the Hemiprocnidae (treeswifts), and the Trochilidae (hummingbirds); however, in the Sibley–Ahlquist taxonomy, this order is elevated to the superorder Apodimorphae, in which hummingbirds are separated into a new order, the Trochiliformes. With nearly 450 species identified to date, it is the most diverse order of birds after the Passeriformes (the "perching" birds).
Description, etymology and taxonomy As the name apodiforme—meaning "footless" in Greek—suggests, the legs of hummingbirds, swifts and treeswifts (or 'crested' swifts) are rather small and offer limited functional uses, aside from perching; their feet are covered with bare skin, rather than the scutes seen on many birds. Another commonality amongst Apodiformes is their evolving longer wings with short and stout humerus bones,Hyman, Libbie Henrietta (1992). Hyman's Comparative Vertebrate Anatomy. Chicago: University of Chicago Press, page 39. a development which has provided the hummingbirds, in particular, with the ideal wings for hovering.Mayr, Gerald (2003): Phylogeny of early tertiary swifts and hummingbirds (Aves: Apodiformes). Auk 120(1): 145–151. PDF fulltext
Together, the hummingbirds, swifts and treeswifts share several anatomical commonalities with their likely-closest extant relatives in the genus Aegotheles—the owlet-nightjars; in particular, similarities are noted between the birds' skull structures.Mayr, Gerald (2002): Osteological evidence for paraphyly of the avian order Caprimulgiformes (nightjars and allies). Journal für Ornithologie 143: 82–97. PDF fulltext
The owlet-nightjars are, apparently, convergent with the closely related Caprimulgiformes (nightjars), which form a clade known as Cypselomorphae together with the Apodiformes and the frogmouths, oilbirds and potoos.
Evolution The Apodiformes evolved in the Northern Hemisphere. Eocypselus, a primitive genus known from the Late Paleocene or Early Eocene of north-central Europe, is somewhat difficult to assign; it is considered a primitive hemiprocnid. This would suggest that the major apodiform lineages diverged shortly after the Cretaceous–Paleogene boundary. However, the perching adaptation of the foot of Eocypselus on which this theory rests may just as well be a symplesiomorphy. Most researchers believe that presently this genus cannot be unequivocally assigned to either the Apodiformes or the Caprimulgiformes.
The Early Eocene Primapus, found in England, is similar to both a primitive swift and the aegialornithids, which are in some aspects intermediate between swifts and owlet-nightjars. Fossil evidence demonstrates the existence of swifts during that period in Europe. At that time, most of Europe had a humid, subtropical climate, possibly comparable to modern-day southern China. For a map of Early–Middle Eocene Earth, see the Paleomap project; here note that both the Caucasus Mountains and the Alps did not exist yet and aegialornithids were possibly present in North America.Somewhat doubtful, as this is difficult to reconcile with the other aegialornithid fossils and Primapus. By the late Eocene (around 35 MYA), primitive hummingbirds started to diverge from the related jungornithids; the Middle Eocene Parargornis (Messel, Germany) and the Late Eocene Argornis, found in today's southernmost Russia, belong to this lineage. Cypselavus (Late Eocene – Early Oligocene of Quercy, France) was either a primitive hemiprocnid or an aegialornithid.
The placement of the Aegialornithidae is not quite clear. Various analyses place them sufficiently close to the Apodiformes to be included here, or into the unique owlet-nightjar lineage in the Cypselomorphae.
Taxonomy
ORDER APODIFORMES Family †Aegialornithidae Lydekker, 1891 [Primapinae Harrison, 1984c] Genus †Primapus Harrison & Walker, 1975 Genus †Aegialornis Lydekker, 1891 Suborder Apodi Genus †Procypseloides Harrison, 1984c Genus †Laputavis Dyke, 2001b Genus †Scaniacypselus Harrison, 1984 Family †Eocypselidae Harrison 1984 Genus †Eocypselus Harrison, 1984 Family Hemiprocnidae Oberholser, 1906 (treeswifts) Family Apodidae Olphe-Galliard, 1887 (swifts) Suborder Trochili Genus †Palescyvus Karchu, 1988 Family †Cypselavidae Mourer-Chauviré, 2006 Genus †Argornis Karchu, 1999 Genus †Cypselavus Gaillard, 1908 Genus †Parargornis Mayr, 2003 Family †Jungornithidae Karchu, 1988 Genus †Jungornis Karchu, 1988 Family Trochilidae Vigors, 1825 (hummingbirds)
See also List of Apodiformes by population
References