As the nameapodiforme—meaning "footless" inGreek—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 thescutes seen on many birds. Another commonality amongst Apodiformes is theirevolving longerwings with short and stouthumerus bones,[1] a development which has provided the hummingbirds, in particular, with the ideal wings forhovering.[2]
Together, the hummingbirds, swifts and treeswifts share several anatomical commonalities with their likely-closest extant relatives in the genusAegotheles—the owlet-nightjars; in particular, similarities are noted between the birds'skull structures.[3]
The Apodiformes evolved in the Northern Hemisphere.Eocypselus, a primitive genus known from the LatePaleocene or EarlyEocene of north-central Europe, is somewhat difficult to assign; it is considered a primitive hemiprocnid.[4] This would suggest that the major apodiform lineages diverged shortly after theCretaceous–Paleogene boundary. However, the perching adaptation of the foot ofEocypselus on which this theory rests may just as well be asymplesiomorphy.Most researchers believe that presently this genus cannot be unequivocally assigned to either the Apodiformes or the Caprimulgiformes.
The Early EocenePrimapus, found in England, is similar to both a primitive swift and theaegialornithids, which are in some aspects intermediate between swifts and owlet-nightjars. Fossil evidence demonstrates the existence of swifts during that period inEurope. 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;[5] here note that both theCaucasus Mountains and theAlps did not exist yet and aegialornithids were possibly present in North America.[6]By the late Eocene (around 35MYA), primitive hummingbirds started to diverge from the related jungornithids; the Middle EoceneParargornis (Messel, Germany) and the Late EoceneArgornis, found in today's southernmostRussia, 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.
^Mayr, Gerald (2002): Osteological evidence for paraphyly of the avian order Caprimulgiformes (nightjars and allies).Journal für Ornithologie143: 82–97.PDF fulltext
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