[2] 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.

[3] 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.

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 the Caucasus Mountains and the Alps did not exist yet and aegialornithids were possibly present in North America.

Various analyses place them sufficiently close to the Apodiformes to be included here, or into the unique owlet-nightjar lineage in the Cypselomorphae.