Phylogenetic relationships of the paraphyletic 'caprimulgiform' birds (nightjars and allies)

In the past years, paraphyly of the traditional 'Caprimulgiformes' (nightjars and allies) with respect to Apodiformes (swifts and hummingbirds) has been well established, but the relationships between the five 'caprimulgiform' family-level taxa remain controversial. These crepuscular or nocturnal birds differ in numerous anatomical features, and here an analysis of 69 morphological characters is performed. Except for the position of the Nyctibiidae (potoos), the topology of the single most-parsimonious tree agrees with the results of a recently published large-scale 'phylogenomic' study. Whereas molecular data support a clade including Nyctibiidae and the Steatornithidae (oilbird), potoos were shown to be the sister taxon of Caprimulgidae (nightjars) in the present analysis. A sister group relationship between Nyctibiidae and Caprimulgidae is strongly supported, both in terms of bootstrap robustness and the number of synapomorphies, and it is detailed that the morphological data are more likely to reflect the true relationships of these birds. A classification is proposed, and the term Strisores is introduced for a clade including all 'Caprimulgiformes' and Apodiformes. It is most parsimonious to assume a single origin of dark activity in the stem lineage of Strisores and a reversal to diurnal activity in Apodiformes. However, a fourfold origin of dark-activity in the stem lineages of Steatornithidae, Podargidae, Aegothelidae and the Caprimulgidae/Nyctibiidae cannot be conclusively excluded with the data at hand.