Classification Caprimulgiformes, as defined below, are not a monophyletic, but rather a paraphyletic order.This is so because the most recent Caprimulgiforme ancestor includes the monophyletic Apodiform order within its progeny. Elevating the 4 families cited below to their own orders while retaining the apodiforme order, as proposed in the order-level tree by Boyd J (2012), would remove paraphyly. However, monophyly could be restored more parsimoniously by moving the 4 apodiforme families together with the four families below into one enlarged "caprimulgiformes-apodiformes" order, termed "Strisores" by Mayr (2011).

The caprimulgiform birds are primarily crepuscular, their activity being largely limited to the periods of dawn and dusk, although they are also nocturnal when there is sufficient illumination, especially by moonlight.

Unlike the nightjars, the frogmouths (Podargidae) and potoos (Nictibiidae) are arboreal (tree-dwelling). The last normally sit crosswise on a branch and fairly upright, both when active and at rest, resembling small, long-tailed owls. However, there are numerous differences, mostly internal, between owls and caprimulgiformes. Externally, caprimulgiforms possess bills and feet that are not raptorial, a flatter head with eyes placed laterally rather than in a frontal facial disk, relatively shorter tarsi, and longer tails. 

Although aerial feeders, most of the true nightjars (Caprimulgidae) roost on the ground, rocks, or fallen trunks, but some prefer horizontal branches of trees, in which case they usually perch lengthwise along the branch.

Oilbirds (Steatornithidae), not represented here, possess a system of echolocation that permits them to fly freely in total darkness, an adaptation related to their roosting and nesting in caves. They leave their caves at dusk and return at dawn and are highly gregarious in their foraging behaviour as well as in roosting and nesting.

Boyd J (2012): Bird Taxonomy

Hackett et al (2008): Phylogenomic study of birds

Mayr G (2011) Strisores et al

caprimulgiform (2011). Encyclopędia Britannica Ultimate Reference