They made detailed studies of the oilbirds and the Bearded bellbird. Oilbirds and most Daddy longlegs are trogloxenes. The eyes of oilbirds are highly adapted to nocturnal foraging. The order Caprimulgiformes includes four families; oilbirds, frogmouths, potoos, and the nightjars. The palm tree forests produce ample fruit that attracts oilbirds. These caves are inhabited by oilbirds, the only nocturnal bird in the world, and bats. Although related to owls, their closest relatives are the oilbirds, potoos, owlet-nightjars, and true nightjars. Oilbirds are reported to eat its fruit and disperse its seeds. Oilbirds are related to the nightjars and usually placed with these in the order Caprimulgiformes.