NYCTIBIIDAE Potoos FAUNA PARAGUAY

NYCTIBIIDAE - POTOOS
Three Paraguayan species in a single genus. Though modern day Potoos are confined to the Neotropics, fossil evidence suggests they were once more widespread, with the oldest fossil dating from the Upper Eocene of Europe, some 25 million years ago. Strictly nocturnal they do not fly during the day under normal conditions.
Visually Potoos are arguably the most bizarre of all Paraguayan birds, having a huge head, long wings and tail and very short legs. They are stocky-looking with a fluffy plumage that make the bird seem much larger than it really is, and are intermediate between owls and nightjars in appearance. The posture is erect, and combined with the cryptic plumage enables the bird to imitate a broken stump when roosting. Potoos always roost alone. It is not unusual for a Potoo to remain completely motionless throughout the entire day, in fact they move so little that algae may grow on the tail feathers. When disturbed the bird sleeks the plumage and raises the head.
The bill is short with a tooth-like projection either side of the maxilla, but the gape is massive and frog-like. Potoos lack rictal bristles but do possess thin bristles on the loral area which either assist in the tactile capture of prey or protect the huge eyes. They feed almost exclusively on flying insects, sallying from a perch, and it is thought that the toothed bill may help to break the exoskeleton. The tough flexible skin of the inside of the mouth and oesophagus may prevent it from damage when swallowing spiny insects. The skin of the gape is highly vascularised and Potoos pant when hot.
Like nightjars, Potoos show eye-shine under artificial light, and the eyes are located laterally, high on the head to give a wide field of vision. The pupils dilate widely to allow maximum light entry, an adaptation to their nocturnal habits. Interestingly the eyelid contains two or three notches of uncertain purpose, though it has been hypothesised that they may enable the bird to see, even when the eyes are apparently closed, a useful skill given their cryptic defence. Comparatively the brain is tiny, of similar size to that of a hummingbird. Potoos have anisodactyl feet with long fleshy toes. There is a thickened pad of skin between the innermost toe and the hallux, an adaptation for spending long periods perched. There are five phalanges in the outer toe and three in the hallux. The claw of the middle toe is not pectinated. The wings are long and broad and despite their clumsy appearance Potoos are capable of agile, silent flight. Broad, fluffy feathers with aftershafts assist in noiseless flight. They have 10 primaries, 11 to 13 secondaries and 10 retrices. A large uropygial gland is present and powder down is located on either side of the rump. The primaries are moulted in descendant series.
Voice may be important in determining species limits. They call mostly at twilight and on moonlit evenings. Skeletally they have a schizognathous palate, a small basypterygoid process, holorhinal nostrils and a metasternum with two notches on each side. Large amounts of fat are stored in the abdominal cavity, possibly to help them survive short periods without feeding. Potoos are host to a unique genus of feather mite (Nyctibiolichus) and louse (Nyctibicola).
Potoos are monogamous breeders, laying a single elliptical egg directly into an unlined depression on a branch. Both sexes incubate, and the limited evidence suggests that males incubate during the day and females at night.

REFERENCES
Campbell B & Lack E 1985 - A Dictionary of Birds - T & AD Poyser.
Cleere N & Nurney D 1998 - Nightjars: A Guide to the Nightjars and Related Nightbirds - Pica Press.
Cohn-Haft M 1999 - Nyctibiidae Handbook of the Birds of the World Volume 5 - Lynx Ediciones.