Potoos roost by day on top of a snapped branch or fence post, body held vertical, head tipped back and bill closed so the outline reads as continuous with the wood. The plumage is mottled grey and brown in a pattern that matches lichen-covered bark. Movement would destroy the effect, so the bird simply does not move.
The eyelid slits solve the resulting problem. Small notches in the upper lid allow light through when the eye is closed, giving enough visual information to track a nearby animal without opening the conspicuous yellow eye. If the threat presses closer the potoo opens its eyes and its very wide gape in a startle display.
By night the strategy reverses: potoos are sit-and-wait hunters, sallying from a perch after large flying insects and, in the great potoo, sometimes bats. The mournful call carries far, and in several regions has attracted folklore. How much detail the eyelid slits actually resolve has not been measured optically.