For the first time the unique coloured pattern of the iris of buttonquails (Turnicidae) is described. This unique pattern is due to the presence of a dark-brown crescent in the iris below the pupil, whose form and extent varies in response to light conditions. This dark crescent is present in the eyes of all individuals of Turnix species at every life stage, a consistency that has not been previously observed for the iridal marks found in other avian groups.