In ornithological terms, a juvenile bird (Japanese: yōchō) generally refers to a bird in the period after hatching and developing full plumage until it completes its first molt. This means it is more developed than a chick (Japanese: hina), which is newly hatched and lacks full plumage, but younger than an immature bird (Japanese: wakadori), which has reached a physical size comparable to an adult. However, the term 'hina' is sometimes used to refer to all pre-fledging juveniles and immature birds, making usage inconsistent; in such cases, a juvenile is considered one stage of a chick's growth. Furthermore, as seen in the term 'juvenile ratio' (yōchō-ritsu), which refers to the proportion of immature individuals (including so-called 'immature birds' capable of flight) within a flock's total population, it is sometimes used as a general term for 'birds in an immature stage of growth.' Since the 'juveniles' targeted by the juvenile ratio are often 'birds that have fledged (independent from parents but not yet fully mature),' one must conclude that there are too many exceptions to provide a definitive definition. For these terms, refer also to the list under 'Ornithological Terms #Breeding.' It chirps with sounds like 'Piyo,' 'Pi,' or 'Piyo!'