Used for all developmental stages of an organism, preceding the attainment of sexual maturity. In birds this period ranges from hatching to usually several months, but in some species it takes a few years before sexual maturity is reached. From hatching on one normally refers to the young birds as 'nestling' (when remaining in the nest during the first weeks), 'chick' (especially for quick nest-leaving, active young), or 'hatchling' (general term). Technically a young bird is called a 'pullus' until it is full-grown and flying. After the nest or pullus stage a young bird that is wearing its first plumage of true feathers is called 'juvenile' until attaining sexual maturity. Other terms describe the condition of the hatchlings in different kind of birds. 'Precocial' young are capable of locomotion more or less immediately after hatching, while others are 'altricial', i.e., incapable of locomotion. 'Ptilopaedic' young are more or less densely covered with down when hatched, while 'psilopaedic' young are naked or only have some dorsal down (and are usually blind). Young leaving the nest immediately or soon after hatching are 'nidifugous' (and necessarily ptilopaedic and precocial), while those remaining in the nest are 'nidicolous'. Intermediate conditions also exist of course, e.g., semi-precocial (well-developed but stay at nest because of feeding habits).

Alternative forms for immature : altricial, chick, hatchling, immatures, juvenile, nestling, nidicolous, nidifugous, precocial, psilopaedic, ptilopaedic, pullus, young.