The aims of this study were to examine thermogenic mechanisms, their development and utilization in juvenile birds during the ontogeny of endothermy. The main focus of interest is shivering thermogenesis in muscle, when and where it develops and how its use is modified by the co-existence of the other forms of thermogenesis. The role of exercise, postprandial heat production and putative nonshivering thermogenesis is studied from this point of view. The perspective of the thesis ranges from the description of the ontogeny of thermogenesis and shivering to an investigation of the role of the other forms of thermogenesis. Paper I describes the endothermic development and attainment of homeothermy in the grey partridge. In Paper II, the development of shivering thermogenesis is studied in three precocial galliform species and in one altricial species. In Paper III, the existence of nonshivering thermogenesis after cold acclimation is investigated in Japanese quail chicks and domestic ducklings. In Paper IV, the participation of postprandial heat production in thermoregulation is examined in Japanese quail chicks. Paper V elucidates the role of exercise thermogenesis in thermoregulation of Japanese quail chicks. The specific subjects to be investigate were:
the efficiency of incipient shivering in heat production;
the utilization of nonshivering thermogenesis as a possible general phenomenon in cold-acclimated precocial chicks during cold spells;
the manner in which postprandial heat production influences shivering thermogenesis and the possible occurrence of substitution; and
exercise thermogenesis both as a potential benefit for young birds and as a possible influence on shivering.