||The discovery of avian uncoupling protein (avUCP), a homologue of the highly thermogenic mammalian UCP1, in avian skeletal muscle may suggest a mechanism of nonshivering thermogenesis (NST) in birds. If avUCP generated heat by uncoupling respiration as does UCP1, however, active NST by this protein would limit ATP production for muscle contraction. Differential bioenergetics of subsarcolemmal (SSM) and intermyofibrillar (IFM) mitochondria in a muscle cell may allow for simultaneous contraction during shivering and avUCP-mediated NST. This study investigated adaptive physiological changes of cold-acclimated (CA) Japanese Quail compared to warm-acclimated quail. Prolonged cold exposure led to lower body temperatures and adversely affected growth. Despite these differences, however, all birds in both acclimation groups possessed avUCP mRNA and protein. Avian UCP was more prevalent in CA quail, but, contrary to predictions, was found only in IFM, suggesting that avUCP likely plays a supportive role during shivering than a central role in NST.