Thyroid hormones (TH) are key regulators of vertebrate metabolism and are thought to regulate development of endothermy in mammals and birds. To better understand TH effects on development of metabolic physiology in a small, precocial avian, we treated embryonic and neonate King quail (Coturnix chinensis) with triiodothryronine (T3) or the thyroperoxidase inhibitor, methimazole to induce hyper and hypothyroidism, respectively. Preliminary results suggest King quail hatchlings (up to 1dph) maintain constant oxygen consumption (VO2) as ambient temperature decreases from 35oC to 20oC. VO2 of older birds (>1dph) began to increase as ambient temperature cooled below 20oC. Previous work with other avian species (Red-winged Blackbird (Agelaius phoeniceus) and Double-crested Cormorant (Phalacrocorax auritus)) suggests hypothyroid treatment will delay maturation of endothermic capacity by affecting organ mass, such as heart and liver, as well as oxygen consumption rates and mitochondrial respiration of permeabilized skeletal muscle fibers. We expect T3 treated animals to obtain endothermic capacity more quickly after hatching than control, while the embryonic ectothermic phenotype typical of birds will persist for a longer duration after hatching in hypothyroid animals. Supported by NSF IOS 1146758 (EMD).