RESUMEN
The transition between the breeding and nonbreeding states is often marked by a shift in energy balance. Despite this well-known shift in energy balance, little work has explored seasonal differences in the orexigenic neuropeptides that regulate food intake in wild animals. Here we tested the hypothesis that free-living male song sparrows (Melospiza melodia) show seasonal changes in energetic state, circulating steroids, and both neuropeptide Y (NPY) and orexin (OX) immunoreactivity. Nonbreeding song sparrows had more fat and muscle, as well as a ketone and triglyceride profile suggesting a greater reliance on lipid reserves. Breeding birds had higher plasma androgens; however, nonbreeding birds did maintain androgen precursors in circulation. Nonbreeding birds had more NPY immunoreactivity, specifically in three brain regions: lateral septum, bed nucleus of the stria terminalis, and ventral tegmental area. Furthermore, nonbreeding birds had more OX immunoreactivity in multiple brain regions. Taken together, the data indicate that a natural shift in energy balance is associated with changes in NPY and OX in a region-specific manner.