Hatching-year (HY) and presumed HY Flammulated Owls (Otus flammeolus) were captured during a period of pre-migratory activity in central New Mexico from 2000 to 2003. Mass gains were evident through the pre-migratory period. Fat deposition was an important component of these mass gains; muscle growth appeared to contribute to a lesser degree. Fat scores and pectoral-muscle scores were positively related to body mass and to each other, and, from first to last capture, most recaptured owls showed increases in body mass that were accompanied by fat deposition and growth in pectoral muscles. These data add to a growing body of research indicating that pre-migration increases in fat and muscle mass may be interdependent, but the magnitude of increased muscle mass may be too small to be detected at certain scales.