Sexual size dimorphism in Tengmalm's owls Aegolius funereus outside the breeding season was investigated by trapping owls on migration during autumn. Owls were accurately sexed using PCR-based molecular techniques. Females were 4% heavier than males, but were c. 30% lighter than during the breeding season, when they carry large fat deposits and can be twice as heavy as males. Thus sexual mass dimorphism outside the breeding season was substantially lower than reported previously for the breeding season in this species. Females also had 2.5% longer wings and tail than males, but there was no difference in wing-loading, indicating that flight proficiency was similar between the sexes. Though the degree of sexual size dimorphism is small, large female size might be an adaptation for accumulating larger fat reserves, while smaller male size allows more efficient flight and hunting. This study highlights the importance of investigating sexual size dimorphism in owls outside the breeding season to avoid overestimation of female body mass.