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Can migrating Tengmalm's Owls Aegolius funereus be reliably sexed in autumn using simple morphometric measurements?

I investigate whether three standard, easily‐taken body measurements (wing length, tail length and mass) can be used in combination to determine accurately the sex of Tengmalm's Owls Aegolius funereus in autumn. A total of 135 migratory Tengmalm's Owls, caught in autumn 1999 on a small island between Sweden and Finland, were weighed, measured and accurately sexed using PCR‐based molecular techniques. While females were on average larger than males, discriminant analysis using wing length, tail length and mass could not satisfactorily classify males and females. Ringers are advised not to use these common morphometric measurements to sex individual Tengmalm's Owls in autumn, as the degree of overlap is too great, and even the largest and smallest individuals might be incorrectly sexed.