Mass variation in Northern Saw-whet Owls: implications for current sexing criteria

ABSTRACT.   Currently, a protocol using body mass as the primary criterion for assigning sex of Northern Saw-whet Owls ( Aegolius acadicus ) is being used by banders. We assessed mass variation of owls using paired measurements of those recaptured ( N = 372) during fall migration along the lower Delmarva Peninsula (1994–2005) to assess the stability of gender assignments resulting from the protocol in current use. Mass variation ranged from 0 to 26.0 g, with a mean of 6.0 g. There was a positive relationship between body mass and the magnitude of the mass change between measurements. Variation in mass between captures caused a considerable shift in gender designations. Changes were most pronounced for birds classified as male ( N = 43), with 58% remaining either definite or probable male and the remainder changing to either unknown (28%) or female (14%). Female designations ( N = 317) were more stable, with 76% remaining either definite or probable female. Variation in mass and associated changes in gender assignments caused a two-fold shift in perceived sex ratio from 1:13 to 1:6 (M:F). Given that meals cause variation in mass that is large relative to the differences between sexes, we suggest that mass carries an unacceptably high level of uncertainty to be useful in assigning gender. Sex ratios and other demographic parameters generated using the current technique should be used with caution.