Determining the Sex of a Saw-whet Owl

Our 116 owl captures during our 2012 banding efforts resulted in a typical skewed sex ratio with high numbers of females. This is common across saw-whet owl banding stations and it is assumed that territorial males might not migrate or are not as attracted to the audiolure as are females, thus capturing fewer males.  

Our BCCER population looked like this: 77 females (66%), 13 (11%) males, and 26 (22%)of unknown sex.  So how do we determine an owl's gender anyway?  A method used by all Project Owlnet banders is to measure the wing chord and the mass (weight) of the bird. Together the results of two variables can help determine the sex. 

Measuring the wing chord distance; the distance from the wrist
to the longest  primary feather using the natural curve of the wing.

weighing the saw-whet owl in a tube to contain the bird

Of course there can be some overlap between the size of the male and female owl, and that becomes the "unknown" zone.  Below is a the discriminant analysis function using the wing chord and weight of known gender birds, to help banders determine sex. The red points are females, the blue  points are males. If an individual's measures fall in an area of overlap, we have to identify its gender as unknown.

a discriminant analysis function using wing chord and 
weight to predict the gender of a  saw-whet owl

To learn more about this function, or sexing and ageing saw-whets, go to the direct source, project owlnet!