Sex and age determination of Short-eared Owl nestlings

We studied plumage patterns of known-sex nestling and juvenile Short-cared Owls (Asio flammeus) to develop a sexing technique for nestlings in the fie ld. Markings on the secondaries varied according to sex, and differences we re apparent from about 10-15 days of age. We also provide aging formulas ba sed on mass for nestlings up to 15 days of age and on wing length for nestl ings older than that age. Finally, we evaluate growth parameters according to sex. The asymptotes of body mass, wing length, and tarsus length growth curves were higher in females than males. The inflexion point was attained earlier by males than by females.