A New Method to Sex Barn Owls Tyto alba

Nominate Barn Owls Tyto a. alba can be sexed based on flecking patterns of the underwing and underbody, but in the Dutch population consisting of the nominate subspecies and T. a. guttata, overlap in flecking exists between the sexes. In this paper we describe a new method to sex Barn Owls based on apical bar widths of the primaries. Data were obtained from 240 dead Barn Owls that were sexed by autopsy. The apical bars of the primaries 10 and 8 were narrower than 7.5 mm in males and wider than 7.5 mm in females. The widths of more basal bars of the primaries and secondaries overlapped between the sexes. The method was validated by providing 20 carcasses to 10 instructed volunteers, who measured the apical bar widths to sex the birds. The measuring error per owl specimen averaged at 2.2% (range 0.8–3.9%), and the proper sex was assigned in all cases.