Validation of the genetic sex assignments
Individuals with the combination of homozygote calls for Z-linked SNP and amplifying for W-linked SNP were assigned as females. Male individuals could show both homozygote or heterozygote calls for the Z-linked SNP. However, since males lack the W-chromosome, they should not show calling variants for the W-linked SNP. We validated these assignments using information on 36 adult breeding individuals (18 distinct females and 18 distinct males) that could be reliably sexed by morphological measures. Although males and females show certain overlap in body size, within a couple the male tends to have consistently larger tarsus and wings than the female (Henderson 1991; authors pers. obs.). During the breeding season and in early stages, we were able to identify breeding pairs because jackdaws are monogamic species forming long-term pair bonds and both sexes participate in building nests within cavities and in parental care (Hahn et al. 2021). To validate the genetic sex assignments, we selected ringed, sized and genotyped individuals from all pairs we identified in our population. We used these 36 individuals of known sex to assess how many were correctly sexed using our SNP approach.