Primary sex ratio adjustment to experimentally reduced male UV attractiveness in blue tits

The study of primary sex ratio adjustment in birds is notorious for inconsistency of results among studies. To develop our understanding of avian sex ratio variation, experiments that test a priori predictions and the replication of previous studies are essential. We tested if female blue tits Parus caeruleus adjust the sex ratio of their offspring to the sexual attractiveness of their mates, as was suggested by a previous benchmark study on the same species. In 2 years, we reduced the ultraviolet (UV) reflectance of the crown feathers of males in the period before egg laying to decrease their attractiveness. In contrast to the simple prediction from sex allocation theory, we found that the overall proportion of male offspring did not differ between broods of UV-reduced and control-treated males. However, in 1 year, the UV treatment influenced offspring sex ratio depending on the natural crown UV reflectance of males before the treatment. The last result confirms the pattern found in the previous blue tit study, which suggests that these complex patterns of primary sex ratio variation are repeatable in this bird species, warranting further research into the adaptive value of blue tit sex ratio adjustment to male UV coloration.