Absence of dispersal-mediated selection on the expression of a female ornament – a long-term study in a wild bird population

There is increasing evidence that female ornamentation is not merely a genetic by-product of male ornamentation and has evolved through sexual and/or social selection processes. However, the magnitude and direction of phenotypic selection on ornamental traits may fluctuate according to environmental conditions and life-history traits such as conspecific density and natal dispersal, affecting the covariation between ornament expression and fitness. Natal dispersal is assumed to be costly due to unfamiliarity and local maladaptation to the new environment. Since both dispersal and the expression/maintenance of ornamental traits are behavioural and energetically costly, selection pressures on ornamental traits are expected to differ depending on dispersal strategies. Breeding density, as a proxy of sexual and social competition, is also expected to influence the selection dynamics on ornaments. Here, we used a 34-year individual-based dataset of pied flycatchers (Ficedula hypoleuca) to analyse phenotypic selection on the occurrence and size of a heritable ornamental trait (the white forehead patch) in females in relation to their natal dispersal behaviour and overall breeding density. We found no differences in the relative number of recruits produced by female with and without a forehead patch. Moreover, neither dispersal nor breeding density influenced the magnitude or direction of phenotypic selection on the white forehead patch in female pied flycatchers. Lastly, we documented a temporal change in female ornament size, which may be explained by a genetic correlation between the sexes. We encourage further work to fully understand the factors that shape the temporal dynamics in female ornamentation.