Mutual ornamentation, age, and reproductive performance in the European starling

Ornamental traits expressed in both sexes are widespread among birds. Many studies have indicated that male ornaments develop through sexual selection. Female ornaments may be correlated effects of selection on males or have signaling functions in their own right. In the facultatively polygynous starling Sturnus vulgaris, both sexes possess iridescent throat feathers, which are actively displayed during Courtship by males. This paper investigates the reproductive correlates of throat feather ornaments in both sexes. Bird-perceived hue and components of feather length covaried strongly, and a summary variable (the first principal component [PCI]) was extracted using Principal component analysis. Sex and age-related differences were found for PCI. Positive assortative mating was found with respect to age and PCI. However, the relative influences of ornamentation and age on breeding variables were hard to separate, so effects may be related to other age-related variables. This provides a cautionary note for studies of ornamentation where age is unknown. However, we argue that throat feathers in starlings may signal age and age-related quality measures in both sexes. Older females with higher PCI scores bred earlier and laid larger clutches with higher hatching, success, older males with higher PCI scores had higher chances of becoming polygynous and attracting high reproductive quality females. PCI showed no relationship with parental care in either sex. Direct reproductive benefits for males and indirect genetic benefits for either sex may drive mate choice based on these age-related characters. However, only experimental manipulation can determine whether ornamentation signals quality variation within age classes.