Within-individual age variation of two melanin-pigmented traits convey different information in adults of common kestrels (Falco tinnunculus)

Abstract

Secondary sexual traits displayed by males and females may have evolved to inform about individual quality. Individuals of primer quality are expected to have benefits in intra- and inter-sexual interactions and ultimately in fitness. However, the covariation of the expression of sexual traits and individual fitness as individual’s age is still poorly known and controversial. Within a life-history perspective, ornament expression is expected to increase as individual’s age, despite decreasing their residual reproductive value. Nevertheless, environmental conditions may also drive both reproduction and the expression of phenotypes as individual’s age. However, little is known about the environmental influence on age-dependence variation of sexual traits. Here we study the age-dependence of the number and size of back spots, a melanin-pigmented and sexual trait in adults of both males and females of common kestrels (Falco tinnunculus). We also analysed the age-dependence of reproductive traits and the environmental influence, stated as food availability (vole abundance), using a 10-year individual-based dataset. We decomposed age-related changes into within- and between-individual variation to assess their contribution to population-level patterns. Our results showed a within-individual decrease in the number but not the size of back spots, only in males. Instead, size but not number of back spots was positively associated with food availability. In addition, reproductive performance of males increases as they age. No significant relationship was found for females neither in the age-dependence nor in the reproductive performance. We suggest that the number of back spots in males can be an age indicator and perhaps an index of reproductive quality. We suggest that genetic and environmental factors may explain the expression of two different characteristics of a same trait. The differences in the responses to age and environment between males and females studied traits may arise by different behavioural functions.