Temporal changes in sex-specific cryptic sexual dimorphism and allometric scaling in the long-lived Alpine swift Tachymarptis melba

Morphological differences between the sexes are frequently reported in wild populations, ranging from subtle to extreme. These differences can extend beyond body size itself and result in differences in the size and/or shape of specific traits in relation to overall body size, as revealed by allometric scaling. Sexually selected traits have historically been expected to display positive allometric scaling (i.e., relatively larger trait size in bigger individuals), although recent works rather suggest that sexually selected traits can also display negative allometric scaling (i.e., relatively larger trait size in smaller individuals). We used a long-term dataset to quantify the sexual dimorphism and sex-specific allometric scaling of morphometric traits in a wild bird generally characterized as monomorphic, the Alpine swift. We identified subtle sexual dimorphisms suggesting that the Alpine swift is rather a cryptically dimorphic species. Fork length was the most sexually dimorphic trait, with males displaying 7% longer forks than females. Interestingly, we found that the extend of this sexual dimorphism has changed over the course of the study, such that male and female feather traits are getting more similar. We further identified sex- and trait-specific allometric scaling, as well as possible sex-specific changes in allometric scaling of fork length over the course of the study. The negative allometric scaling of morphometric traits in the Alpine swift suggests that selection has favoured reduced investment in these body structures for larger individuals, and confirms that, even in subtly dimorphic species, sexually selected traits can display negative allometric scaling.