Assessing cranial morphology for sex determination in juvenile alpine swifts (Tachymarptis melba)

The cranial morphology of juvenile alpine swift (Tachymarptis melba), a species adapted for an aerial lifestyle, has received limited attention. This study examined skull shape and size variation in 100 juveniles (57 females, 43 males), collected from 2020 to 2024 at Istanbul University-Cerrahpaşa after natural death. Using 3D scanning and 18 landmarks, we applied Procrustes ANOVA and principal component analysis (PCA) to assess the effects of sex, centroid size, body weight, and cranial length. Sex significantly influenced skull shape but not size. Centroid size showed no significant effect on shape in females or males. Body weight significantly impacted shape, more so in males than in females, whereas cranial length had no effect. PCA revealed minor sex-based shape differences along PC1, though not statistically significant. These results indicate subtle sexual dimorphism in juvenile skull shape, driven partly by weight, with no variation in size. This work highlights the role of early developmental factors in rapid morphological development, setting the stage for further ontogenetic studies.