Size variation in the Mute Swan <i>Cygnus olor</i>

Sexual dimorphism in body size is a well-known phenomenon in the Mute Swan Cygnus olor, likely influenced by different selection pressures associated with its bi-parental care system. This study investigates geographic differences in morphological traits of Mute Swans wintering across four regions of Poland, focusing on variation by age and sex. Across all linear body measurements, males were consistently larger than females, with the most marked sexual dimorphism observed in tarsus-plus-toe length. This trait increased with age only in males, potentially reflecting functional adaptations related to sex-specific parental roles, such as enhanced take-off ability, locomotion, and territorial displays in heavier males. In contrast, total head length showed no age-related change in either sex, suggesting early developmental completion due to its role in foraging. The 4th primary length exhibited the greatest age-related increase, supporting the need for expanded wing area as body mass increases with age. The Sexual Dimorphism Index exceeded the Age Dimorphism Index for all traits by severalfold, indicating stronger influence of sexual selection over ontogenetic growth in shaping body size. Among measured traits, tarsus-plus-toe length, forearm length, and total head length demonstrated the highest measurement repeatability, whereas wing length and 4th primary length demonstrated reduced consistency in repeated measurements. Our findings highlight sex- and age-specific morphological variation shaped by differing selective pressures and underscore the importance of considering both biological and geographic factors in studies of avian body size.