Environmental versus endogenous drivers of body condition in the Mute Swan (Cygnus olor): a comparison of coastal and inland wintering populations

Body condition reflects energy acquisition and is a key trait in studies of animal behaviour, ecology, and conservation. We examined age-, sex-, and region-related variation in the body condition of Mute Swans ( Cygnus olor ) wintering in coastal and inland regions differing in mean daily temperature and harshness index of winter. The study was conducted in areas where swans had constant access to artificial food resources provided by the public. Winter body condition varied significantly with age, region, and season, but not with sex. The adults consistently exhibited higher body condition than juveniles, with immatures generally showing intermediate values, although this pattern varied regionally. Swans wintering in inland regions had higher body condition than those in coastal regions across age classes measured by scaled mass index. Body condition declined progressively over the winter season in all regions. This decline was steeper in adults wintering inland than in those in coastal areas, while juveniles showed similar rates of decline across regions. Despite widespread artificial feeding, marked spatial, seasonal, and age-related differences in body condition remained, indicating that winter energy reserves are regulated by both endogenous processes and regional environmental conditions. These findings demonstrate persistent energetic constraints during wintering, even under high anthropogenic food availability, suggesting that supplemental feeding does not fully compensate for climatic differences between wintering areas.