Coeffects of temperature and photoperiod on the age-related timing of spring migration of Whooper swans via satellite tracking

Extrinsic and endocrine factors are important mechanisms influencing the timing of spring bird migration. Whooper swans (Cygnus cygnus), including 122 adults and subadults over one year old and 98 juveniles under one year old, were satellite tracked to obtain detailed locational data at a wintering ground from 2015 to 2020. We assessed the effects of temperature, wind, photoperiod, available food resources, and sex hormones on age-related spring departure time. Our results suggested that both photoperiod and temperature had the greatest impacts on the spring departure time of adults, subadults and juveniles when food resources and wind conditions were favourable. With increasing photoperiod and temperature, the peak sex hormone concentrations of adults and subadults occurred one month earlier than that of juveniles, which corresponded to the peak of their departure time. Our study showed the synergistic impact of photoperiod and temperature and their variation among adults, subadults and juveniles on the initiation of spring migration. Compared with juveniles, adults and subadults with spring migration experience and reproductive needs could adjust their behaviours more quickly before departure based on changes in environmental conditions.