Spring stopover patterns of migrating Whooper Swans (Cygnus cygnus): temperature as a predictor over a 10-year period

The decision by birds to stop or not (i.e. stopover decisions) during migration can affect their fitness and also the function of ecosystems, as well as, in a detrimental way, agricultural activities by grazing, trampling and nutrient translocation. We studied how temperature influences the stopover pattern of a large grazing waterfowl. Specifically, we compared the variation in the duration, start and end of the stopover period of Whooper Swans (Cygnus cygnus) with temperatures during the pre-arrival period at Lake Tysslingen in south-central Sweden during a 10-year period. The start day of the stopover period was negatively correlated to the mean temperature of the pre-arrival period. However, the temperature during the pre-arrival period did not explain the variation in the end of the stopover period. Furthermore, the duration of the stopover period was negatively correlated to the start but not to the end of the stopover period. This pattern follows observations that temperature is closely linked to food supply and availability of open water in other waterfowl. In the light of our findings, we conclude that an increased temperature will result in a spatial and temporal shift in the stopover pattern of Whooper Swans along the migration route, and in our specific case the birds will probably arrive earlier in spring and consequently stay longer within the studied area. Moreover, the prolonged stay at the stopover site will probably also result in increased grazing pressure and risk of damage to crops.