River and sea: foraging range of freshwater and marine Common Terns

The foraging ranges of breeding seabirds differ between species and colonies, depending on species locomotion traits, food availability, environmental characteristics, and social factors. The Common Tern (Sterna hirundo) is a generalist seabird that breeds and forages in a wide variety of habitats in both freshwater and marine environments. Using GPS loggers, we compared the foraging range, foraging patterns, and the colony attendance of Common Terns from freshwater and marine colonies during the incubation and chick-rearing periods. Common Terns from freshwater colonies foraged at lower distances from colonies and had higher daily colony attendance than conspecifics from marine colonies in the Adriatic Sea. Maximum distance to the colony and 50% and 95% utilisation distributions were higher during the chick-rearing phase. Colony attendance was higher during the incubation and among females. Compared to freshwater terns, marine birds foraged later in the evening. Freshwater terns mainly foraged along rivers but also visited smaller lakes. Terns from marine populations were not recorded to forage in freshwater sites within the foraging range. Tracking studies with finer temporal scale and detailed spatio-temporal data on wind and tide dynamics are needed to understand environmental factors affecting foraging dynamics of Common Terns in the Adriatic Sea.