LUXURY IN FRESH-WATER AND STRESS AT SEA - THE FORAGING OF THE COMMON TERRA STERNA-HIRUNDO

We compared the foraging strategies of Common Terns Sterna hirundo in freshwater (Lake Jeziorsko, Brzeg, Poland) and marine environments (Mi nsener Oldeoog, German Wadden Sea). Body mass changes, nest relief and duration and number of feeding trips per day were studied by automati cally weighing the adults, using electronic balances under the nests. At the freshwater site, adults were lighter both before and after feed ing and gained less mass during a trip. In the Wadden Sea, single feed ing trips lasted longer than at the freshwater site and the terns made fewer trips per day. To achieve the same mass gain per day as in bird s in freshwater, trips at sea had to be longer and food intake per tri p was higher. The daily duration of absence for feeding and the daily mass gain were about the same in both areas. The limnetic feeders fini shed foraging earlier in the evening than the terns foraging at sea. T hese differences are consistent with the hypothesis that limnetic prey availability was consistent, whereas the tides limited the availabili ty of marine prey. In consequence, foraging over freshwater presents s everal advantages, such as higher colony attendance, better mate coord ination and better parental care.