Succesvolle broedgevallen van Visdief Sterna hirundo op nestvlotten in het Friesche Veen

In Drenthe the Common Tern is a rare breeding bird (25-30 pairs). In 2003 several pairs were noticed to fight about an old tree stub as a breeding site in the Friesche Veen (Northern-Drenthe). As the stub was the only (vulnerable) nesting site available, we supplied the terns of a breeding raft in spring 2004. The raft measured 1.2x2.5 m and was built of PVC-tubes, a PVC surface and hod some additional floating tanks made of plastic. The surface was covered with seashells and the raft was provided with small sheds in the corners where the chicks could seek shelter from sun and rain. In 2004 we failed to obtain accurate data about breeding number and success, but from a distance terns were seen to attack approaching Egyptian Geese, Alopochen aegyptiacus, so probably breeding took place. In 2005 we installed a second raft and on 29 June did a check by boat. While approaching we were attacked by several adults. On one raft two birds were incubating a clutch, on the other were two chicks near fledging, two half mature chicks and one small chick. We assume that the chicks belonged to three different pairs. On the old stub one bird was incubating, so the colony existed of at least six pairs. During ourvisit three adults harassed one swimming chick. They pecked the chick on back and head. It is very unlikely that the chick finally survived the attacks and it is unclear why the adult birds attacked it.