Re-pairing – taking a new mate after failure of the first breeding attempt – within the breeding season is unusual for seabirds (Schreiber & Burger 2002). In the monogamous Common Tern Sterna hirundo, mate change may happen between, but not within the season (Gonzáles-Solís 1999a). For a long-term study on the population ecology of Common Terns, since 1992, all fledged chicks in the Banter See colony in Wilhelmshaven (North-West Germany) have been fitted with transponders (TROVAN ID 100, 11 x 2 mm), allowing remote identification of individuals. The transponders are subcutaneously implanted, require no battery and have an unlimited life. Since 1993, all breeders in the colony were checked by placing portable antennas at the nests to identity the marked breeders (see Fig. 1). Around the colony, there is a registration system consisting of 44 resting platforms equipped with antennas to record marked terns whenever they use them (for more details see Wendeln & Becker 1997; Becker et al. 2001; Ludwigs & Becker 2005), allowing to gather information on individual arrival. In 2001, the male “Birk” (transponder ID 0015FE81, ring number 7729576, fledged in 1996) bred with the female “Ronja” (ID 0134EB35, ring 7745054, fledged 1997), just as in the previous year. “Ronja” arrived at the colony on 4th May, and “Birk” on 8th May, and terns were observed on nest number 529 during day and night. The first egg was laid on 20th May. After the clutch was completed (3 eggs), the female was observed incubating for the last time on 30th May, and disappeared during the beginning of June (she probably died). “Birk” incubated the clutch for a few days, but then deserted it. Later in the season, “Birk” was observed incubating a new clutch of nest no. 917 with a new partner (the female “Hanni”, ID 01D207AF, ring 7728684, fledged in 1998).