J. Ornithol. 140,491--498 (1999) @ Oeutsche Ornithologen-GesellschaftlBlackwell Wissenschafts- Verlag, Berlin ISSN 0021-8375 Within and between season nest-site and mate fidelity in Common Terns (Sterna hirundo) Jacob González-Solís *, Helmut Wendeln and Peter H. Becker Institut für Vogelforschung "Vogelwarte Helgoland", 0-26386 Wilhelmshaven * Presentaddress and correspondingauthor: JacobGonzález-Solís,Marine Life Sciences Oivision, British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 OET, United Kingdom. E-mail: jacob@porthos.bio.ub.es \ Summary We studied nest-site and mate fidelity in renesting Common Tems (Sterna hirundo), from 1993 to 1997, in a Common Tem colony breeding on six small artificial islands in the har- bour afea in Wilhelmshaven (Gerrnan North Sea coast). Implanted transponders made pos- sible individual recognition of the adults throughout their lifetime. We compared intra- ¡:;.; season rates of nest-site and mate fidelity with between-year rates. Intra-season divorce I was never observed. On the other hand, inter-year divorce was estimated at about 25%, suggesting that the costsof intra-season divorce are higher or opportunities for divorce are lower than between years. For 75% of 26 pairs that renestedwithin the sameseason, the distancemoved was les s than 4.3 m. For 75% of 57 faithful pairs betweentwo consecutive seasons, the distancemoved was les s than 1.25 m. Dispersal distances betweenand within yearsdid not differ significantly but were clearly shorter than a paired random distribution of nests.Most of the birds changedthe nest-site either when renesting or between years, ~;C but most of the new scrapes can be consideredas Iying within the original territory. Thus, failure of the first breeding attempt increased neither divorce nor nest-site dispersal as I compared with between-year rates. We also studied the influence of parental age and the date of breeding failure on distances moved in renestingbirds. Late renestingpairs did not ..' changethe nest-site.A multiple regression analysisrevealedthat the longer the duration of the first breedingattempt and the older the pair, the shorterthe nestdispersal distance. , Key words: age, breeding failure, divorce, nest dispersal, renesting Zusammenfassung Neststandort- und Partnertreue ersatzbrütender Flu8seeschwalben (Sterna hirundo) Van 1993-1997 untersuchtenwir Neststandort- und Partnertreueersatzbrütender Flu6- seeschwalben (Sterna hirundo) in einer Kolonie, die im Banter See,Wilhelmshaven, auf sechs künstlichen Inseln brütet. Transponder errnoglichten die lebenslangeindividuelle Erkennung der Altvogel. Neststandort- und Partnertreue innerhalb der Brutsaison und zwischen den labren wurden verglichen. Innerhalb der Brutsaison kam Scheidung nicht varoEine Trennungsrate von 25% zwischen den labren deutet darauf hin, da6 die Kosten einer intrasaisonalenScheidung hoher sind als die diejenigen einer Scheidung zwischen den labren. 75% der ersatzbrütenden Paare wiihlten den neuen Neststandortweniger als 4,3 m vom Platz des Erstgeleges entfemt (n = 26). 75% der partnertreuen Paare brüteten U.S.Copyright Clearance Center Code Statement: 0021-8375/99/14002--0491 $ 11.00/0