Within and between season nest-site and mate fidelity in Common Terns (Sterna hirundo)

We studied nest-site and mate fidelity in renesting Common Terns (Sterna hi rundo), from 1993 to 1997, in a Common Tern colony breeding on six small ar tificial islands in the harbour area in Wilhelmshaven (German North Sea coa st). Implanted transponders made possible individual recognition of the adu lts throughout their lifetime. We compared intra-season rates of nest-site and mate fidelity with between-year rates. Intra-season divorce was never o bserved. On the other hand, inter-year divorce was estimated at about 25%, suggesting that the costs of intra-season divorce are higher or opportuniti es for divorce are lower than between years. For 75% of 26 pairs that renes ted within the same season, the distance moved was less than 4.3 m. For 75% of 57 faithful pairs between two consecutive seasons, the distance moved w as less than 1.25 m. Dispersal distances between and within years did not d iffer significantly but were clearly shorter than a paired random distribut ion of nests. Most of the birds changed the nest-site either when renesting or between years, but most of the new scrapes can be considered as lying w ithin the original territory. Thus, failure of the first breeding attempt i ncreased neither divorce nor nest-site dispersal as compared with between-y ear rates. We also studied the influence of parental age and the date of br eeding failure on distances moved in renesting birds. Late renesting pairs did not change the nest-site. A multiple regression analysis revealed that the longer the duration of the first breeding attempt and the older the pai r, the shorter the nest dispersal distance.