Change of mate in a Greylag Goose Anser anser population effects of timing on reproductive success

The population of the West Baltic Greylag Goose Anser anser is migratory,
breeding in southwest Scania, southernmost Sweden and wintering in the
Guadalquivir Marismas, southwest Spain, or in the Dutch Delta. This population
has been the subject of a long-term study o f a sample o f neck cottared
individuals, and observations o f marked individuals were used to assess
rates of mate fid e lity in this supposedly long-term monogamous species.
The annual divorce rate during the years 1985-2000 was 10.5% [n-415], with
29.7% of alt pairs ending in a divorce [n=158¡. A fte r divorce or death of the
partner, birds re-paired assortatively with individuals from th e ir own breeding
area, using the same wintering area. New p a ir bonds formed on the
breeding area, during one of three periods; ju s t a fte r re tu rn in spring, ju s t
a fte r m o u lt and ju s t before departure in autumn. Divorce o r p a rtn e r death
during the period 15 October - 28 February, when breeders were away from
the breeding grounds, resulted in a significantly tower reproductive success
during the following breeding season, compared to mate losses during the
period 1 June - 14 October.