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

Abstract

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 of a sample of neck collared individuals, and observations of marked individuals were used to assess rates of mate fidelity 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 all pairs ending in a divorce (n= 158). After divorce or death of the partner, birds re-paired assortatively with individuals from their own breeding area, using the same wintering area. New pair bonds formed on the breeding area, during one of three periods; just after return in spring, just after moult and just before departure in autumn. Divorce or partner death during the period 15 October - 28 February, when breeders were away from the breeding grounds, resulted in a significantly lower reproductive success during the following breeding season, compared to mate losses during the period 1 June - 14 October.