The between-season divorce rate of the European Blackbird was studied in 1997–2003 in two city parks in northwest Poland. Within the population studied, 52 and 51% of pairs in each park were observed to divorce. Of the eight parameters included in the analysis, the divorce rate was found to depend on marriage training and the time of territory acquisition. Territory quality, breeding success, synchrony in territory acquisition, age and size of the partners were unimportant. 19.5% of pairs with marriage training were observed to divorce, whereas 65.5% of pairs without marriage training did so. Birds acquiring their territories earlier divorced their partners significantly less frequently than those acquiring a territory at a later date. During the first post-divorce breeding season, young males that stayed in the park started breeding earlier than in the preceding year; old males started breeding later, whereas the difference was not significant in young or old females. Regardless of their age, the divorced birds showed a reduced number of fledglings raised with a new partner. The reduced breeding success may be a result of poor adaptation to increased pressure from predators. Similar divorce rates in pairs with and without breeding success and the rarer divorces among pairs claiming their territories earlier strongly support the habitat-mediated hypothesis. However, the more frequent desertion of poor territories by females, as well as differences found between older males and females in the timing of the onset of breeding before and after divorce, indicate that an individual may choose divorce to maximize fitness.
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Vol. 41 • No. 1