Abstract
1. The breeding success of fieldfares Turdus pilaris was studied in 56 colonies and 16 solitary pairs in a mountain birch area of N Sweden, and in 40 colonies and 70 solitary pairs in S Sweden. The analysis is based on a statistical method that treats colonies, not nests, as independent sample units. 2. The probability of adult survival was the major difference between the areas. Adult breeding birds ran a much higher risk of predation in S than in N Sweden; the risk increased with colony size. 3. Predation on eggs and nestlings decreased with increasing colony size in both areas. 4. The total risk of offspring loss owing to predation (of either parents or offspring) decreased with increasing colony size in the north, but not in the south
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