Snow D.W. (1969) An analysis of breeding success in the Blackbird, Turdus merula. ARDEA 57 (3-4): 163-171

In British Blackbirds, a minority of the nests started are successful. Due to a combination of factors, nesting success starts moderately high in March, decreases to a minimum in April, and then increases to a maximum at the end of the season. The factors which combine to produce this trend (stimulation of very early and late nesting by favourable weather, earlier and later nesting by older birds, maximum of predation about a month after breeding has started, increasing cover as the season advances) are to a large extent independent of the average suitability of environmental conditions for rearing young. The analysis of partial losses, of death of young in the nest, and of total losses caused by bad weather, shows however that conditions for rearing young are in fact most favourable in the middle part of the season, if predation is excluded, and thus supports the hypothesis that the breeding season, and clutchsize variation within the season, are both ultimately adapted to the seasonal changes in environmental conditions that affect the birds' ability to rear young. The factors that tend to depress nesting success in the middle of the breeding season are not likely to be peculiar to the Blackbird. It may therefore be expected that other species with long breeding seasons may show seasonal variation in overall nesting success that runs counter to the variation which would be obtained if the suitability of environmental conditions for rearing young were the only factor involved.