Seasonal Variation of Clutch Size in the European Blackbird Turdus merula: A New Ultimate Explanation

Abstract

Seasonal patterns of clutch size, partial hatching, fledging and breeding losses were analysed in an urban blackbird population in Budapest during four successive breeding seasons from 1986 to 1989. Average clutch size was maximal mid-season, but timing of the clutch size peak varied from year to year according to the beginning of the breeding season. Contrary to expectation, partial losses both in egg and nestling stages were greatest mid-season, and seasonal patterns of successes corresponded well with the pattern of precipitation preceding egg laying. The amount of rainfall proved to be a good indicator of earthworm abundance, the main food of urban blackbirds. Seasonal patterns of hatching success proved to be quite different between four-and five-egg clutches, while fledging success did not differ significantly. The pattern of breeding success of females laying five eggs followed precipitation closer and earlier, while females laying only four eggs lost eggs or nestlings only when the amount of precipitation fell below a threshold level. Productivity of clutch size four reached that of clutch size five at the beginning and at the end of the season but not in the middle. Our results suggest a new ultimate explanation for the seasonal clutch size maximum in this species. Although the key factor seems to be changes in food supply during egg formation, it affects clutch size indirectly by changing the trade-off between fertility and hatching success throughout the season.