ARE LARGE CLUTCHES COSTLY TO INCUBATE - THE CASE OF THE PIED FLYCATCHER

The costs of incubation were studied in a population of Pied Flycatchers Ficedula hypoleuca both by gathering non-manipulative data and performing clutch size manipulations in three successive seasons. Enlarging clutch size caused a prolonged incubation and increased hatching failures in every year. The length of incubation did not differ between control and reduced clutches but was extended by 0.8 d in enlarged clutches. The manipulation of clutch size did not affect the weight of incubating females. However, in 1993 the incubation weight of females was negatively correlated with the length of incubation. Adverse spring weather most likely caused the many abnormalities and failures during incubation in 1993. Seven females disappeared and three dead females were found in nest-boxes during the incubation period. Moreover, in six nests one or two eggs were rolled to the rim of the nest cup presumably by the incubating female. All these observations suggest that incubation might be an energetic bottleneck for breeding and thus could be a contributory factor in the determination of clutch size in this species.