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

The costs of incubation were studied in a population of Pied Flycatche rs Ficedula hypoleuca both by gathering non-manipulative data and perf orming clutch size manipulations in three successive seasons. Enlargin g clutch size caused a prolonged incubation and increased hatching fai lures in every year. The length of incubation did not differ between c ontrol and reduced clutches but was extended by 0.8 d in enlarged clut ches. The manipulation of clutch size did not affect the weight of inc ubating females. However, in 1993 the incubation weight of females was negatively correlated with the length of incubation. Adverse spring w eather most likely caused the many abnormalities and failures during i ncubation in 1993. Seven females disappeared and three dead females we re 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 presumabl y by the incubating female. All these observations suggest that incuba tion might be an energetic bottleneck for breeding and thus could be a contributory factor in the determination of clutch size in this speci es.