Askenmo C. (1982) Clutch size flexibility in the Pied Flycatcher Ficedula hypoleuca. ARDEA 70 (2): 189-196

The relations between female weight, clutch size and breeding success were studied in a population of Pied Flycatchers Ficedula hypoleuca. Clutch size increased with female weight, assumed to be a function of foraging conditions. This relation was interpreted as a phenotypical adjustment of the reproductive rate to the breeding conditions encountered by individual females. Heavy females produced more fledglings, and fledgling output increased with clutch size, which lend further support to the hypothesis. The interpretation of weight differences is discussed as well as implications of the proposed flexibility for the relations between clutch size, female age, season and reproductive effort.