Effect of timing and female quality on clutch size in the Collared Flycatcher Ficedula albicollis

Capsule: Laying date and female age appear to be related to clutch size. Aims: To test two hypotheses ('date' and 'quality'), which might explain why fewer eggs are laid late in the season. Methods: Four years of data and multivariate analysis were used to test the effects of timing of breeding and female quality reflected by morphological variables and age on clutch size in the Collared Flycatcher Ficedula albicollis. We estimated food supply during parental care by measuring diet composition of nestlings. Results: We distinguished the independent effects of date and age of females on clutch size. The type of prey fed to nestlings was different early and late in the season. Hence food supply during the nestling care period may be a limiting environmental factor that indirectly determines clutch size. Conclusion: Our results are consistent with the predictions of the date hypothesis, but the quality hypothesis was also partially supported. Depending on year effects, 30-50% of the variance in clutch size may be related to the timing of breeding and an additional 5-10% may be due to quality (age) differences between early- and late-breeding birds.