Egg size is a particularly important life-history trait mediating maternal influences on offspring phenotype. Females can vary their egg-size investment in relation to environmental circumstances, their own breeding condition, and the quality of their mate. Here we analyzed inter- and intraclutch variation in egg size in the Collared Flycatcher (Ficedula albicollis) on the basis of eight years of data. According to our results, mean egg size increased with female condition, but did not differ among young, middle-aged, and old females. The male’s age, body size, and forehead patch size did not influence egg size; thus, we found no evidence for differential investment in egg size in relation to male quality. We found no effect of laying date on egg size when controlling for ambient temperature during the egg formation period, yet temperature had a significant effect on egg size. That result indicates proximate constraints on egg formation. Furthermore, we report on annual differences in intraclutch egg-size variation. Egg size increased within clutches in years with a warm prelaying period; whereas in years when the weather during that period was cold, there was no significant intraclutch trend. Proximate considerations seem to explain the observed patterns of intraclutch egg-size variation; however, we cannot reject the adaptive explanation. Mean egg size and intraclutch egg-size variation were unrelated to clutch size. Therefore, we found no evidence for a trade-off between size and number of eggs within a clutch.
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Vol. 122 • No. 2