The effects of age (1 year-old vs older females) on laying-date and clutch-size of the Great Tit Parus major (in eight independent study areas) and the Pied Flycatcher Ficedula hypoleuca (five areas) were meta-analysed using both published and unpublished records. To standardize comparisons between areas and species, the standardized difference between the means of the two age groups was used as a measure of the magnitude of the age-effect (effect size).

In Great Tits the laying-date of ‘young’ females was 0.30 standard deviation units later and clutch-size 0.31 units smaller than that of ‘old’ females. In the Pied Flycatcher the corresponding figures were 0.45 and 0.81, respectively. All the effect size measures were highly significant but for clutch-size the effect size of the Pied Flycatcher was significantly greater than that of the Great Tit. In the Great Tit the effect size of clutch-size was heterogeneous across the different study areas.

It was estimated that the following numbers of unpublished studies showing null results would have to have accumulated in file-drawers before we could say that the results concerning effect sizes are due to sampling bias: for Great Tits 121 studies of laying-date and 196 studies of clutch-size; and for Pied Flycatchers 45 studies of laying-date and 139 studies of clutch-size.