Supplementary Methods
Is clutch size a sex-limited trait?
To test whether clutch size can be considered as a trait of the laying female, we ran a general linear random effects model including male and female identity, restricted to males and females that bred more than once and with different partners. Clutch sizes of the same pair over multiple years were averaged. All clutch sizes were first corrected for year and female age. This data set consisted of 318 combinations of 226 males and 208 females. Whereas female identity explains 39 % of the total phenotypic variance (z = 4.27, p<0.001), male identity explains a non-significant 7.7 % (z = 0.56, p=0.29).
Experimental manipulations.
In several years clutch and brood size manipulations (either reduced or enlarged) were performed. In all analyses the clutch size before manipulation was used. Local survival nor local recruitment differed significantly between manipulated and unmanipulated clutches (generalised linear models with binomial and poisson errors, respectively: χ 2
(1)
=0.73, p=0.39 and χ 2
(1)
=0.81, p=0.37). Furthermore, including manipulated or not into any of the analyses did not change the significant terms in the final model, and the term itself was never significant. The a priori exclusion of manipulated clutches would reduce statistical power and result in a biased subset of the data since early or large clutches, and the clutches of immigrant females were more likely to have been manipulated (generalised linear model: χ 2
(1)
=3.85, p=0.050
,
χ 2
(1)
=9.62, p=0.002, χ
2
(1)
=7.00, p=0.008, respectively).
Pedigree reconstruction.
The pedigree used included all birds that have been recorded breeding on Vlieland.
The pedigree was established by assuming that both sexes are the genetic parents of all offspring to which they provide parental care, and that all individuals that a

nestling shares a nest with are its full siblings. This is assumption is probably reasonable since both extra-pair fertilisations and intraspecific brood parasitism were found to be very rare in this population 1 . Since both phenomena will introduce errors in the pedigree they are likely to make estimates of genetic parameters more conservative.
Animal model.
First year breeders lay significantly smaller clutches than older females (old – young
= 0.18 ± 0.059) (general linear mixed model: year: F
20,636
=4.83, p<0.001; age:
F
1,636
=10.1, p=0.016). Both age and year were included as fixed effects. Female identity was fitted as an additional random effect to account for permanent environment effects. There is no evidence for either a maternal effect or a genetic correlation with laying date (unpublished results). Clutch size data on females from
1965 up until 2003 were used to prevent that breeding values of individuals that bred around 1975 and 1995 where predicted less accurately. No distinction was made between the East and West, and the subpopulation of breeding was thus not included in the model, nor was the origin of the bird. This was done to prevent biases in predicted breeding values as a result of the fact that by definition less information is available for immigrants. Furthermore this would make our tests unnecessarily conservative. Analyses were performed using breeding values for males. These are predicted from the clutch sizes of their female relatives, and are not biased by the observations on the individuals themselves, as opposed to predicted breeding values for females.
1. Verboven, N. & Mateman, A. C. Low frequency of extra-pair fertilizations in the Great Tit Parus major revealed by DNA fingerprinting. J. Avian Biol. 28, 231-239
(1997).