Data on egg size and their sources.

Data on within-clutch variation in egg size used in this study. Marginal eggs represent the later-laid, asynchronously-hatching eggs and were identified by dividing the hatching spread by the laying interval between subsequent eggs. If these calculations produced non-integers, we rounded the values up. Core eggs represent the early-laid, synchronously-hatching eggs and were identified by subtracting the number of marginal eggs from the average clutch size. Sizes of core eggs and marginal eggs were averaged to produce mean core egg size and mean marginal egg size, respectively. In our statistical models, we included proportional difference in egg size between core and marginal eggs as a covariate. This was calculated by dividing the difference in egg size between core and marginal eggs (mean marginal size minus mean core size) by mean core egg size.