1. Starling (Sturnus vulgaris) clutches were manipulated so that sibling eggs were incubated within either natural-sized clutches, or clutches that had been experimentally enlarged but were still within the natural range of variation. Thus the consequences of incubated clutch size for hatching success were investigated.
2. Eggs incubated within enlarged clutches hatched less successfully than eggs incubated within natural-sized clutches, suggesting that clutch size affects the conditions experienced by embryos during incubation.
3. Eggs incubated in enlarged clutches may have hatched poorly because clutch enlargement altered nest microclimate, causing increased water loss during the incubation period.
4. There was no evidence that enlargement altered nest microclimate by energetically constraining parents from incubating effectively. Instead, intrinsic physical properties of enlarged clutches affected clutch temperature directly.
5. Parents that had incubated experimentally enlarged clutches subsequently fledged fewer chicks than control parents, suggesting that constraints imposed during incubation may influence the optimal number of eggs that parents should lay.
6. Future studies should investigate whether parents laying naturally large clutches can minimize the problems of incubating many eggs by adaptively tailoring the shape and composition of their eggs to their expected clutch size.