Optimizing Great Tit Clutch Size in a Fluctuating Environment

Great Tits (Parus major) nesting Wytham Wood near Oxford, England, lay clutches averaging 8.53 eggs, although the most productive clutch size is 12. We evaluated various mechanisms that have been suggested to account for this apparent disparity, including (1) cost of reproduction, (2) Mountford's "cliff—edge effect," and (3) year—to—year environmental variability. Data on 4489 clutches, including 603 manipulated clutches, from the past 24 yr, offer weak or contradictory support for the first two mechanism. There are marked annual environmental fluctuations, and a geometric mean estimate of relative fitness best predicts the observed mean clutch size. Years that are poor for survival of young affect individuals laying layer larger clutches much more than they affect those laying smaller clutches. This "bad—years effect" reduces the mean and increases the variance in fitness for individuals laying large clutches more than it does for individuals laying smaller clutches. Hence it is more advantageous, in the long term, for birds to lay clutches smaller than the most productive clutch size. The cost of reproduction as it affects adult survival is neither necessary nor sufficient for clutch size optimization in Great Tits.