PHENOTYPIC ADJUSTMENT OF CLUTCH SIZE DUE TO NEST PREDATION IN THE GREAT TIT

Nest predation may decrease the optimal clutch size for many reasons. In particular, a smaller clutch size may permit more investment in re-laying or in survival to the next reproductive event. The response to nest predation may be either genetic, as shown by comparative studies on birds, or phenotypic, as found in several aquatic species (invertebrates and fish). We have studied the effects of several years of high nest predation on clutch size variation in a Great Tit (Parus major) population. We tested whether nest predation was associated with selection on clutch size or with any phenotypic changes. In the year following peak nest predation, mean clutch size was reduced by about one egg. For nest predation, adult survival, and local recruitment, we failed to detect any dependence on clutch size; the reduction in clutch size was mostly phenotypic. Similarly, the return of clutch size to pre-predation values was explained as a phenotypic change. Surprisingly, individuals that did not experience nest predation themselves also modified their clutch size. This suggests that information on past predation persisted in the population for several years. In years with reduced clutch size, adult survival at the population level was higher, suggesting that a reduction in clutch size would be adaptive in case of nest predation, and that females adjusted their clutch size toward the optimum.