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 (inverteb rates and fish). We have studied the effects of several years of high nest predation on clutch size variation in a Great Tit (Pants major) p opulation. We tested whether nest predation was associated with select ion on clutch size or with any phenotypic changes. In the year followi ng 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-pre dation values was explained as a phenotypic change. Surprisingly, indi viduals that did not experience nest predation themselves also modifie d their clutch size. This suggests that information on past predation persisted in the population for several years. In years with reduced c lutch size, adult survival at the population level was higher, suggest ing that a reduction in clutch size would be adaptive in case of nest predation, and that females adjusted their clutch size toward the opti mum.