Is the small clutch size of a Corsican blue tit population optimal?

In an attempt to test predictions of the optimisation hypothesis of life hi story traits in birds, we estimated fitness consequences of broad size mani pulations. Experiments were carried out over a period of 4 years in a Medit erranean population of blue tits Parus caeruleus which is confronted with a particular set of environmental constraints. Effects of brood size manipul ation were investigated in relation to year-to-year variation in environmen tal conditions, especially caterpillar abundance. There was a strong variat ion in the effects of brood size manipulation depending on year. Most effec ts were on offspring quality (fledging mass, tarsus length). The absolute n umber of recruits did not significantly differ among categories (reduced, c ontrol, enlarged broods) but varied considerably among years. Females recru ited from enlarged broods were of lower quality, started to breed later and laid fewer eggs than those recruited from control and reduced broods. Neit her parental survival nor reproductive performances of adults in year n + 1 was affected by brood size manipulation in year n. Thus there was no evide nce for a cost of reproduction in this population. Since the number of recr uits did not depend on brood size manipulation (recruitment rates were high er in reduced broods), but recruits from reduced broods were of better qual ity compared with other groups, we conclude that adults lay a clutch that i s larger than that which is predicted by the optimisation hypothesis. Produ cing more young could incur some penalties because offspring from large bro ods are of lower quality and less likely to recruit in the population. Two possible reasons why decision rules in this population seem to be suboptima l are discussed.