Experimental Evidence for a Causal Relationship between Timing and Success of Reproduction in the Great Tit Parus m. major

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SUMMARY (1) We investigated experimentally whether timing of reproduction is causally involved in the seasonal decline in reproductive success in the great tit Parus m. major L. This is important because of its implications for the optimization of reproductive decisions of the individual bird. (2) Timing of breeding was manipulated by inducing repeat clutches through removal of the first clutch. The experiment was performed in 1987 in two forests (W and 0) and followed up the year after. It was assumed that parental quality was unaffected by the ex-periment. Reproductive success of the experimental and undisturbed control clutches were compared. For forest 0, reproductive success of the experimental clutches was also compared with the expected reproductive success of first clutches laid at the same time. (3) Clutch size, fledging success and nestling weight in experimental clutches were reduced as compared with controls in forest W but not in forest 0. We conclude that late breeding causes reduced pre-fledging reproductive success in forest W but not in forest 0. (4) Recruitment of fledged young in the breeding population was strongly reduced by the experiment. We conclude that late breeding caused reduced post-fledging reproductive success in both forests. (5) Survival of females was reduced by the experiment in forest 0 relative to controls and this corresponded with the natural seasonal trend. Survival of females in W and of males in both forests was unaffected. In experimental females, clutch size in the next breeding season was reduced by 16 eggs compared with controls. The clutch size of the males was unaffected, as was the following year's laying date for both sexes. We conclude that future reproductive success of females, but not of males, is also related to the timing of current reproduction. (6) Possible mechanisms underlying the relationship between date and reproductive success are discussed. It is suggested that differences in pre-fledging reproductive success induced by the experiment are related to food availability. Differences in post-fledging reproductive success are probably caused by competition between early and late young.