Brood size manipulations were carried out to test whether clutch size variation in individual great tits (Parus major) controlled for laying date was tuned to their phenotypic quality and/or local food abundance (individual optimization hypothesis; IOH). Broods with different original clutch sizes, but equal hatching dates, were manipulated to a common brood size. A third brood was kept as a control. Under the IOH, we expected a positive association between reproductive success and original clutch size. Fledgling production varied in an inconclusive way after manipulation, with data from 1 out of 3 years favoring the IOH. The effect of manipulation on the probability of a second clutch was consistent with the IOH in another 1 out of the 3 years. When fitness accrued to second broods was also taken into consideration in terms of annual fledgling production, results from 2 out of 3 years tended to support the IOH. There was no effect of the manipulation on fitness (estimated as the number of recruits plus parents breeding in the next season). Both the clutch component (local recruitment) and the parental component (survival till next breeding season) varied inconclusively with respect to the IOH. On the basis of fitness measurements, the IOH could not be confirmed as an explanation for clutch size variation in this population. In 2 out of 3 years one of the three fitness components measured varied in accordance with the IOH. Overall the evidence for the IOH in this data set is therefore weak.