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In birds, the annual mean clutch size is often negatively correlated with population density. This relationship is at least in part due to adjustment by individuals. We investigated whether this response is adaptive in two ways. First we used an optimality model to predict how optimal clutch size (the clutch size that maximizes the number of breeding birds [recruits and surviving parents] in the next season) varies with density. We parameterized the model using data on fitness consequences of experimental variation in brood size and natural variation in population density in a Great Tit (Parus major) population. Predicted optimal clutch size decreased with density, but the predicted relationship was stronger than the observed relationship. Second, we investigated the relationship between the annual selection differential for clutch size and density. We found no relationship, indicating that there is no selection for a steeper than observed relationship between clutch size and density. This implies that the observed response is adaptive, and that the prediction of the optimality model lacks some important constraints or selection pressures. We further used the optimality model to examine the sensitivity of the optimal clutch size to density at different stages in the reproductive cycle. This analysis suggested that the nestling stage was most important. To the best of our knowledge this is the first study that makes quantitative predictions of optimal dutch size in relation to population density.