Great tits often lay clutches larger than they rear to independence. Although there are extensive studies on brood reduction in large obligate brood reductionists such as raptors and wading birds, knowledge on the ecological significance of brood reduction for most facultative brood reductionists is lacking. In this study, we investigated a population of the great tit in Tomakomai, Hokkaido to explore the ecological functions of facultative brood reduction. 300 nestboxes were set up and checked periodically to determine the number of eggs and nestlings in the breeding season of 2009. Out of 36 nests, brood reduction was observed in 14 (38.9%) nests, and it was observed more among lighter (less-conditioned) parents than heavier (better-conditioned) ones. When males and females are analyzed separately, only females’ weights were significantly related to the number of nestlings. Body weight of nestlings was significantly heavier for those reared in the nests without brood reduction than those with brood reduction. The results suggest that the rate of brood reduction in this population of great tits varies among pairs, and surplus eggs seem to serve as optimistic size adjustment of brood size, expecting a suitable condition for nestling rearing.