That the clutch size in nidicolous species is adapted to the largest number of young for which the parents can provide enough food, is known as Lack's theory (Lack 1954). It follows that the number of chicks that the parents can effectively rear must be dependent on the food requirement of each chick. This point has been neglected both by those who support Lack's theory and by those against it, perhaps because it has been assumed that the energy requirement by nestlings is more or less independent of the area, the season, or the brood size itself. Hence, some authors, e.g. von Haartman (1954) and Skutch (1967), thought that the fact that there was no appreciable reduction in clutch size in those species in which a single parent tends the young, as compared with those in which both parents share the task, was contradictory to Lack's theory. My observations (Royama 1966 a) showed that the food requirements per chick in broods of great tits Parus major L. under natural conditions, and those of blue tits P. caeruleus L. under laboratory conditions, varied inversely with the number of chicks in the nest, and this was considered to be due to greater heat loss by the chicks in small broods than in large ones. The inverse relationship is such that there would be no appreciable reduction in the total food requirement by the brood when its size is halved, and thus natural selection would not favour those individuals which lay distinctly reduced clutches when they have to feed the young single-handed.