In this paper I review what is known about the relationship between the tits' clutch-size and the timing of their breeding season and the numbers of their caterpillar prey. The results come from the Edward Grey Institute's long-term study of Great and Blue Tits in Wytham Wood, near Oxford. This study was started in 1947 by Drs D. Lack and J. A. Gibb (Lack 1966, Perrins 1979)- These results could only have been obtained from a long-term study and emphasize the value of such studies for our understanding of certain aspects of ecology. In addition, they could not have been obtained by a study of the birds alone, being dependent on a knowledge of their insect food; most of this information was made available as a result of a parallel long-term study of the insect fauna of the oak trees, made by a number of people from the Hope Department of Entomology, University of Oxford.