Family-Size in Titmice of the Genus Parus

In previous papers the hypothesis was advanced that, in birds, family-size is ultimately limited by natural selection, and that the normal clutch-size corresponds to the maximum number of young which can be successfully raised to maturity.It was shown that, as brood-size increases, so does the proportionate mortality among the young, either in the nest as in Apus melba (Lack and Arn, 1947), or shortly after the young leave the nest, as in Sturnus vulgaris (Lack, 1948).In the above two species, the increased mortality was sufficient to offset the initially higher broodsize, but it was not sufficient to do so in two species of Turdus, in which the position is complicated by the existence of phenotypic and adaptive modifications in clutch-size (Lack, 1949).Knowledge of these papers is assumed, and references in the present paper to the above species are cited from them.In Western Europe, there are six common species of Parus; the Great Tit P. major} Blue Tit P. caeruleus, Coal Tit P. ater, Marsh Tit P. palustris} Willow Tit (Chickadee) P. atricapillus and Crested Tit P. cristatus.Each of these has a somewhat different ~lutch-size.