Latitudinal gradients in clutch size: an extension of David Lack’s theory

In the European robin, Erithacus rubecula, there is a gradient in mean clutch size from the northern to the southern limits of the range of the species. Mean clutches of 6.3 occur in Scandinavia, 5.9 in central France, 4.9 in Spain, 4.2 in North Africa, and 3.5 in the Canary Islands (Lack, 1954). There is, in addition, a small longitudinal gradient within Europe, amounting to a decrease of about 0.4 from east to west, and in Britain clutches are slightly smaller than in areas immediately across the Channel in continental Europe. The gradient is, however, mainly latitudinal, and the pattern of geographical variation in clutch size in the robin is repeated in many species of birds, most of them passerines.