Avian clutch size of increasing clutch size with increasing latitude. The proximate and ultimate causes for this pattern have been a subject of intense debate involving the ideas of group and individual selection. David Lack observed a direct relationship between latitude and avian clutch size. Birds near the equator laid approximately half as many eggs as those that resided in northern temperate habitats. He observed an increasing clutch-size from the equator towards the poles (something he referred to as the “latitude trend”) for many passerine (perching) birds, near-passerine (tree-dwelling) birds and in various other groups: Strigiformes, Falconiformes, Ciconiiformes, Laridae, Ralliformes, Galliformes, Podicipedifomes, and
A History of British Birds as closely as I could", but admits that he had "in several cases" to rely on the stuffed "preserved skins" of his neighbour Richard Routledge Wingate. The grouping of species gave Bewick difficulty, as the scientific sources of the time did not agree on how to arrange the species in families, or on a sequence or grouping of those families. Bewick for example uses family groups like "Of the Falcon", in which he includes buzzards and sparrowhawks as well as what are now called falcons. The families of land birds are further grouped into birds of prey, omnivorous birds, insectivorous