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 correspondents. Those who provided skins or information are acknowledged. The species are grouped into families such as "Of the Falcon", using the limited and conflicting scientific sources of the time. The families of land birds are further grouped into birds of prey, omnivorous birds, insectivorous birds, and granivorous birds, while the families of water birds are simply listed, with related families side by side. Each species entry begins on a new page; any spaces at the ends of entries are filled with tail-pieces, small, often humorous woodcuts of country life. "British Birds" remains in print, and has attracted the attention