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
The Birds of America framed. They are all hanging throughout the public areas of the Woodstock Inn, in Woodstock, Vermont, which he built in 1969. Paisley Museum and Art Gallery, in Paisley Scotland has the four volume elephant folio of Birds of America. The Royal College of Physicians and Surgeons in Glasgow, Scotland holds one volume which is on display in the library, together with an interactive version. Though individual prints are commonly available, only one hundred twenty complete sets are known to exist. One complete copy of "The Birds of America" exists as part of The Darlington Collection at the University of Pittsburgh.