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
British Birds (magazine) possible to bind 19 into one volume. In 2007 a DVD-ROM disk containing the first 100 years' worth of content (Vols 1-100) was released in conjunction with BirdGuides Ltd. Subsequent volumes are available as paid-for updates. In 2010, the magazine launched a blog, Facebook group and a Twitter account. From October 2011, the magazine's first 100 years' of back issues are available on-line, free. More recent issues are available to subscribers to the print edition. British Birds (magazine) British Birds is a monthly ornithology magazine that was established in 1907. It is now published by BB 2000 Ltd, which is