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
Birds of Australia tail for rapid changes of direction. In consequence, the unrelated birds that fill that role in the Americas and in Australia look and act as though they are close relatives. Australian birds which show convergent evolution with Northern Hemisphere species: Australian birds can be classified into six categories: For comprehensive regional lists, see: For Australia's endemic species, see: Other regional, state and island bird lists: National organizations Australian regional and state organisations Important regional references include: Full-coverage field guides in print are as follows, in rough order of authority: Birds of Australia Australia and its offshore islands and territories have