A great many different classifications of birds have been proposed. As reference to any series of bird books will make clear, there is no simple agreement. However, almost all are based on the so-called Wetmore order that is used in Checklist of Birds of the World by J. L. Peters and others (Museum of Comparative Zoology, Cambridge, Massachusetts). This was written over a long period, 1931–1987, and through that time views changed. Even then, no one thought that this was likely to be the final story. In part, this is because birds fossilize so poorly; they are small and have hollow bones. As a result, we have a very incomplete knowledge of their evolution and the relationships between many groups were simply not clear.
From The New Encyclopedia of Birds in Oxford Reference.
Subjects: Zoology and Animal Sciences.