Birds have traditionally been termed the best known group of vertebrates. This cliché, however, relates to the fact that living species are well described, even to the subspecies level. Birds are well known because they are diurnal (therefore easily observed) and because of their aesthetic appeal—birds are beautiful. Because of this appeal, birds have been subject to persistent scrutiny by amateur and professional naturalists alike, and their study has consequently contributed to the development of not only evolutionary thought, but also such diverse fields as ecology, behavior and even comparative physiology. However, while these disciplines have advanced, our understanding of bird phylogeny and evolution has simply not kept pace and the relationships of the higher categories of birds are still very poorly understood.