Classification of birds is a contentious issue and keeps changing. Sibley & Ahlquist's Phylogeny and Classification of Birds (1990) is a landmark work on the classification of birds (although frequently debated and constantly revised). Evidence for the various orders seems to be fairly good, but the relationships between the orders are in a state of disarray. Evidence from modern bird anatomy, fossils and DNA have all been brought to bear on the problem but no strong consensus has emerged.
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