A scientific interest in birds dates back at least to the time of Aristotle. In his book Historia Animalium he describes over 140 species, their habits and physiology. At the present day, the number of people interested in ornithology, with and without scientific backgrounds, exceeds those interested in other vertebrates except perhaps humans, dogs, cats and horses. An indication of an affectionate attitude towards birds by scientists can be detected from the names of research publications. Auk, Condor, Ibis, Emu are important avian biology journals, not solely devoted to a single avian genus, but there are at present no journals with titles like Rat, Mouse, Guinea-pig, Hamster or Monkey, in spite of very substantial amount of research on these species.

Birds have been classified into nearly 10,000 different species. Nevertheless, when compared with the other classes of vertebrate, birds show a greater degree of uniformity of shape and size, while there is less diversity within an order of birds than within a family of the other classes of vertebrate. The latter is usually attributed to the selective pressure that the evolution of flight has imposed on them. The vast majority of present-day birds are capable of flight. The class of vertebrate to which birds are most closely related are reptiles.