Linnaeus first included birds in the 6th edition of his Systema Naturae which was published in 1748. In it he listed 260 species arranged into 51 genera and six orders. The entries for each species were very brief; he did not include a description but instead provided a citation to an earlier publication, often to his own Fauna suecica which was published in 1746. Linnaeus generally followed the classification scheme introduced by the English parson and naturalist John Ray which grouped species based on the characteristics of their bill and feet.
The 10th edition appeared in 1758 and was the first in which Linnaeus consistently used his binomial system of nomenclature. He increased the number of birds to 554 species which filled 116 pages compared with only 17 in the 6th edition. For each species he included both a brief description and also citations to earlier publications. He maintained 6 orders as in the 6th edition but renamed Scolopaces to Grallae. He rearranged some of the genera, dropping several and adding others to bring the total to 63.
The International Commission on Zoological Nomenclature chose 1 January 1758 as the "starting point" for zoological nomenclature, and asserted that the 10th edition of Systema Naturae was to be treated as if published on that date. In 2016 the list of birds of the world maintained by Frank Gill and David Donsker on behalf of the International Ornithologists' Union included 448 species for which Linnaeus's description in the 10th edition is cited as the authority. Of the species 101 have been retained in their original genus and 347 have been moved to a different genus. In addition, there are five species on Linnaeus's 1758 list that are now considered as subspecies. Of Linnaeus's 63 genera, only Tantalus and Colymbus are not now used.
In the 12th edition of his Systema Naturae published in 1766, Linnaeus described many additional birds that had not been included in the 10th edition. The 12th edition included 931 bird species divided into 6 orders and 78 genera. The 12th edition is cited as the authority for 257 modern species of which only 25 have been retained in their original genus. There are now believed to be around 10,000 extant species.
Linnaeus described the class Aves as:
A beautiful and cheerful portion of created nature consisting of animals having a body covered with feathers and down; protracted and naked jaws (the beak), two wings formed for flight, and two feet. They are aereal, vocal, swift and light, and destitute of external ears, lips, teeth, scrotum, womb, bladder, epiglottis, corpus callosum and its arch, and diaphragm.