1998. For the first time a full checklist of world species contains around 18,000 alternative scientific and over 27,000 alternative English names which are used in modern field guides and scientific literature to describe 9941 bird species. Hundreds of identical names applied to two or more (up to six or seven) different species are also indicated.
A status column is used to indicate: general conservation status where relevant, e.g. globally threatened or possibly extinct; 1,647 species which are recognized by many specialists but not by all relevant authorities.
The introduction explains some relevant ornithological issues in simple terms, including taxonomy and species definition, also expressions such as gene pool, patricity, allospecies and superspecies. Full English and Scientific name indices indicate all species using any particular name, rather than giving a page number only.