How many species of birds have existed?

Bibliographic Details
Main Author:	Brodkorb, Pierce, 1908-1992
Format:	Book
Language:	English
Published:	Gainesville : University of Florida, 1960.
Series:	Bulletin of the Florida Museum of Natural History. Biological sciences ; v. 5, no. 3.
Subjects:	Birds > Classification. Birds, Fossil.

Description
Physical Description:	42-53 pages
Bibliography:	Bibliography: pages 51-53.