IN THIS ISSUE OF THE AUK, two papers provide a fascinating illustration of the state of studies of avian biodiversity. First, Whitney and Alvarez (1998) describe a new species of Herpsilochmus antwren from Peru and Ecuador. Second, Isler, Isler, and Whitney (1998) present an interesting application of vocal characters to delimiting species. This pair of papers provides material for reflection on the true dimensions of world avian diversity and its study. Slightly more than 50 years ago, Ernst Mayr (1946) stated "The total figure of 8,616 [bird] species [in the world] is ... probably within five per cent, and certainly within ten per cent, of the final figure . . . I doubt that in the entire world even as many as 100 new species remain to be discovered." A steady stream of descriptions of species new to science from diverse regions of the world nevertheless suggests that the supply is not yet exhausted. The Andes and adjacent regions of South America have proved to be a reliable source, as have Asia, southeastern South America, Africa, and even the Philippines and Mexico. The period 1941 to 1997 yielded at least 163 valid species descriptions, with no sign of exhausting the supply (Amadon and Short 1992). In fact, my estimates for a 1990s rate actually suggest an increase over rates from the 1970s and 1980s (Fig. 1), probably as a result of increased awareness of the importance of song characters in identifying new species (Parker 1991). Thus, Mayr's prophecy of