This article describes bird species discovered since 1900. Before the 20th century, and into its early decades, the pace of discovery (and "discovery") of new species was fast; during this period, with numerous collecting expeditions into species-rich areas not previously visited by western ornithologists, up to several hundred new species per decade were being described. Many of these were of course not new to the local people, but since then, the pace has slowed, and new species are generally only being found in remote areas, or among cryptic or secretive groups of species. Nonetheless, several tens of species were described for the first time even during the 1990s. Considerable time can pass between discovery and publication, for a number of reasons. Individual countries particularly rich in species newly described during this period are: Brazil Colombia Peru Indonesia Philippines A number of individuals have been particularly prolific in describing new species, such as: Niels Krabbe Paul Coopmans Bret Whitney Species described that were not valid species A number of species described during this period have turned out not to be valid species. There are a number of reasons for this. The following is a list of these species: Cox's sandpiper, Calidris paramelanotos, in 1982, now known to be a stereotyped hybrid between males of the pectoral sandpiper and female curlew sandpipers. Bulo Burti boubou, Laniarius liberatus, from Somalia, in 1991, found to be a color morph of Laniarius nigerrimus. Kenyon's shag or Amchitka cormorant, Phalacrocorax kenyoni, in 1991, now considered to refer to small females of the pelagic cormorant. Mascarene shearwater, Puffinus atrodorsalis, in 1995, now considered indistinguishable from the tropical shearwater, Puffinus lherminieri bailloni (or P. bailloni bailloni) Brigida's woodcreeper, Hylexetastes brigidai, in 1997, now usually considered a subspecies of the red-billed woodcreeper pending thorough study. The Meise and AMNH reviews During the 20th century, ornithologists published a number of periodic reviews of newly described species. The purpose of each of these was to collect together in a single paper, for ease of reference, all new species' descriptions published in the period of study, and to present an analysis of these, indicating which represent valid species, and which, for various reasons, do not. The first such review was published in 1934, by the ornithologist Wilhelm Meise, covering the period 1920 to 1934. Meise presented his review to the Eighth International Ornithological Congress (IOC) in Oxford. The review listed 600 new