SummaryAfter a comparison of the numbers of avian species counted by Kikkawa&Pearse (1969), Pianka&Schall (1981) and Blakerset al.(1984), statistical methods are used to evaluate the closeness of the relationship between these numbers and the main environmental parameters such as rainfall (used among other climatic parameters by Pianka&Schall) and the chief characteristics of vegetation and the land surface. While it is acknowledged that rainfall is of vital importance, it is shown here that its relevance acts indirectly through its influence on the vegetation, and particularly the height of the vegetation, which offers the greatest variety and number of avian niches. The closest statistical correlation is obtained from a combination of relative relief of the land surface, variety of vegetation types, height of the vegetation and closeness of the plant cover. Gradual spatial variations (clines) in the density of avian species are examined and the concept of eco-biological escarpment (steep spatial change in numbers) is introduced. Maps are presented to show the geographical patterns of clines and escarpments (latitudinal, longitudinal and oblique) in the numbers of avian species in Australia. Map 8 shows a synthesis of avian species clines, revealing the dominance of NE-SW and NW-SE discontinuities along the western and north-eastern Torresian boundaries.