Quantification of species turnover rates on islands in the context of the equilibrium theory of island biogeography (MacArthur and Wilson 1967) depends on accurate inventories of the biota taken at appropriate intervals in time. Inaccurate historical data concerning species composition is a source of error that must be avoided when calculating turnover rates (Lynch and Johnson 1974). Johnson’s (1974) analysis of historical changes in species composition and abundance exemplifies the importance of sound avifaunal inventories for later comparison. In this paper, we present data to provide future researchers with a baseline from which to calculate turnover rates for the breeding avifaunas of two conifer “islands” in the Mojave Desert in California.