7 AVIFAUNAL CENSUSES SPANNING ONE-HALF CENTURY ON AN ISLAND OF WHITE FIRS (ABIES-CONCOLOR) IN THE MOJAVE DESERT

I compared total counts of summer resident birds conducted in 1939, 19 73 to 1977, and 1989 on a pristine isolated island of white firs (Abie s concolor) on Clark Mountain, San Bernardino County, California. Avia n species richness and biomass fluctuated dramatically during the enti re 50-year interval, as well as between 1973 and 1977. Only three spec ies, Mountain Chickadee (Parus gambeli), Yellow-rumped Warbler (Dendro ica coronata) and Western Tanager (Piranga ludoviciana), were recorded in all seven censuses. The chickadee, the sole permanent resident, wa s relatively stable in numbers over all censuses. In contrast, summer resident species showed pronounced annual changes in abundance. Even d uring years of peak numbers, density of nesting birds was the lowest r eported for a montane coniferous island in the western United States. Relative abundance in the 1970s was correlated with regional range exp ansions of several species, possibly in response to increased precipit ation. Conversely, low numbers in 1989 were associated with widespread drought. In agreement with insular equilibrium theory, the small Clar k Mountain avifauna is vulnerable to chance extirpation and high turno ver. Species richness, however, showed no hint of equilibrium. Specime n-based identifications of subspecies and species of colonists demonst rated that the intermediate location of Clark Mountain encourages colo nists from three major mountain systems-Great Basin Ranges-Rocky Mount ains, Sierra Nevada-Coast Ranges, and southern Arizona-Sierra Madre Oc cidental.