THE ROLE OF RECRUITMENT IN THE DYNAMICS OF A SIERRAN SUB-ALPINE BIRD COMMUNITY

The species richness and number of territories of all species combined in a l-km2 Sierran subalpine study area fluctuated dramatically over 7 summers, and were highly correlated with the timing of snowmelt. Over the same period, productivity (number of young fledged per breeding territory) remained relatively constant, with dominant species tending to show higher and more constant productivity than did rare species. No significant correlations were found, at least for dominant species, between productivity and either the total number of breeding territories or species richness, suggesting that productivity did not strongly depend on either the potential intraspecific or interspecific competitive regime. Data collected annually on the return rates of 96 breeding adult birds banded in 1979 failed to detect changes in their winter survival rates. These patterns suggest that the recruitment of new breeders, particularly young, is a major factor controlling the population dynamics of subalpine birds and that natal and breeding dispersal may involve an altitudinal component dependent upon environmental conditions at the start of the breeding season. -Author