Habitat associations of the San Clemente sage sparrow (Amphispiza belli clementeae)

I studied habitat associations of the San Clemente Sage Sparrow (Amphispiza belli clementeae), an endemic resident to San Clemente Island, California, between 2005 and 2006 at the population and territory spatial scales. At the population scale, habitat variables and Sage Sparrow densities were measured along 25 line transects in each year; at the territory scale, habitat variables and territory size were measured within 30 Sage Sparrow territories in each year. A high proportion of the variation in Sage Sparrow density among the transects was explained by habitat variables in 2005 and 2006 and the variables were generally consistent across the two years. Density in both years was positively correlated with increasing shrub and Lycium californicum cover, overall vegetation cover, and a decrease in grass ground cover, and horizontal heterogeneity. Conversely, a relatively low proportion of the variation in Sage Sparrow territory size was explained by habitat variables in 2005 and 2006, and associated variables were largely inconsistent between years. Larger territories were generally associated with less vegetation cover, lower shrub cover, and increases in sub-shrubs and grasses. Strong differences in density and territory size habitat associations indicate that Sage Sparrows select or respond to different resources at the two spatial scales measured in this study. Sage Sparrow population size and vegetation structure and composition changed greatly between years due to dramatic difference in rainfall. However, sparrow densities on transects in 2005 and 2006 were highly correlated. The strong correlation in bird densities between years is likely due to the consistent cover and distribution of perennial shrub species, especially Lycium, on the transects. Lycium and other perennial shrubs are an integral part of Sage Sparrow habitat and are their primary nesting substrate. In 2006, habitat was likely saturated, especially within areas of high Sage Sparrow densities, and territory sizes appeared to reach a threshold size of approximately 1.1 ha when plot densities reached 1.0 birds/ha. This suggests that there may be an apparent minimum territory size necessary for breeding sparrows in years of high population pressures and low resource availability.