Does Nest Placement Affect the Fate or Productivity of California Gnatcatcher Nests?

Abstract

I monitored 384 California Gnatcatcher (Polioptila californica) nests at five study areas in western Riverside County, California, from 1992 through 1995. Overall, 78.9% of the nesting attempts failed (52.9% from nest predation), and 21.1% fledged young. Vegetation measurements were taken at 207 of 384 nest locations and at 207 randomly selected locations within gnatcatcher territories. Nest placement was not random in that gnatcatchers placed their nests in locations with greater cover and height of perennial vegetation, increased horizontal structural homogeneity, and increased vertical structural homogeneity relative to random locations. The volume of gnatcatcher nest shrubs was lower than that of random shrubs, and concealment was higher at nests than at random shrubs. Nevertheless, variables that differed significantly between nests and random locations were not significantly associated with nesting success, nest predation, nest abandonment, or productivity. The use of specific perennial shrub species as nest substrates was not proportional to the availabilities of the shrub species within the study areas, yet I found no relationship between nesting success and the species of nest shrub. Nesting success appeared to be unrelated to the abundance of perennial shrubs at four of five study areas. My results suggest that factors affecting the fate of California Gnatcatcher nests operate at scales larger than the nest site or nest patch.