Association of Within-Territory Vegetation Characteristics and Fitness Components of California Gnatcatchers

Abstract

-Correlations among habitat characteristics and fitness components of California Gnatcatchers (Polioptila californica californica) were examined using within-territory vegetation and life-history data for 41 pairs of gnatcatchers at four sites from 1993 through 1995. Gnatcatchers nested earlier, had more successful nests, produced more fledglings, had a longer nesting period, and had lower fledgling costs when their territories were associated with increased grass and forb cover, increased perennial structure, increased horizontal perennial homogeneity, decreased vertical perennial homogeneity, and decreased perennial diversity. Within-territory vegetation variables, derived from the correlation of vegetation and life-history variables, were able to discriminate among gnatcatcher pairs grouped by site. Survival of adult gnatcatchers was significantly higher in the gnatcatcher group that had average within-territory vegetation characteristics associated with maximization of other life-history variables and was significantly lower in the gnatcatcher group that had average within-territory vegetation characteristics associated with minimization of other life-history variables. Survival of juveniles was not associated with within-territory vegetation. The finite rate of population increase (X) was >1 for the gnatcatcher group where within-territory vegetation and life-history variables were maximized, approximately equal to 1 for gnatcatcher groups where within-territory vegetation and life-history variables were moderate, and <1 for the gnatcatcher group where within-territory vegetation and life-history variables were minimal. Our study suggests that within-territory vegetation characteristics are associated with adult survival and X, but site characteristics independent of vegetation also could have contributed to such an association. Received 3 September 1996, accepted 1 April 1997. SPECIES ABUNDANCE can be a misleading indicator of habitat quality and reproductive success (Van Horne 1983, Pulliam 1988, Martin 1992, Vickery et al. 1992a). Assuming that habitat features associated with fitness components (i.e. increased reproductive output and survival) will be chosen more often over time, preservation of (or management for) such features should increase fitness and promote short-term increases in abundance (Martin 1992). For most bird species, however, specific habitat features associated with increased fitness are poorly understood (but see Vickery et al. 1992b). Most studies have taken a qualitative approach to linking fitness components with habitat or nest location (e.g. Walkinshaw 1953, Graber 1961, Nolan 1963, Thompson and Nolan 1973, Stewart et al. 1977, Knapton 1978, Middleton 1979, Fischer 1980, Zimmerman 1982). 3E-mail: gbraden@co.san-bernardino.ca.us Although these studies provide valuable information on life history, they lack quantitative assessments of the association between habitat and fitness. This study focuses on identifying the association between habitat features and fitness components for the California Gnatcatcher (Polioptila californica californica), a small, sedentary passerine endemic to the coastal sage scrub community of southern California and northern Baja California (Atwood 1991). The California Gnatcatcher is listed as a threatened species. The current population estimate for the United States is approximately 2,000 pairs (Atwood 1992). The decline of this species has been concomitant with the disappearance and degradation of the coastal sage scrub habitat throughout southern California (Atwood 1993). The coastal sage scrub community is a Mediterranean habitat characterized by facultatively drought-deciduous, mesophilic shrubs from