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 (λ) 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 λ, but site characteristics independent of vegetation also could have contributed to such an association.