SURVIVAL AND BREEDING ADVANTAGES OF LARGER BLACK BRANT (BRANTA BERNICLA NIGRICANS) GOSLINGS: WITHIN- AND AMONG-COHORT VARIATION

Abstract

We examined the relationship between mass late in the fi rst summer and survival and return to the natal breeding colony for 12 cohorts (1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997) of female Black Brant (Branta bernicla nigricans). We used Cormack-Jolly-Seber methods and the program MARK to analyze capture-recapture data. Models included two kinds of residuals from regressions of mass on days aft er peak of hatch when goslings were measured; one based on the entire sample (12 cohorts) and the other based only on individuals in the same cohort. Some models contained date of peak of hatch (a group covariate related to lateness of nesting in that year) and mean cohort residual mass. Finally, models allowed survival to vary among cohorts. The best model of encounter probability included an eff ect of residual mass on encounter probability and allowed encounter probability to vary among age classes and across years. All competitive models contained an eff ect of one of the estimates of residual mass; relatively larger goslings survived their fi rst year at higher rates. Goslings in cohorts from later years in the analysis tended to have lower fi rst-year survival, aft er controlling for residual mass, which refl ected the generally smaller mean masses for these cohorts but was potentially also a result of population-density eff ects additional to those on growth. Variation among cohorts in mean mass accounted for 56% of variation among cohorts in fi rst-year survival. Encounter probabilities, which were correlated with breeding probability, increased with relative mass, which suggests that larger goslings not only survived at higher rates but also bred at higher rates. Although our fi ndings support the well-established linkage between gosling mass and fi tness, they suggest that additional environmental factors also infl uence fi rst-year survival.