Is fledging success a reliable index of fitness in Northern Goshawks?

Abstract

Fledging success is often assumed to be a reliable index of reproductive success (i.e., fitness) in the ornithological literature. We examined the validity of this assumption in a large population of Northern Goshawks (Accipiter gentilis) on the Kaibab Plateau, Arizona, at both the population and individual levels. We used mark-recapture data from 558 fledglings produced at 494 nests over a 10-yr period to assess the hypothesis that the number of fledglings returning to breed from an annual fledgling cohort is positively correlated with the size of the cohort. Natal philopatry was low and recruitment was gradual: only 48 fledglings (8.6%) returned to breed between 2–8 yr of age (x = 3.5 yr). We found no evidence that the breeding population produced more local recruits in years of high fledgling production than in years of low fledgling production. At the individual level, however, fledgling production for 290 breeding adults was related to their contributions to the future breeding population. Variation in fitness potential among territorial adults was high, as only 20% of the breeding population produced nearly 50% of the fledglings and 84% of the local recruits during the study. Our results indicate that measures of annual productivity for a large breeding population were not reflective of reproductive success, whereas measures of individual productivity were. We conclude that fledging success of individual goshawks is a reliable index of fitness, but that population productivity is a poor predictor of local recruitment.

Citation

Wiens, J. David; Reynolds, Richard T. 2005. Is fledging success a reliable index of fitness in Northern Goshawks? Journal of Raptor Research. 39(3): 210-221.