Fledging success as an index of recruitment in red-winged blackbirds

We used data from an 11-year study of Red-winged Blackbirds (Agelaius phoeniceus) to test the hypothesis that fledging success is a reliable index of recruitment at the population and the individual level. Natal philopatry was only 2.37% overall (3.51% for males and 1.49% for females) in our study population. However, the number of fledglings that returned as breeding adults from an annual fledgling cohort was significantly correlated with the size of the cohort. The correlation was also significant when males and females were analyzed separately despite sex differences in natal philopatry, age of first breeding, and probable differences in mortality factors. Recruitment increased disproportionately with the size of the fledgling cohort. Thus, years of high production produced proportionately more breeding adults. At the individual level, the number of fledglings sired by a male in his lifetime was significantly correlated with the number of his descendants that eventually returned to breed in the study population. These results support the widely held assumption in avian field studies that fledging success is a reliable index of fitness.