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

We used data from an 11-year study of Red-winged Blackbirds (Agelaius phoen iceus) to test the hypothesis that fledging success is a reliable index of recruitment at the population and the individual level. Natal philopatry wa s only 2.37% overall (3.51% for males and 1.49% for females) in our study p opulation. However, the number of fledglings that returned as breeding adul ts from an annual fledgling cohort was significantly correlated with the si ze of the cohort. The correlation was also significant when males and femal es were analyzed separately despite sex differences in natal philopatry, ag e of first breeding, and probable differences in mortality factors. Recruit ment increased disproportionately with the size of the fledgling cohort. Th us, years of high production produced proportionately more breeding adults. At the individual level, the number of fledglings sired by a male in his l ifetime was significantly correlated with the number of his descendants tha t eventually returned to breed in the study population. These results suppo rt the widely held assumption in avian field studies that fledging success is a reliable index of fitness.