In southern Michigan, nearly half of the male Indigo Buntings, Passerina cyanea, returning from a yearling season settled on a new territory in their second breeding season, whereas 95% of the older adult males returned to the same territory in consecutive years. First-year males were significantly more likely to disperse after a failed breeding season than after a successful one; no such difference was found in females. Males were more likely to disperse after their first year than after a later year. Females were more likely to disperse than were males. Female breeding dispersal was more closely associated with a site than with the return of an individual mate, and more with the return of an individual male than with their previous breeding success. In the two populations, 49.5% of the returning females whose mate also returned (n = 222) re-united with their former mate on the old territory, and the other 50.5% took a new mate, usually when the female settled on a new site. The dispersal behavior of 30 buntings that were banded as nestlings and returned to their natal area in both their first and second breeding seasons was independent of the behavior of their parent of the same sex when it was the same age; we found no trend indicating heritability of differences in dispersal behavior. Site choice in females and social constraints in males appear to explain much of the difference in dispersal observed with sex, age, and breeding success. Seasonal and adult lifetime breeding successes generally were independent of whether an individual had dispersed from first to second year. Birds that dispersed were as successful as birds that returned to their earliest breeding site. Breeding success in a later year did not differ from breeding success in the previous year, either in the dispersers or in the birds that returned to the same territory. However, the males that dispersed to a new territory in their second year had marginally higher (P = 0.058) mean adult lifetime success in one population. In the two study areas, about 30% of all fledged buntings were produced after breeding dispersal by the male parent, and 70% were from broods where at least one parent had changed its breeding territory. Dispersing birds generally were successful after they dispersed, at least in the sample observed, which consisted of birds that found a suitable breeding site. The estimated effective population size of buntings was increased slightly by breeding dispersal, but natal dispersal has an impact greater by orders of magnitude, both in the number of young produced and in the proportion of breeding birds that dispersed into the local population from other natal areas. The dispersal variance effective number N for bunting populations is estimated at about 1 million birds, which is an order of magnitude less than the total population size for the species.
