Over 7 yr nesting success, nest mortality and reproductive behavior of female Boat-tailed Grackles (Quiscalus major) nesting in a freshwater impoundment in South Carolina were examined. Most (96% of 1368) nests were built in colonies on cattail (Typha spp.) islands. Average colony size ranged from 7 to 10 nests; maximum size ranged from 20 to 37 nests. Average distance between nearest-neighbor colony nests was 3.5 m. Breeding was synchronized, and at least 50% of all nests in the population were started within 12-16 d of the first. Females used dead (overwintering) vegetation for nesting, but colony sites did not appear to be limited. The breeding biology of Boat-tailed Grackles in South Carolina differs little from that reported for a population nesting in cattail marshes in central Florida, 600 km SW. In each region, 61% of nests produced at least one fledgling. Daily mortality rates of eggs were slightly lower in South Carolina, but mortality rates of nestlings were the same. Unlike in Florida, no difference was found between the survival of two-egg and three-egg clutches. In both regions, predation was the main cause of nest loss. Starvation was the second most important mortality factor in South Carolina, where it was relatively more important than in Florida. Most predation in South Carolina was by snakes. In Florida rodents were most important. In both areas, predation appears to be the main selective force molding females' nesting behavior.
