In a South Carolina cattail marsh, female Boat-tailed Crackles (Quiscalus major) concentrated to nest on predator-safe islands patrolled by American alligators (Alligator mississippiensis). I examined several hypotheses regarding colonial nesting behavior of Boat-tailed Crackles. (1) I reject the null hypothesis that colony-nesting and solitary females do not differ in reproductive success. Solitary-nesting females had lower reproductive success (37%) than colony females (63%), a difference perhaps attributable to males' defense of colonies from predatory birds. (2) I reject the hypothesis that the physical characteristics of sites used for Colonies differ from those that are not used. Islands occupied by colonies were the same as unoccupied islands in measurements of water depth and vegetation structure. Colony site use is therefore not a result of habitat heterogeneity. (3) I reject the hypothesis that breeding success does not differ between contemporaneous colonies. In two of four years, I found significant variation among colonies in percentage of nests that produced young. Nesting success was independent of colony size. (4) I accept the hypothesis that the reproductive success of colonial females is related to extrinsic factors rather than to differences in female quality. Predation was the main cause of nest mortality, and varied between colony sites, but its intensity was not related to colony size. Although clutch size remained constant over the nesting period, number of fledglings per nest decreased, mainly as a result of the increased proportions of nests lost to predators. Starvation, the second most important cause of mortality, did not vary according to colony size nor to time of breeding. Early synchronized nesting of the whole population may reduce the effects of predation by both a "head-start" effect, and by predator satiation.
