We hypothesized that whereas domestic fowl in small groups establish a dominance hierarchy through aggressive interactions those in large groups adopt a low-aggression (tolerant) social strategy. To test this hypothesis, we examined the effect of group size on the ontogeny of aggressive behavior in groups of 15, 30, 60 and 120 female White Leghorn chickens housed in enclosures of 3, 6, 12 and 24m2 at a constant density of 5 birds/m2. Focal observations of 12 birds per group were carried out from 3 to 18 weeks of age. At all group sizes, aggressive pecking peaked between 6 and 12 weeks and then declined. In support of our hypothesis, regression analysis on means across all ages showed a linear reduction in the frequency of pecks and threats given per focal bird with increasing group size. However, the frequency of pecks and threats received per focal bird showed the opposite pattern, being higher in larger than smaller groups. This discrepancy between aggression given and received may have been due to the focal birds receiving a disproportionate amount of aggression in comparison to unmarked individuals. Alternatively, it could be interpreted as evidence of birds adopting different social strategies with increasing group size. In this case, we speculate that, while the majority of birds may adopt a tolerant strategy in larger groups, a minority may be despotic, directing aggression indiscriminately towards other birds. In either case, the results have implications for future studies of aggression where often only a sample of birds is observed.