Sexually mature young males of many dichromatic species begin breeding while expressing female-like or incomplete adult phenotypes, which contradicts sexual selection theory. It is thought that by delaying the acquisition of adult appearance, young males may avoid aggression from other males, but reduce mating opportunities, resulting in trade-offs between these 2 life-history traits. Thus, an optimal young male phenotype should allow mating while reducing aggressiveness. Differential character maturation (i.e., the acquisition of adult appearance does not occur in all traits at the same time) has been described in bird species, but has largely been neglected in the study of delayed plumage maturation. Using the common kestrel Falco tinnunculus, we tested the prediction that maturation of different adult traits depends on the aggressiveness elicited by both male and female individuals and that young male phenotypes should be a combination of traits triggering low levels of aggressiveness while still revealing sex. We explored the pattern of delayed plumage maturation observed in 4 different adult male traits (head, rump, tail, and back plumages) in young breeding males captured over 8 years. We then analyzed the intensity of agonistic responses against these traits in breeding males and females by presenting young male decoys showing these mature characters. We found that those characters eliciting high aggressiveness in males (head and rump) were partly expressed in young males and only 1 (tail) of the 2 traits (tail and back) eliciting high aggressiveness in females was poorly expressed. Our study suggests that conspecific aggressiveness of both males and females can play a significant role in determining patterns of delayed plumage maturation.