Both male and female peafowl grow crests on top of their head - iridescent blue in males, dull iridescent green and brown in females - but the potential signal function of this plumage ornament is unknown. In this study, peafowl crests were measured in three feral populations, and morphological variation in this ornament was studied in relation to body condition (body mass in relation to tarsus length) and health (white blood cell concentration and ectoparasite load). Prior to the start of the breeding season, male crests are wider with greater pennaceous area, and are more likely to have all feathers grown out compared with female crests. Only crest length changed with measurement date, increasing over time; in males, crest measurements were not related to the extent of train feather development. Crest morphology is a potential signal of individual health and condition in both sexes, but in different ways. In females, the amount of crest plumage grown out to its full extent was related to body condition at the start of the breeding season, whereas in males, the size and pennaceous area of the ornament were related to ectoparasite load. Observations of within-sex agonistic behaviour suggest a possible role for the crest ornament in status signaling in males, because males that engage in more aggressive interactions tend to have wider crests. There was no evidence for a relation between crest morphology and agonistic behaviour in females.
Journal of Avian Biology

Dakin, R. (2011). The crest of the peafowl: A sexually dimorphic plumage ornament signals condition in both males and females. Journal of Avian Biology, 42(5), 405–414. doi:10.1111/j.1600-048X.2011.05444.x