Bill color
- Testosterone activates sexual dimorphism including male-typical carotenoid but not melanin plumage pigmentation in a female bird
Highlighted Article: Female red-backed fairy-wrens express male-typical traits in response to exogenous and endogenous testosterone, including carotenoid-pigmented plumage, darkened bill and an enlarged sperm storage organ, but are constrained in production of melanin-pigmented plumage.