In her early research using the Japanese quail, Adkins-Regan performed a number of fundamental experiments on the mechanisms of sexual differentiation of behavior in birds, which have a ZW sex determination system. Her work demonstrated that, unlike what is observed in mammalian sexual differentiation, female-typical reproductive behavior can be activated in both male and female Japanese quail by estrogen treatment.[5][6] She also demonstrated that treatment of male Japanese quail embryos with estrogens de-masculinizes behavior, resulting in adult males that fail to show male-typical courtship behavior even when later treated with testosterone.[7] These experimental findings suggested a model of sexual differentiation for birds in which the steroid hormone estradiol secreted by the heterogametic sex (in this case, females) is responsible for the process of differentiation.