In order to determine the critical period(s) during which estrogen alters sexually dimorphic behavior and neuroanatomy in zebra finches (Poephila guttata), nestlings were injected daily with 20 micrograms estradiol benzoate (EB) during posthatching week 1, week 2, week 3, or weeks 1, 2, and 3. At 7 months of age, birds were implanted with testosterone propionate and tested with female partners for singing, dancing, and copulatory mounting. Brains were subsequently processed for morphometry, and the volumes of the song system nuclei HVC, area X, and RA and the soma sizes and densities of neurons in RA were determined. Males given EB during week 1 failed to mount. Females given EB during week 1 were fully masculinized with respect to dancing and RA neuron soma size and density, and were partially masculinized with respect to song nuclei volumes and singing. Treatment beginning after week 1 was ineffective or less effective for all measures. Only for RA neuron measures was treatment for all three weeks more effective than week 1 treatment. Thus the first post-hatching week is the most influential period of those tested for effects of exogenous estrogen on sexual differentiation in this species, and is a period during which both masculinization of females and demasculinization of males is possible.

In order to test hypotheses about the organizational role of early estrogen in the sexual differentiation of behavior in zebra finches (Taeniopygia guttata), newly hatched birds were given one of three treatments: (1) daily injection with an aromatase inhibitor (fadrozole hydrochloride) for the first week; (2) the same treatment for the first two weeks; (3) daily injection of the vehicle for one or two weeks. As adults, birds were implanted with testosterone propionate and underwent a three-phase testing procedure designed to measure singing, dancing, copulation, and preferences for male vs female partners. Males were completely unaffected by fadrozole hydrochloride treatment; they sang, danced, and mounted at control levels, and like control males they preferred female partners. Females were similarly unaffected, with one important exception: those treated with fadrozole hydrochloride during the first week preferred to be spend time near females instead of males in choice tests, unlike control females. These results suggest that a model of zebra finch sexual differentiation in which estrogen organizes male-typical behavior is unlikely to be correct. At the same time, they provide evidence that a key dimorphic feature of mate choice, preference for opposite-sex birds (sexual orientation), may result from organizational hormone actions during the early post-hatching period in this pair-bonding species.

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