The form, function, and evolutionary significance of neural aromatization

•Songbirds are exceptional models in behavioral neuroendocrinology.•Aromatase is expressed at multiple regions and at high levels in the songbird brain.•Aromatase is expressed in dendrites, soma, axons and axon terminals in brain.•Neuroestrogens participate in many brain functions based on their unique cellular distribution.•Elevated expression of brain aromatase may have boosted the songbird radiation.