I am interested in the proximate mechanisms controlling animal behavior, particularly in how hormones are involved. You might have thought that hormones are always slow-acting, but this is not true. For example, we have, for the first time, measured rapid changes in the amounts of the sex-steroid estradiol in the auditory system of birds as they interact with other birds or hear the sounds of other birds. Our study species is the zebra finch, a model species for most laboratory studies of birds because they are very easy to maintain in the lab. Rapid synthesis of estrogen occurs in the brains of zebra finches, and the estrogen quickly increases the strength of nervous system responses to songs of other zebra finches. We have discovered that estradiol functions almost like a rapid-acting neurotransmitter, modulating core functions such as song learning and song expression in birds. Similar changes appear to occur in the hippocampus of scrub jays as they form and retrieve memories of sites where they store food. Birds are exceptional model organisms for understanding hormones and complex cognitive behaviors.