There are sex differences in the behavior and brain of budgerigars (e.g. Eda-Fujiwara et al., 2016). We artificially paired male and female budgerigars and kept them for 5 weeks, then separated the male and female birds and continued keeping them. Immediately after isolation, we conducted an experiment in which birds were alternately presented with the calls of mates and non-mate calls, and compared how they responded. Both males and females learned to recognize of their mate during the mating period and responded more to their mate's calls. However, after one month of separating the males and females, the males responded more to the calls of a non-mate female than their mate. Furthermore, after one month of isolation, we investigated a region called CMM, which is the higher auditory center in the cerebrum, and discovered that there were sex differences in cerebral hemispheric dominance. Comparative research between bird and human brains has shown that the CMM is a region like the　auditory association area of the human cerebrum. Furthermore, some human studies reported sex differences in cerebral hemispheric dominance. The study of the budgerigar brain helps to reveal the biological basis of sex differences in human language function.

Regarding vocal imitation learning (learning to produce vocalizations by imitating the vocalizations of other individuals), males imitate the call of their mate, but females do not imitate the male's call. However, if the male does not engage in imitative learning, females will imitate the male's vocalizations (Plummer & Striedter 2002). Additionally, in cages containing only females, females will imitate the vocalizations of other females (Hile & Striedter 2000). Such an interesting phenomenon has been discovered in which female behavior changes depending on the social environment, but the mechanism is unknown. How does the social environment influence female behavior and do sex differences in behavior occur? We are conducting research to answer these questions.