Figure 1: Experimental designs to explore a sensitive period of peer-social learning a. Five developmental conditions of peer-social environment Besides the bipolar sets, isolated (Iso) and grouped (Grp), three kinds of five-day grouped with ten-day isolated conditions (GII, IGI, and IIG) were evaluated b. Parameters of local preference to interpret a subject’s social distance to the reference peers (see also e) c. The scene of a behavioral test on P15 d. Four types of chicks’ typical calls e. Fifteen parameters to be extracted from video-recorded behavioral data