In this study, we found that the recognition of familiarity was expressed in three ways by 15 day old subject chicks. Firstly, the rate of dj-calls in the v−a+ context increased when emitted to familiar peers relative to unfamiliar peers. Secondly, D-call morphology showed greater complexity in calls emitted to familiar peers in the v−a+ context. Lastly, the rate of J-calls increased when emitted to familiar peers, with concomitant approach and floor pecking behavior in the v+a+ context from the v−a+ context. This result suggested that acoustic cues of familiar peers elicited dj-calls as well as D-call complexity and that visual cues elicited J-calls irrespective of familiar or unfamiliar perception. Chicks were deprived of the somatosensory cues resulting from pecking and floor scratching, by placing the boxes containing a subject and reference peers on separate platforms. Similarly the sensory cue of olfaction was also removed or at least diminished by covering the test box with a transparent plastic sheet, to prevent air flow between the cages. These combined results suggested that dj-call was a unique behavior elicited from subject chicks toward familiar peers in the v−a+ context. It may be related to Morton's motivation-structure rules 
that call type shift from D-call to dj-call during isolation to v−a+ and dj-call to j-call during v−a+ to v+a+. Our finding suggests that peer affiliation can be established by acoustic recognition, independent of visual face recognition 
and when integrated, acoustic recognition is modulated. Widely divergent vertebrates possess similar domain-relevant biases toward visual facial cues and similar arguments have been made for auditory cues 
. However, the precise mechanisms for visual recognition dominance are undetermined, The mechanisms relating to how cross modal sensory integration develops at behavioral and neurobiological levels, is a subject for further research.
Subject chicks met reference chicks either through being reared in the same cage from hatching (familiar reference) or reared in a different group (unfamiliar reference). In the familiar reference tests, we observed a decreased call rate and intensity (f2−f1 value), even though the subject and reference chicks had shared the same cage until the day of testing. The reason for this call modulation in the repeated familiar reference test may be related to the fact that in unfamiliar cages, subject chicks suffered stress, as indicated by roving behavior and an accompanying D-call. In this situation, meeting familiar peers may reduce this stress, while unfamiliar peers may impose additional stress 
. As repeated testing affected call behavior, we used a single test to evaluate chick behavior towards familiar and unfamiliar references. As mentioned earlier, the highly dynamic nature of the contact call has already been recognized and the development of individual chick recognition proceeds in parallel with the segregation of inter group call perception 
. Chicks recognize a number of maternal calls, including the food call, follow me call, roosting call, predator call, and fear call 
. It is unknown if chicks are capable of recognizing familiar peer calls based on individual chick recognition. In this study, we did not address the issue of whether subject chicks recognized the calls of individual reference chicks in familiarity cognition, nor did we examine the type of call that induced calls expressing familiarity. Since our test situation presented reference chicks in groups of two to four chicks, a different type of study at a future date is needed examine these intriguing questions.
The dj-call was the major call type elicited to familiar reference chicks in the v−a+ context. The intermediate call has not been as well characterized in literature as the J-call (pleasure note, twittering call) or D-call (distress call, peeps). Marx et al., 
showed that intermediate calls (short peeps) were elicited during a step-wise isolation paradigm, in a particular starting group size. Call types in the last step, where subject chicks were left alone, differed significantly depending on the initial peer group size. If the group size was greater than four, the major call type emitted was a D-call, with no intermediate calls. Where subject chicks started the step-wise isolation test in groups of two or three, intermediate and D-calls were emitted. The interpretation of this behavior is currently uncertain. Considering that dj-calls are emitted from chicks in step-wise isolation and independent of group size, this call may be related to search attention 
and alert conditions. Electrical stimulation of the intercollicular nucleus (ICo) in the mesencephalon, induced distress calls in control chicks, and crowing in testosterone-treated chicks 
. However, it is unclear whether D- and dj-calls are variants, derived from the same call output center, or whether there is a specified motor control center for each call, which is regulated differentially by emotional and cognitive neural networks 
. Studies examining immediate early gene expression, and multi-point in vivo recording and micro-dialysis using awake animals, should shed light on these important questions 
In this study, integrative analysis of multi-behavioral parameters was effective in identifying the characteristic structure of complex expressions, such as social behavior, even though the analysis is based solely on the extraction of behavioral parameters from video recordings, without any assumption of inter-parameter correlation. It is feasible that PCA could be applied to the objective translation of social non-verbal communication and/or non-social interaction with the environment by animals, including humans. This method may well provide support to intra and inter-species communication studies, as an information processing interface.