Why could continuous partnership in monogamous species be worth to the individual when reproduction is not restricted to the pair-bond? Could the stable pair-bond of flock-forming bird species be considered as a valuable social relationship? Valuable relationships (clear reciprocal friendly behaviours between individuals with none or little agonistic conflict) are widespread in maintaining cohesion in species forming stable social groups. Zebra finches (Taeniopygia guttata) are gregarious songbirds establishing life-long pair-bonds. Using Network theory, we quantify and analyze well-defined behaviours that describe this species’ social links. In a group with balanced sex-ratio, birds form reproductive male-female pair-bonds that also represent real valuable relationships. To test the stability of this pair-based social organization, we study the relationships established in groups of varied sex-ratio: birds form pair-bond with the available partner, regardless of the sex. So, before a reproductive link, zebra finch pair-bond is a real valuable social relationship. Social grooming is the main way to establish and maintain partnerships, but vocal communication could also service group cohesion and should thus reflect zebra finches social network. Using a homemade threshold-based automatic system, we show that pair-bonding affects the group acoustic dynamic: the more paired birds, the less the group vocalizes. To confirm this pair-bond effect, we prevented males from establishing relationships by physical isolation. Isolated males show more burst-like vocal activity and use differently their vocal repertoire. Because zebra finches behaviour and acoustic communication dynamic only depend on pair-bonding regardless of the sex, monogamy could be more a valuable relationships than a reproductive strategy.
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