Vocal duets are joint displays where two birds, generally a mated pair, produce temporally coordinated vocalizations. Duets may participate in pair-bond maintenance, mate guarding or collaborative defence of resources. Duetting shows a great diversity in the degree of coordination between mates and the variety of vocalizations. Only 3 to 7% of world-wide species have been reported to duet, perhaps because studies generally focused on conspicuous duets with high temporal precision. Thus, more private forms of duet might have been overlooked. Here we study private vocal communication between mates in wild zebra finches (Taeniopygia guttata). Zebra finches are Australian gregarious songbirds that form life-long pair-bonds. The partners are inseparable unless nest building, incubating or brooding. Using microphones inside nest boxes, we monitor interactive communication between partners at the nest and its variation along the reproductive stages. We show that, after separation periods, partners perform coordinated mutual vocal displays involving specific soft vocal elements. Using playback experiments, we demonstrate that these soft calls allow mate recognition. Thus, we propose that nest mutual displays in zebra finches represent private vocal duets and may function to mediate pair-bond maintenance.