We investigated the process of pair formation in Zebra Finches. Two males and two females, unfamiliar with each other, were put in a flight cage and we observed these four birds for 30 min each day for 10 days. In total, five sets of four birds were used. During the observation period, the position of each individual was recorded every minute. The occurrences of songs, agonistic behavior, and other behaviors were recorded in real time and additional video analyses were performed off line. On introducing the birds, the males immediately began fighting and a firm dominance hierarchy was established, usually within a few minutes. Most of the directed songs occurred during the first day and gradually the proportion of undirected songs increased. In each set, birds formed at least one intimate heterosexual pair. We analyzed the properties of the individuals that formed the pair. The results indicated that the heavier females pair-bonded quicker, and the males that won dominance bonded sooner and four out of five dominant males weighed more than the other male in each set. By contrast, the song duration and song quality of each male did not differ between the paired male and the other male. In our experimental setting, females did not pay as much attention to the song as to the social dominance. Therefore, the functional significance of courtship songs appears after the dominance hierarchy is established among males and is used as a secondary cue for mate choice in females.