Duet displays within a flock function as a joint resource defence signal in the red-crowned crane

Avian duets performed in an exclusive territory have been studied intensively. By contrast, only a few studies have been conducted on duets with multi-modal displays performed within a fission-fusion flock in which individuals can participate in face-to-face social interactions. Here, we examined the duet displays (DDs) of the red-crowned crane (Grus japonensis), an endangered species in Japan, in a wintering flock in which individuals gathered for artificially provisioned foods. We tested two hypotheses: that DDs function as a signal of joint resource defence and for pair bond maintenance. Our behavioural analyses showed that the total frequency of DDs within a flock was positively associated with the number of joining cranes. This result indicated that the change in within-flock competition positively affected DD performance because an increase in flock size can reduce the per capita gain of resources. At the individual level, pairs accompanying their young (families) performed DDs more frequently than pairs that did not have their young. To care for their young, access to food resources is thought to be more important for pairs accompanying their young than pairs that did not have young. This result suggests that the DDs reflected the motivation to compete for resources. The performance of DDs increased the probability of staying in a favourable area. These results support the hypothesis that DDs function as a signal of joint resource defence in the flock. This study provides rare data on the function of coordinated vocal displays within a flock of birds.