Arch displays signal threat intentions in a fission-fusion flock of the red-crowned crane

Ritualised displays are used by group-living animals in various contexts, such as when individuals join a group or flock. Only a few quantitative studies of the function of displays in species with fission-fusion social dynamics have been conducted to date. Here, we examined the arch display in red-crowned cranes (Grus japonensis) immediately after joining a flock. Behavioural observations indicated that the arch functions as a signal of both threat motivation and individual strength. Singletons had disadvantages in terms of competition over resources and were, therefore, expected to have higher threat motivation than pairs or families. Indeed, singletons performed the arch more frequently than did pairs or families. Performance of the arch was related to dominance: males and adults were more likely to perform the arch than females and sub-adults. The likelihood of performing the arch was positively associated with local group density, indicating that joiners arched in more competitive situations. Contextual analyses indicated that subsequent behaviour by a joiner was more aggressive and that nearby individuals more frequently showed behavioural responses when a joiner arched than when it did not. Together, this study shows that cranes demonstrate functional displays to potential competitors, and represents a rare example of the functional analysis of ritualised signals in non-songbird species with fission-fusion social dynamics.