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Abstract: Vigilance is used to detect predators and monitor rivals. We studied red-crowned cranes (Grus japonensis) in Yancheng Biosphere Reserve, China, to examine changes in the allocation of time to vigilance as a function of flock size in areas with different levels of human disturbance. There was low level of human activity in the core area of the reserve whereas more human activity occurred in the buffer zone. Vigilance decreased linearly with flock size but to significantly different extent in the two areas, with a more pronounced decrease in the more disturbed area. In smaller crane flocks, vigilance was higher in the more disturbed area. Vigilance also varied in a non-linear fashion with flock size first decreasing and then increasing in larger flocks. Increase in vigilance in larger flocks was accompanied by an increase in fighting suggesting that vigilance in large flocks was aimed partly at monitoring rivals. The effect of human disturbance on vigilance suggests that less time is available for foraging in more disturbed areas. Nevertheless, moving to less disturbed areas may not be an option for the cranes given the increase in vigilance that would probably occur in areas with more cranes. [ABSTRACT FROM AUTHOR]