Individual animals living in group with stable membership may gain a long-term fitness benefit through sharing predation risk with other partners, but meanwhile undertake a potential cost of competing for food resources. This cost often depends on the type of food resources. Individuals do not keep together all the time and a few may split themselves from the current flocks when good foods are available for only themselves. In evolutionary ecology, it is interesting to know how those temporary extra-range individuals organize their anti-predation vigilance. Tibetan eared pheasant Crossoptilon harmani is typically gregarious throughout the non-breeding season in alpine shrub vegetation around mid-Yalong Zangbo River, Tibet. Sometimes a few individuals were observed to leave the current flocks if they discovered good food resources. Separation from the flock was more likely when original flock size was larger but less likely if more extra-range foragers were present. The duration of separation events was longer when there were more extra-range foragers. Similar to a widely reported relationship in natural size flocks of many species of birds, vigilance levels of the free individual eared pheasants declined as the extra-range flock size increased. Whether to leave current flocks and how long to keep solitary were believed to be a result of individual’s trade-off between current food reward and predation risk. The trade-off also provided an explanation for strong grouping behavior of the eared pheasants
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