暂时与群体分离的个体藏马鸡的反捕食警戒

通过与同伴分担捕食风险 ,生活在稳定群体中的个体动物可以获得长期的适合度利益 ,但同时它们不得不承担食物竞争所带来的潜在代价。这种代价常常取决于食物资源的类型。当好的食物资源出现时 ,一些个体可以离开群体而独享这种资源。了解这些临时游离者如何组织其反捕食行为 ,在进化生态学上是有意义的。藏马鸡 (Crossoptilonharmani)是西藏雅鲁藏布江中游高山灌丛植被的一种典型的非繁殖季节集群鸟类。野外观察表明 ,为了独享好的食物资源 ,一些个体常常远离当前群体的活动范围。分离事件更可能发生于大的群体 ,但其发生率与参与者的数量呈负相关 ;而参与分离的个体愈多 ,分离持续的时间就愈长。分离者的个体警惕水平随着临时群体大小的增加而下降 ,遵从在其它自然大小鸟类群体所发现的一般性规律。分离行为的发生和持续时间被认为是个体对当前食物回报和捕食风险进行权衡的结果。这种利益 -代价权衡也可以解释藏马鸡所具有的强烈集群行为

Anti-predation vigilance of individual Tibetan eared pheasants temporarily separated from the flocks

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 midYalong 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.