Raihani N J, Ridley A R
LARG, Department of Zoology, University of Cambridge, Cambridge CB2 3EJ, UK.
Biol Lett. 2007 Dec 22;3(6):624-7. doi: 10.1098/rsbl.2007.0435.
Group living can provide individuals with several benefits, including cooperative vigilance and lower predation rates. Individuals in larger groups may be less vulnerable to predation due to dilution effects, efficient detection or greater ability to repel predators. Individuals in smaller groups may consequently employ alternative behavioural tactics to compensate for their greater vulnerability to predators. Here, we describe how pied babbler (Turdoides bicolor) fledging age varies with group size and the associated risk of nestling predation. Nestling predation is highest in smaller groups, but there is no effect of group size on fledgling predation. Consequently, small groups fledge young earlier, thereby reducing the risk of predation. However, there is a cost to this behaviour as younger fledglings are less mobile than older fledglings: they move shorter distances and are less likely to successfully reach the communal roost tree. The optimal age to fledge young appears to depend on the trade-off between reduced nestling predation and increased fledgling mobility. We suggest that such trade-offs may be common in species where group size critically affects individual survival and reproductive success.
群居可为个体带来诸多益处，包括合作警戒和较低的被捕食率。由于稀释效应、高效的察觉能力或更强的驱赶捕食者的能力，较大群体中的个体可能较不易受到捕食。因此，较小群体中的个体可能会采用其他行为策略来弥补其更高的被捕食脆弱性。在此，我们描述了双色蚁鵙（Turdoides bicolor）的出飞年龄如何随群体大小以及雏鸟被捕食的相关风险而变化。雏鸟被捕食在较小群体中最为常见，但群体大小对出飞雏鸟的捕食没有影响。因此，小群体的雏鸟更早出飞，从而降低被捕食的风险。然而，这种行为存在代价，因为较年幼的出飞雏鸟比年长的出飞雏鸟行动能力更弱：它们移动的距离较短，且不太可能成功抵达公共栖息树。雏鸟出飞的最佳年龄似乎取决于雏鸟被捕食风险降低与出飞雏鸟行动能力增强之间的权衡。我们认为，这种权衡在群体大小严重影响个体生存和繁殖成功的物种中可能很常见。
