Data from: Predation can select for later and more synchronous arrival times in migrating species

For migratory species, the timing of arrival at breeding grounds is an important determinant of fitness. Too early arrival at the breeding ground is associated with various costs, and we focus on one understudied cost: that migrants can experience a higher risk of predation if arriving earlier than the bulk of the breeding population. We show, using both a semi-analytic and simulation model, that predation can select for later arrival. This is because of safety in numbers: predation risk becomes diluted if many other individuals, either con- or heterospecific, are already residing in the area. Predation risk dilution can also select for more synchronous arrival because deviating from the current population-wide norm to earlier or later dates leads to higher predation risk or to failures in territory acquisition, respectively. The fact that selection for high arrival synchrony can in some cases be more important than selection for a specific date (early or late) within the season is an example of an ‘evolutionary priority effect’: whichever strategy – in this case a particular arrival time – becomes established in a population can remain stable over long periods of time; there are many possible equilibria (multiple stable states) which the population can remain at. Mixed arrival strategies are also possible under some circumstances.对于迁徙物种而言，抵达繁殖地的时间是影响其适合度（fitness）的重要决定因素。抵达繁殖地过早会带来多种代价，本研究聚焦于其中一种尚未得到充分研究的代价：若迁徙个体的抵达时间早于繁殖种群的主体群体，其遭遇捕食的风险会更高。本研究通过半解析模型与模拟模型两种方法证明，捕食压力会促使个体选择更晚的抵达时间。这一现象源于"群体安全效应"：若已有大量同种或异种个体栖息于该区域，捕食风险会被稀释。捕食风险稀释效应还会促使种群选择更同步的抵达时间：偏离当前种群整体的时间规范，提前抵达会面临更高捕食风险，延后抵达则可能无法成功获得领地。对高抵达同步性的自然选择，在某些情况下会比选择季节内特定的抵达时间（提前或延后）更为重要，这一现象正是"进化优先效应"的典型案例：即种群中一旦确立某一策略——此处特指特定的抵达时间——便可长期维持稳定；种群可存在多种稳定平衡点（多重稳态）。在部分情境下，混合抵达策略也有可能出现。