A game-theoretical model of kleptoparasitic behaviour in an urban gull (Laridae) population.

Kleptoparasitism (food stealing) is a significant behaviour for animals that forage in social groups as it permits some individuals to obtain resources whilst avoiding the costs of searching for their own food. Evolutionary game theory has been used to model kleptoparasitism, with a series of differential equation based compartmental models providing significant theoretical insights into behaviour in kleptoparasitic populations. In this paper we apply this compartmental modelling approach to kleptoparasitic behaviour in a real foraging population of urban gulls (Laridae). Field data was collected on kleptoparasitism and a model developed that incorporated the same kleptoparasitic and defensive strategies available to the study population. Two analyses were conducted: 1. An assessment of whether the density of each behaviour in the population was at an equilibrium. 2. An investigation of whether individual foragers were using Evolutionarily Stable Strategies (ESS) in the correct environmental conditions. The results showed the density of different behaviours in the population could be at an equilibrium at plausible values for handling time and fight duration. Individual foragers used aggressive kleptoparasitic strategies effectively in the correct environmental conditions but some individuals in those same conditions failed to defend food items. This was attributed to the population being composed of three species that differed in competitive ability. These competitive differences influenced the strategies that individuals were able to use. Rather than gulls making poor behavioural decisions these results suggest a more complex three-species model is required to describe the behaviour of this population.

盗寄生行为（Kleptoparasitism，即食物窃取）是群居觅食动物的重要行为，该行为使部分个体无需付出自行搜寻食物的成本即可获取资源。进化博弈论已被应用于盗寄生行为的建模研究，一系列基于微分方程的分室模型为盗寄生种群的行为分析提供了重要的理论洞见。本文将该分室建模方法应用于真实的城市鸥类（鸥科（Laridae））觅食种群的盗寄生行为研究。研究团队采集了该种群盗寄生行为的野外观测数据，并构建了纳入研究种群实际可采用的盗寄生与防御策略的模型。本研究开展了两项分析：其一，评估种群内各类行为的密度是否处于均衡状态；其二，探究个体觅食者是否在适配的环境条件下采用了进化稳定策略（Evolutionarily Stable Strategies，ESS）。研究结果显示，在处理时长与争斗时长的合理取值范围内，种群内不同行为的密度可达到均衡状态。个体觅食者在适配的环境条件下可有效运用攻击性盗寄生策略，但部分个体在相同环境下却无法守护其获取的食物资源。该现象的成因在于该种群由三种竞争能力存在差异的物种所构成，这些竞争能力差异影响了个体可采用的行为策略。上述研究结果表明，相较于认为鸥类做出了不当的行为决策，更复杂的三物种模型才能够准确刻画该种群的行为特征。