Data from: The effect of kleptoparasite and host numbers on the risk of food-stealing in an avian assemblage

Kleptoparasitism involves the theft of resources such as food items from one individual by another. Such food-stealing behaviour can have important consequences for birds, in terms of individual fitness and population sizes. In order to understand avian host–kleptoparasite interactions, studies are needed which identify the factors which modulate the risk of kleptoparasitism. In temperate European intertidal areas, Eurasian oystercatchers Haematopus ostralegus feed primarily on bivalve molluscs, which may be stolen by kleptoparasitic species such as carrion crows Corvus corone and herring gulls Larus argentatus. In this study we combined overwinter foraging observations of oystercatchers and their kleptoparasites on the Exe Estuary, UK, with statistical modelling to identify the factors that influence the likelihood of successful food stealing behaviour occurring. Across the winter, 16.4% of oystercatcher foraging attempts ended in successful kleptoparasitism; the risk of theft was lowest in February (10.8%) and highest in December (36.3%). Using an information theoretic approach to compare multiple logistic regression models we present evidence that the outcome of host foraging attempts varied with the number of kleptoparasites per host within the foraging patch for two out of five individual months, and for all months grouped. Successful, kleptoparasitism was more likely to occur when the total number of all kleptoparasites per host was greater. Across the entire winter study period, oystercatcher foraging attempts that resulted in kleptoparasitism were associated with a mean number of kleptoparasites per host that was more than double that for foraging attempts that ended in the oystercatcher successfully consuming the mussel. Conversely, the stage of the tidal cycle within the estuary did not affect the outcome of oystercatcher foraging attempts. Our study provides evidence that bird numbers influence the risk of kleptoparasitism within avian assemblages.

盗寄生（Kleptoparasitism）是指一个个体从另一个个体处盗取食物等资源的行为。这类盗食行为会对鸟类的个体适合度与种群规模产生重要影响。若要深入理解鸟类宿主与盗寄生者之间的互作关系，需开展相关研究以明确调控盗寄生风险的各类关键因素。在欧洲温带潮间带生境中，欧亚蛎鹬（Haematopus ostralegus）主要以双壳类软体动物为食，其获取的食物可能会被小嘴乌鸦（Corvus corone）、银鸥（Larus argentatus）等盗寄生物种盗取。本研究结合英国埃克斯河口越冬期内对蛎鹬及其盗寄生者的觅食观测数据与统计建模，旨在识别影响盗食行为成功发生概率的相关因素。整个越冬周期内，16.4%的蛎鹬觅食事件最终以成功盗寄生告终；盗寄生风险在2月最低（10.8%），12月最高（36.3%）。研究采用信息论方法对比多元逻辑回归模型，结果显示：在5个单独观测月份中的2个月份，以及所有月份合并分析时，宿主觅食事件的结果会随觅食斑块内单只宿主对应的盗寄生者数量发生显著变化。当单只宿主对应的盗寄生者总数量更高时，盗寄生成功的概率显著升高。在整个越冬研究周期中，发生盗寄生的蛎鹬觅食事件，其单只宿主对应的盗寄生者平均数量是蛎鹬成功取食贻贝的觅食事件的两倍以上。与之相反，河口的潮汐周期阶段并不会对蛎鹬觅食事件的结果产生影响。本研究证实，鸟类种群数量会影响鸟类群落内的盗寄生风险。