Data from: The costs of kleptoparasitism: a study of mixed-species seabird breeding colonies

Mixed-species assemblages are common in nature, providing mutual benefits to associating species including anti-predator advantages or resource facilitation. However, associating with other species may also impose costs through kleptoparasitism (food theft). Identification of these costs, and how they vary when different species breed alongside one another, is essential to understand the payoffs of mixed-species assemblages. We explore the costs of kleptoparasitism for greater crested terns Thalasseus bergii provisioning offspring at a single-species colony, where individuals suffer kleptoparasitism from conspecifics, and at a mixed colony where terns breed alongside Hartlaub’s gulls Chroicocephalus hartlaubii and are vulnerable to both intra- and interspecific kleptoparasitism. Gull presence increased both kleptoparasitic attacks and the proportion of prey lost or stolen during provisioning, relative to the single-species colony. Provisioning adults suffered additional energetic costs in response to gull kleptoparasitism, requiring more attempts to deliver prey, taking longer to do so, and swallowing more prey (to the detriment of their offspring). Gulls also increased the duration of tern vulnerability to kleptoparasitism, because they continued to steal food from adults and chicks after precocial chicks left the nest, when intraspecific kleptoparasitism is negligible. Terns breeding in a mixed colony therefore suffer direct and indirect costs through decreased provisioning and increased provisioning effort, which may ultimately impact reproductive success, resulting in colony decline where kleptoparasitism is frequent. This study illustrates how forming a mixed-species seabird breeding assemblage has costs as well as benefits, potentially fluctuating between a parasitic and a mutualistic relationship.

种间混合集群在自然界中极为常见，可为参与集群的物种带来互利益处，包括反捕食优势与资源促进效应。然而，与其他物种形成集群也可能通过盗寄生（kleptoparasitism，即食物窃取行为）带来成本。明确此类成本，以及不同物种同域繁殖时成本的变化规律，是理解种间混合集群收益的关键所在。本研究以大凤头燕鸥（Thalasseus bergii）为研究对象，探究其在单物种繁殖群与混合繁殖群中遭遇盗寄生的成本：在单物种繁殖群中，个体仅遭受同种个体的盗寄生；而在混合繁殖群中，燕鸥与哈劳氏鸥（Chroicocephalus hartlaubii）同域繁殖，同时面临种内与种间盗寄生的威胁。与单物种繁殖群相比，哈劳氏鸥的存在不仅提升了盗寄生攻击频次，还增加了育雏过程中猎物丢失或被窃取的比例。面对哈劳氏鸥的盗寄生，育雏成鸟需承担额外的能量成本：需更多次尝试才能递送猎物、花费更长时间完成递送，且会吞下更多猎物，这将损害雏鸟的生存利益。此外，哈劳氏鸥还延长了大凤头燕鸥遭遇盗寄生的窗口期：当早成雏离巢后，种内盗寄生已基本可忽略，但哈劳氏鸥仍会继续窃取成鸟与雏鸟的食物，从而延长了燕鸥受盗寄生威胁的时长。因此，在混合繁殖群中繁殖的燕鸥，会因育雏效率下降与育雏投入增加，同时承受直接与间接成本，这最终可能影响其繁殖成功率，在盗寄生频发的区域甚至会导致繁殖群规模下降。本研究表明，海鸟种间混合繁殖集群的形成兼具收益与成本，其物种间关系可能在寄生与互利之间动态波动。