Data from: Helper effects in the azure-winged magpie Cyanopica cyana in relation to highly-clumped nesting pattern and high frequency of conspecific nest-raiding

In avian cooperative breeding systems, many benefits obtained by social pairs from the presence of helpers have been uncovered. However, until now, the factors that determine the type of assistance helpers provide and the responses of social pairs have not been well illustrated. We examined the contribution of helpers to cooperative groups and the relevant responses of dominant pairs in the azure-winged magpie Cyanapica cyana which breeds on the Tibetan Plateau. We used the capture–mark–recapture method to identify helpers. Results showed that helpers were mostly the yearling sons of dominant pairs. They mainly contributed to the cooperative group in three ways, courtship-feeding the incubation female, provisioning the brood, and defending the nest. For responses of dominant pairs, we unexpectedly found that clutch size was not influenced by the presence of helpers at the nest. However, cooperative groups had higher brood feeding rates than biparental nests and their feeding pattern also differed to that of the latter. Consequently, nestlings in cooperative groups had larger fledging body mass than that in biparental nests. By examining reasons for nest failure, we revealed that conspecific nest-raiding contributed to more nest failure than any other natural predators. Because of the contribution of helpers in defending against both predators and conspecific nest-raiders, cooperative groups had higher survival rate than biparental nests. Thus, our findings suggest that in a highly-clumped nesting pattern, factors concerning the risk of nest predation, rather than that influencing food supply, play an important role in determining helper effects and responses of aided dominant pairs.

在鸟类合作繁殖体系中，社会配偶对因协助者（helpers）参与而获得的多项收益已得到揭示。然而迄今为止，决定协助者所提供援助类型以及社会配偶对响应机制的相关因素，仍未得到充分阐释。本研究针对栖息于青藏高原的蓝翅喜鹊（Cyanapica cyana，Azure-winged Magpie）合作繁殖群体中协助者的贡献，以及优势配偶对的相应响应展开了调查。本研究采用标记重捕法（Capture-Mark-Recapture）对协助者进行甄别。研究结果显示，协助者多为优势配偶对的一岁龄子代雄性。协助者主要通过三种方式为合作繁殖群体提供助力：为孵卵雌鸟进行求偶喂食、为雏鸟提供食物以及开展巢防御行为。针对优势配偶对的响应，本研究意外发现，巢内协助者的存在并未对窝卵数产生显著影响。然而，合作繁殖群体的雏鸟喂食频率显著高于双亲巢，且二者的喂食模式亦存在差异。因此，合作繁殖群体中的雏鸟出飞时的体质量显著高于双亲巢中的雏鸟。通过分析巢失败的原因，本研究发现，同种巢盗引发的巢失败比例高于任何其他自然捕食者。由于协助者可协助抵御捕食者与同种巢盗，合作繁殖群体的巢存活率显著高于双亲巢。综上，本研究结果表明，在高度聚集的筑巢模式下，决定协助者效应以及受助优势配偶对响应的关键因素，是巢捕食风险而非食物供给相关因子。