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.

在鸟类合作繁殖系统（avian cooperative breeding systems）中，繁殖配对（social pairs）借助帮手鸟（helpers）的存在所获得的诸多益处已被广泛揭示。然而直至今日，决定帮手鸟所提供援助类型以及繁殖配对应对策略的相关因素，仍未得到充分阐明。我们以青藏高原繁殖的青翅鹊（Cyanapica cyana，Azure-winged Magpie）为研究对象，探究了帮手鸟对合作繁殖群体的贡献，以及优势繁殖对的相应应对策略。我们采用标记重捕法（capture–mark–recapture）对帮手鸟进行识别。研究结果显示，帮手鸟大多为优势繁殖对的当年生幼雄鸟。它们主要通过三种方式为合作群体提供协助：向孵卵雌鸟进行求偶喂食、育雏投喂以及巢区防御。针对优势繁殖对的应对策略，我们意外发现，巢内帮手鸟的存在并不会对窝卵数（clutch size）产生影响。然而，合作繁殖群体的育雏投喂率显著高于双亲巢，且其投喂模式与双亲巢也存在差异。由此，合作繁殖群体中的雏鸟离巢体重（fledging body mass）显著高于双亲巢中的雏鸟。通过分析巢失败（nest failure）的成因，我们发现同种巢侵袭（conspecific nest-raiding）导致的巢失败比例高于其他所有自然捕食者。得益于帮手鸟对捕食者与同种巢侵袭者的双重防御作用，合作繁殖群体的巢存活率显著高于双亲巢。综上，我们的研究结果表明，在巢址高度聚集的繁殖模式下，决定帮手鸟效应以及受助优势繁殖对应对策略的关键因素，是巢捕食风险（nest predation）而非食物供应相关因素。