Predation is a major cause of mortality and nest failure in birds. Cooperative predator defense can enhance nest success and adult survival but, because it is inherently risky, dynamic risk assessment theory predicts that individuals modify defense behavior according to the risk posed by the predator. Parental investment theory, on the other hand, predicts that reproductive payoffs (brood value) determine investment in nest defense. We propose that, in cooperative breeders, fitness benefits deriving from the survival of other group members may additionally influence defense behavior (social group benefits theory). We tested predictions of these theories in the cooperatively breeding purple-crowned fairy-wren, Malurus coronatus, where brood value is higher for breeders, but social group benefits more important for helpers. We recorded experimentally induced individual defense behaviors in response to predator models presented near nests, representing differing levels of threat to nests and adults. As predicted, 1) individuals engaged in less risky defenses when encountering a more dangerous predator (dynamic risk assessment theory); 2) individuals defended older broods more often, and breeders defended more than helpers (parental investment theory); and 3) helpers were more likely to respond to a predator of adults (social group benefits theory). Our findings highlight that predator defense in cooperative breeders is complex, shaped by the combination of immediate risk and multiple benefits.

中文翻译：

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捕食者防御由合作育种者的风险、育雏价值和社会群体利益塑造

捕食是鸟类死亡和筑巢失败的主要原因。合作捕食者防御可以提高筑巢成功率和成虫存活率，但由于其固有的风险，动态风险评估理论预测个体会根据捕食者带来的风险改变防御行为。另一方面，父母投资理论预测繁殖收益（育雏价值）决定了对巢穴防御的投资。我们提出，在合作育种者中，源自其他群体成员生存的健康益处可能会额外影响防御行为（社会群体利益理论）。我们在合作繁殖的紫冠仙女鹪鹩（Malurus coronatus）中测试了这些理论的预测，其中繁殖者的育雏价值更高，但社会群体利益对助手更重要。我们记录了实验诱导的个体防御行为，以响应巢穴附近出现的捕食者模型，代表对巢穴和成虫的不同威胁水平。正如预测的那样，1）个体在遇到更危险的捕食者时会采取风险较小的防御措施（动态风险评估理论）；2) 个体更频繁地保护较老的育雏，育种者保护的多于帮手（父母投资理论）；3）帮助者更有可能对成年人的掠夺者做出反应（社会群体利益理论）。我们的研究结果强调，合作育种者的捕食者防御是复杂的，由直接风险和多重收益的组合形成。1）个体在遇到更危险的捕食者时会采取风险较小的防御措施（动态风险评估理论）；2) 个体更频繁地保护较老的育雏，育种者保护的多于帮手（父母投资理论）；3）帮助者更有可能对成年人的掠夺者做出反应（社会群体利益理论）。我们的研究结果强调，合作育种者的捕食者防御是复杂的，由直接风险和多重收益的组合形成。1）个体在遇到更危险的捕食者时会采取风险较小的防御措施（动态风险评估理论）；2) 个体更频繁地保护较老的育雏，育种者保护的多于帮手（父母投资理论）；3）帮助者更有可能对成年人的掠夺者做出反应（社会群体利益理论）。我们的研究结果强调，合作育种者的捕食者防御是复杂的，是由直接风险和多重收益的组合形成的。