Data from: Multiple benefits of alloparental care in a fluctuating environment

Although cooperatively breeding vertebrates occur disproportionately in unpredictable environments, the underlying mechanism shaping this biogeographic pattern remains unclear. Cooperative breeding may buffer against harsh conditions (hard life hypothesis), or additionally allow for sustained breeding under benign conditions (temporal variability hypothesis). To distinguish between the hard life and temporal variability hypotheses, we investigated whether the number of alloparents at a nest increased reproductive success or load-lightening in superb starlings (Lamprotornis superbus), and whether these two types of benefits varied in harsh and benign years. We found that mothers experienced both types of benefits consistent with the temporal variability hypothesis, as larger contingents of alloparents increased the number of young fledged while simultaneously allowing mothers to reduce their provisioning rates under both harsh and benign rainfall conditions. In contrast, fathers experienced load-lightening only under benign rainfall conditions, suggesting that cooperative breeding may serve to take advantage of unpredictable benign breeding seasons when they do occur. Cooperative breeding in unpredictable environments may thus promote flexibility in offspring care behaviour, which could mitigate variability in the cost of raising young. Our results highlight the importance of considering how offspring care decisions vary among breeding roles and across fluctuating environmental conditions.尽管合作繁殖脊椎动物（cooperatively breeding vertebrates）在不可预测环境中的分布占比偏高，但塑造这一生物地理格局的潜在机制仍不明朗。合作繁殖既可以缓冲恶劣环境带来的生存压力（艰难生活假说，hard life hypothesis），也可使种群在温和环境下维持持续的繁殖活动（时间变异性假说，temporal variability hypothesis）。为区分这两种假说，我们以栗翅椋鸟（Lamprotornis superbus）为研究对象，探究巢内异亲（alloparents）的数量是否会提升繁殖成功率或产生负载减轻效应，并分析这两类收益在恶劣与温和年份的差异。 研究结果显示，母鸟同时获得了两类收益，符合时间变异性假说：无论降雨条件处于恶劣还是温和状态，更多的异亲个体均能增加出巢幼鸟的数量，同时降低母鸟自身的育雏投喂率。与之相反，公鸟仅在温和降雨条件下才会获得负载减轻效应，这表明合作繁殖或可帮助物种抓住偶发的温和繁殖季以获取收益。 因此，不可预测环境中的合作繁殖或能推动育雏行为的灵活性，从而缓解育雏成本的波动。本研究结果强调，需关注不同繁殖角色的育雏决策如何随环境波动发生变化。