Data from: Maternal allocation in eggs when counting on helpers in a cooperatively breeding bird

For cooperatively breeding birds, it has been proposed that breeders should reduce their investment in eggs when they count on helpers, because this can be compensated for by helpers provisioning of nestlings. Data from some species have supported this prediction, but this is not the case in others. It has also been proposed that mothers should not reduce but rather increase investment if the presence of helpers enhances the reproductive value of offspring, a pattern that might also influence egg production as long as helpers are predictable for laying females. Here, we studied maternal expenditure in eggs and clutches in the Iberian magpie, to see whether mothers reduce their expenditure at the egg stage in the presence of helpers. Our results show that investment in clutches varied depending on the year, date in the season and age of the mother, but there were no reductions in maternal expenditure per individual egg when they counted on helpers. On the contrary, a pattern emerged in the opposite direction of more investment in eggs associated with the future presence of helpers at the nestling stage. Our data suggest that the predictability of helpers, along with the type of benefits accrued from the contribution of helpers, may be crucial to understanding the reaction of mothers at egg production.

针对合作繁殖鸟类，已有假说提出：当繁殖亲鸟可获得帮手协助时，应降低其在鸟卵上的投入，因为该部分投入可通过帮手对雏鸟的育雏行为得到补偿。部分物种的实验数据支持这一预测，但其他物种并未呈现该规律。另有假说指出，若帮手的存在能够提升后代的繁殖价值，则亲鸟非但不应降低投入，反而应增加对卵的投资；只要产卵雌鸟能够预判帮手的到来，这种模式同样可能影响卵的产量。本研究以伊比利亚喜鹊（Iberian magpie）为研究对象，探究繁殖亲鸟在有帮手协助时，是否会降低产卵阶段的投入。研究结果显示，亲鸟的窝卵投入会随年份、繁殖季日期以及亲鸟年龄发生变化，但当亲鸟预判有帮手协助时，其单枚鸟卵的投入并未减少。与之相反，我们观察到一种反向模式：雏鸟阶段未来帮手的到场与更高的卵投入水平相关联。本研究数据表明，帮手的可预判性，以及帮手贡献所带来的收益类型，或是理解亲鸟在产卵阶段应对策略的关键因素。