Data from Experimentally induced increases in fecundity lead to greater nestling care in blue tits

Models on the evolution of bi-parental care typically assume that maternal investment in offspring production is fixed and predict subsequent contributions to offspring care by the pair are stabilized by partial compensation. While experimental tests of this prediction are supportive, exceptions are commonplace. Using wild blue tits (<i>Cyanistes caeruleus</i>), we provide, to our knowledge, the first investigation into the effects of increasing maternal investment in offspring production for subsequent contributions to nestling provisioning by mothers and male partners. Females that were induced to lay two extra eggs provisioned nestlings 43% more frequently than controls, despite clutch size being made comparable between treatment groups at the onset of incubation. Further, experimental males did not significantly reduce provisioning rates as expected by partial compensation, and if anything contributed slightly (9%) more than controls. Finally, nestlings were significantly heavier in experimental nests compared with controls, suggesting that the 22% average increase in provisioning rates by experimental pairs was beneficial. Our results have potential implications for our understanding of provisioning rules, the maintenance of bi-parental care and the timescale over which current–future life-history trade-offs operate. We recommend greater consideration of female investment at the egg stage to more fully understand the evolutionary dynamics of bi-parental care.

有关双亲育幼（bi-parental care）演化的模型通常假定母体投入（maternal investment）在后代生产（offspring production）环节固定不变，并据此预测：配对双方后续对育幼的贡献会通过部分补偿（partial compensation）机制维持稳定。尽管该预测得到了实验验证的支持，但例外情况并不鲜见。本研究以野生蓝山雀（Cyanistes caeruleus）为研究对象，据我们所知，首次探讨了母体在后代生产环节投入增加后，对雌鸟及雄配偶后续雏鸟投喂（nestling provisioning）行为的影响。尽管在孵化（incubation）初期我们已使实验组与对照组的窝卵数（clutch size）保持一致，但被诱导多产两枚卵的雌鸟，其雏鸟投喂频率较对照组高出43%。进一步而言，实验组雄鸟并未如部分补偿机制所预期的那样显著降低投喂频率，反而较对照组略微提升了9%的投喂贡献。最后，实验组巢中的雏鸟体重显著高于对照组，这表明实验组配对整体的投喂频率平均提升22%的做法是有益的。本研究结果有助于深化我们对育幼投喂规则、双亲育幼维持机制，以及当前-未来生活史权衡（life-history trade-offs）发挥作用的时间尺度的理解。我们建议未来研究应更多关注产卵阶段的母体投入，以更全面地解析双亲育幼的演化动力学（evolutionary dynamics）。