Nest design as an adaptative tool to escape brood parasitism

Abstract 1. Nest characteristics vary depending on environmental factors but may also be influenced by between-species interactions. As a defence against avian brood parasites, hosts can evolve specific nest architecture to minimise risks of brood parasitism by preventing access to the nest cup or reducing the success of the parasite chick. 2. The common redstart (<em>Phoenicurus phoenicurus</em>) is a regular host of the common cuckoo (<em>Cuculus canorus</em>), and due to its cavity-nesting behaviour, cuckoo eggs are often laid outside the nest cup and consequently unsuccessful. This allowed us to test (i) if cuckoos prefer certain host nest designs and (ii) if redstart nest design can reduce the probability of successful brood parasitism. 3. While recording brood parasitism events in two redstart populations, we documented nest cup characteristics, such as internal dimensions, materials used and position within the nest-box, as well as the distance and angle between the nest cup and the nest-box entrance hole. 4. We found that cuckoos preferred bigger nest cups, possibly targeting high-quality individuals, and nest cups that were built higher than the rim material, which may contribute to an easier eviction of the host eggs/nestlings. Redstarts, on the other hand, reduced successful brood parasitism by placing nest cups further from the cavity entrance, but also selecting cavities with relatively smaller entrances. 5. We show the importance of host nest design influencing the coevolution between hosts and brood parasites. Redstarts seem to adopt nest design strategies to decrease brood parasitism risk, while cuckoos’ preference for certain aspects of redstart nest design may be shaped by a conflict between female laying success and chick eviction success. Hosts’ and brood parasites’ frontline strategies may then shape host nest characteristics and parasite’s preferences, impacting their co-evolutionary arms-race.

摘要 1. 巢的特征不仅受环境因素影响，同时也可能受到物种间相互作用的调控。作为抵御鸟类巢寄生者（avian brood parasites）的防御策略，寄主物种可演化出特定的巢结构，通过阻断巢杯（nest cup）的进入路径或降低寄生雏鸟的存活成功率，以减少巢寄生发生的风险。2. 普通红尾鸲（Phoenicurus phoenicurus）是大杜鹃（Cuculus canorus）的常规寄主，由于其洞巢栖息的习性，大杜鹃的卵常被产在巢杯之外，因此无法成功孵化。借此实验体系，我们得以验证两个科学问题：（i）大杜鹃是否偏好特定的寄主巢设计；（ii）普通红尾鸲的巢设计能否降低成功巢寄生的概率。3. 在对两个普通红尾鸲种群的巢寄生事件进行记录的过程中，我们还采集了巢杯的多项特征数据，包括内部尺寸、筑巢所用材料、巢箱内的摆放位置，以及巢杯与巢箱入口之间的距离和夹角。4. 研究结果显示，大杜鹃偏好尺寸更大的巢杯，这一选择可能指向优质的寄主个体；同时它们也偏好筑造高度高于巢边缘材料的巢杯，这类巢杯或可更便于寄生雏鸟推出寄主卵或雏鸟。反观普通红尾鸲，则通过将巢杯放置在远离洞巢入口的位置，以及选择入口相对更小的洞巢，来降低成功巢寄生的概率。5. 本研究证实了寄主巢设计在寄主与巢寄生者的协同进化过程中发挥着重要作用。普通红尾鸲似乎会通过调整巢设计策略来降低巢寄生风险，而大杜鹃对普通红尾鸲巢设计特定特征的偏好，可能是由雌鸟产卵成功率与雏鸟推离成功率之间的进化冲突所塑造的。由此可见，寄主与巢寄生者的前沿攻防策略，或共同塑造了寄主的巢特征以及寄生者的寄主选择偏好，进而影响二者的协同进化军备竞赛（coevolutionary arms-race）。