Animals rely on an array of environmental triggers or cues to make their behavioural and life‐history decisions. Brood–parasitic females should prefer the best host to maximize their reproductive success. Despite the fact that Eurasian reed warbler Acrocephalus scirpaceus is one of the most frequent host of the common cuckoo Cuculus canorus, parasitic chicks often prematurely fall out of the nests and drown. To establish whether these events are influenced by the size of the nest, we experimentally replaced original Eurasian reed warbler nests for much bigger great reed warbler Acrocephalus arundinaceus ones and transferred cuckoo chicks into them to record their fate. The chicks in bigger nests showed significantly lower falling‐out rate (4 of 29) compared with those remained in smaller nests (12 of 32). These findings suggest that host nest size plays an important role in the suitability of host species and that smaller host nests may pose a high mortality risk to the relatively big cuckoo chicks. Moreover, cuckoo chicks that fell out of the nests were significantly less fed by foster parents before falling‐out event than those stayed in the nests. This indicates that insufficient parental care may also facilitate falling‐out of cuckoo chicks, probably because hungry chicks are more restless in host nests. The relatively frequent falling‐out events represent a substantial and underestimated cost of brood parasites. Without video surveillance it can incorrectly be confused with predation. We therefore strongly recommend using video‐recording for determining predation costs in birds.

中文翻译：

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掉出寄主巢：一个被忽视的因素，降低了寄生虫雏鸡的存活率

动物依靠一系列环境触发因素或提示来做出行为和生活史决策。繁殖寄生的雌性应该选择最好的寄主，以最大程度地繁殖。尽管欧亚的芦苇莺（Acrocephalus scirpaceus）是杜鹃（Cuculus canorus）杜鹃的最常见寄主之一，但寄生小鸡常常过早地从巢中掉落并淹死。为了确定这些事件是否受巢的大小影响，我们通过实验将原始的欧亚芦苇莺替换为更大的大芦苇莺Acrocephalus arundinaceus并把杜鹃雏鸟转移到它们中以记录它们的命运。与留在较小巢穴中的雏鸡（32的12）相比，在较大巢穴中的小鸡的脱落率（29中的4）显着降低。这些发现表明，寄主巢的大小在寄主物种的适应性中起着重要作用，较小的寄主巢可能会给相对较大的杜鹃雏鸡带来很高的死亡风险。此外，从巢中掉出来的杜鹃雏鸟在掉落事件发生之前比寄养在巢中的养父母少得多。这表明父母的照顾不足也可能促进布谷鸟雏鸡的脱落，这可能是因为饥饿的雏鸡在寄主巢中更加不安。相对频繁的脱落事件代表着大量而被低估的寄生虫成本。没有视频监控，它可能会错误地与掠夺相混淆。因此，我们强烈建议使用视频录像来确定鸟类的捕食成本。