Data used in this study from Egg retrieval versus egg rejection in cuckoo hosts

Before complex nests evolved, birds laid eggs on the ground, and egg retrieval evolved as an adaptation against accidental displacement of eggs outside the nest. Therefore, egg retrieval is an ancient, and likely ancestral, widespread behaviour in birds. However, it has received little attention in studies of avian brood parasitism, perhaps because most parasitism occurs in species with complex nests, a context in which egg retrieval seems irrelevant. However, for cavity-nesting hosts of avian brood parasites, egg retrieval may still play an important role in the coevolutionary interactions between obligate brood parasites and hosts, because egg retrieval can be considered to be antagonistic to egg rejection behaviour in hosts, yet both may involve cognition to recognize eggs. We hypothesized that (1) cavity-nesting hosts should retrieve misplaced eggs from outside the nest cup, (2) brood parasitism has modulated egg retrieval behaviour in cavity-nesting hosts and (3) hosts use the same visual cues for decision-making during egg recognition in both egg retrieval and egg rejection actions. To test these hypotheses, we performed a series of experiments in a cavity-nesting host, the green-backed tit (Parus monticolus). Foreign eggs with different levels of mimicry were placed within or outside nest cups of hosts to test their responses. We found that host decisions about whether to retrieve or reject an egg both depended on the degree of mimicry. However, hosts sometimes first retrieved poorly mimetic foreign eggs and then rejected them. Alternatively, hosts sometimes failed to retrieve highly mimetic conspecific eggs. We suggest that egg retrieval in hosts is likely to be a result of the interaction between ancient retrieval behaviour and subsequent adaptation against brood parasitism.This article is part of the theme issue ‘The co-evolutionary biology of brood parasitism: from mechanism to pattern’.

在复杂巢穴演化形成之前，鸟类均于地面产卵，捡卵行为（egg retrieval）的演化是为了应对卵意外移出巢外的适应性策略。因此，捡卵行为是鸟类中一类古老且大概率属于祖先性状的广泛分布行为。不过，该行为在鸟类巢寄生（avian brood parasitism）的相关研究中鲜有受到关注，这或许是因为多数巢寄生现象发生在拥有复杂巢穴的鸟类类群中，此时捡卵行为看似无关紧要。然而，对于鸟类巢寄生的洞巢宿主而言，捡卵行为仍可能在专性巢寄生鸟类（obligate brood parasites）与宿主间的协同进化互作中发挥重要作用——因为捡卵行为可被视为与宿主的卵排斥行为（egg rejection behaviour）相互拮抗，但二者均需借助认知能力识别卵类。我们提出以下三项假说：（1）洞巢宿主应能将巢杯外的错位卵搬回巢内；（2）巢寄生作用调控了洞巢宿主的捡卵行为；（3）宿主在捡卵与卵排斥这两种行为的决策过程中，使用相同的视觉线索（visual cues）进行卵识别。为验证上述假说，我们以洞巢宿主绿背山雀（Parus monticolus）为研究对象开展了一系列实验。将拟态程度（mimicry）各异的外来卵放置于宿主的巢杯内外，以此测试其行为响应。研究发现，宿主对于是否捡卵或排斥卵的决策，均取决于外来卵的拟态程度。不过，宿主有时会先搬回拟态度较低的外来卵，随后再将其排斥；或是无法识别并搬回拟态度极高的同种卵。我们认为，宿主的捡卵行为很可能是古老捡卵行为与后续针对巢寄生的适应性演化共同作用的结果。本文隶属于主题专辑“巢寄生的协同进化生物学：从机制到模式”（The co-evolutionary biology of brood parasitism: from mechanism to pattern）。