Data from: Cuckoo parasitism in a cavity nesting host: near absent egg-rejection in a northern redstart population under heavy apparent (but low effective) brood parasitism

Brood parasite - host systems continue to offer insights into species coevolution. A notable system is the redstart Phoenicurus phoenicurus parasitized by the “redstart-cuckoo” Cuculus canorus gens. Redstarts are the only regular cuckoo hosts that breed in cavities, which challenges adult cuckoos in egg laying and cuckoo chicks in host eviction. We investigated parasitism in this system and found high overall parasitism rates (31.1% of 360 redstart nests), but also that only 33.1% of parasitism events (49 of 148 eggs) were successful in laying eggs into redstart nest cups. The majority of cuckoo eggs were mislaid and found on the rim of the nest; outside the nest cup. All available evidence suggests these eggs were not ejected by hosts. The effective parasitism rate was therefore only 12.8% of redstart nests. Redstarts responded to natural parasitism by deserting their nests in 13.0% of cases, compared to desertion rates of 2.8% for non-parasitized nests. Our egg parasitism experiments found low rates (12.2%) of rejection of artificial non-mimetic cuckoo eggs. Artificial mimetic and real cuckoo eggs added to nests were rejected at even lower rates, and were always rejected via desertion. Under natural conditions, only 21 cuckoo chicks fledged of 150 cuckoo eggs laid. Adding to this low success, is that cuckoo chicks are sometimes unable to evict all host young, and were more likely to die as a result compared to cuckoo chicks reared alone. This low success seems to be mainly due to the cavity nesting strategy of the redstart which is a challenging obstacle for the cuckoo. The redstart-cuckoo system appears to be a fruitful model system and we suggest much more emphasis should be placed on frontline defences such as nest site selection strategies when investigating brood parasite-host coevolution.

巢寄生（brood parasite）-宿主系统持续为物种协同进化研究提供关键洞见。其中极具代表性的研究体系为普通红尾鸲（Phoenicurus phoenicurus）被“红尾鸲型大杜鹃”（Cuculus canorus寄生宗）寄生的体系。红尾鸲是唯一一种在洞穴中筑巢繁殖的常规大杜鹃宿主，这一特性既对成年大杜鹃的产卵行为构成挑战，也为杜鹃雏鸟的宿主驱逐行为带来了阻碍。我们对该系统的寄生现象展开了调查，结果显示整体寄生率较高：在所调查的360个红尾鸲巢中，寄生率达31.1%；但进一步分析发现，仅有33.1%的寄生事件（148枚寄生产卵尝试中的49枚）成功将卵产入红尾鸲的巢杯内部。绝大多数大杜鹃卵被误产在巢缘而非巢杯内，所有现有证据均表明这些卵并非被宿主主动抛出。因此，实际有效寄生率仅为红尾鸲巢的12.8%。红尾鸲对自然寄生的响应表现为：13.0%的寄生巢会被亲鸟弃巢，而非寄生巢的弃巢率仅为2.8%。我们开展的卵寄生实验显示，对于人工放置的非拟态大杜鹃卵，宿主的拒卵率仅为12.2%；而人工放置的拟态卵以及真实大杜鹃卵的拒斥率更低，且所有拒斥行为均通过弃巢完成。在自然条件下，所产的150枚大杜鹃卵中仅有21枚成功育出雏鸟。除此以外，部分杜鹃雏鸟无法将所有宿主幼鸟驱逐出巢，相较于独自被抚育的杜鹃雏鸟，这类雏鸟的死亡率显著更高。这种低寄生成功率主要源于红尾鸲的洞穴筑巢策略，这对大杜鹃而言是极具挑战性的演化障碍。红尾鸲-大杜鹃系统堪称极具价值的模式研究体系，我们建议在开展巢寄生-宿主协同进化研究时，应更多关注诸如巢址选择策略等前沿防御机制。