巢寄生鸟类与寄主之间的“军备竞赛”是研究物种协同进化的绝佳范例。寄生双方在不同时期进化出不同的寄生与反寄生策略。大杜鹃（Cuculus canorus）是最为典型的巢寄生鸟类之一，因其分布广、寄主种类多、寄生体系复杂而备受重视。大杜鹃在欧洲地区可寄生在125种以上雀形目鸟类的巢中，但主要寄主约有20种，并分化为16个支系或卵型。特定的杜鹃种群选择专门的寄主，这种寄主的专一性在很多杜鹃支系中都得到了证实。杜鹃如何选择与之匹配的寄主？为什么一些适宜的寄主却很少被利用？这是巢寄生研究领域两个备受关注的问题。东方大苇莺（Acrocephalus orientalis）和震旦鸦雀（Paradoxornis heudei）是两种依赖芦苇生境繁殖的鸟类，在局部地区同域分布。在山东黄河三角洲国家级自然保护区，东方大苇莺是大杜鹃的重要寄主，其寄生率为24.8%（61/246巢）；而震旦鸦雀仅偶尔被寄生，寄生率低于1%（1/106 巢）。前期研究认为，本地区杜鹃卵均高度模拟东方大苇莺卵，而非震旦鸦雀卵。本研究不完全支持生境印记假说（Habitat imprinting hypothesis），即杜鹃在印记生境中随机选择寄主。杜鹃对东方大苇莺的倾向性选择可能取决于以下两方面的因素：一是杜鹃的主动性选择机制，如寄主印记（Host imprinting）或其他的寄主选择线索；另一方面则是震旦鸦雀在生活史特征的限制或某方面突出的反寄生能力。本论文主要从第二个方面探讨大杜鹃对两种寄主选择性寄生的机制。主要结果如下：（1）种群数量和分布在山东黄河三角洲国家级自然保护区，东方大苇莺种群数量大（250-300对）、分布广；震旦鸦雀种群数量较小（50对），分布狭窄。二者仅局部性同域分布，如研究区A，两种鸟类的巢址几乎随机镶嵌分布，但另外两个区域震旦鸦雀数量很少，主要以东方大苇莺为主。三个区域东方大苇莺均被寄生，寄生率相似，在20~30%之间。东方大苇莺寄生的巢主要分布在距离电线等栖枝较近的芦苇生境中。（2）繁殖时间震旦鸦雀为留鸟，繁殖较早，开始于5月初，繁殖期与大杜鹃部分隔离。杜鹃产卵之前，55%的震旦鸦雀已经进入孵卵和育雏期，失去寄生的可能。而大杜鹃和东方大苇莺的繁殖期同步，均从5月底开始到7月底或8月初。东方大苇莺被寄生的概率与单位时间内巢的可获得性成反比，繁殖同步性越高的繁殖对，其巢被寄生的概率越低。（3）巢址特征尽管两种鸟类均筑巢于芦苇生境，但二者的巢址特征略具差异。震旦鸦雀喜好具有一定比例旧芦苇、中等高度（1.5-2.5m）的芦苇生境；而东方大苇莺偏好芦苇长势更好(2.5m以上)、无旧芦苇的生境。但这些差异可能不是导致杜鹃倾向性选择寄主的主要原因。震旦鸦雀距离栖枝的距离（67.93±7.64m）显著大于东方大苇莺（42.80±4.45m；Mann-Whitney U：Z=558.0，p=0.002），该因素可能会对杜鹃的寄主选择具有一定影响。另外，震旦鸦雀的巢体积较小，仅为东方大苇莺的一半，对杜鹃雏鸟的生长具有一定影响。距栖枝的距离是影响大杜鹃寄生东方大苇莺的重要因素，这符合栖枝接近假说（Perch approach hypothesis）。但在研究区A和B，寄生巢和非寄生巢距栖枝的距离却没有显著性差异，这主要是由于这两个区域电线网络密集，为杜鹃提供了足够的栖位，一般巢址距电线的距离在30m之内。在这些区域，东方大苇莺被寄生的概率可能由一些其他因素决定的，比如前面提到的单位时间内东方大苇莺巢的可获得性等。本研究不完全支持巢暴露度假说（Nest exporsure hypothesis），寄生巢的可见度略高于非寄生巢，但巢址上方的植被高度和盖度均是寄生巢高于非寄生巢。巢暴露度也是影响捕食的重要因素，大杜鹃是否在寄生与捕食两种因素之间做出权衡，还有待进一步研究。（4）卵模拟和识别大杜鹃卵高度模拟东方大苇莺的卵，而非震旦鸦雀的卵。卵的模拟既包括卵色和斑点，也包括卵的大小。本地区大杜鹃（卵长径*短径：21.97 mm*16.22 mm，n=39）和东方大苇莺（21.21 mm*15.49 mm，n=104)的卵均小于日本的该寄生体系（杜鹃：22.7 mm*17.3 mm；n=59；东方大苇莺：22.1 mm*15.9 mm，n=215），也小于欧洲大杜鹃（22.26 mm*16.57 mm, n=91）与大苇莺(23.05 mm*16.25 mm, n=294)的寄生体系。震旦鸦雀对苇莺支系杜鹃模拟卵的识别率较高，特别是在产卵期，其识别率高达100%（n=8）；而东方大苇莺对高度模拟的杜鹃卵的识别率较低，约为20%左右。震旦鸦雀和东方大苇莺对三组非模拟卵（模型卵、同种涂蓝色卵和异种互换卵）均具有较强的识别能力，但震旦鸦雀对非模拟卵的识别能力（约60%）略低于东方大苇莺（约70%）。这说明震旦鸦雀可能是大杜鹃遗弃的寄主，其依然保留着部分的卵识别能力。但由于寄生压力较小，卵识别能力明显低于现在的常见寄主——东方大苇莺。模型卵的材质对震旦鸦雀的丢卵行为有影响，但对东方大苇莺无影响。东方大苇莺可以直接丢除模型卵和真卵，但震旦鸦雀仅可以丢除真卵，而不丢除模型卵。震旦鸦雀识别模型卵后主要是弃巢或被迫接受。从抓卵指数和喙的张合度分析发现，震旦鸦雀几乎不可能采用抓取方式丢卵，而东方大苇莺存在抓取的可能性，如喙叼卵的最小张合度仅为30°左右，但抓卵指数却小于200mm2，低于抓取杜鹃卵的最低线。震旦鸦雀常采用刺穿的方式丢出真卵，这从已有的视频和模型卵上的啄痕可以证明；但东方大苇莺到底是采用抓卵还是刺穿方式，还需进一步研究。震旦鸦雀对卵的识别能力在产卵期最高，随孵卵期降低；而东方大苇莺对卵识别与孵卵期无关。两种鸟类识别非模拟卵所需要的时间均较短（约1-2天），拒绝模型卵时均具有较高的代价，而拒绝真卵的代价则相对较低。虽然，东方大苇莺更容易丢卵，但两者在卵拒绝代价上无显著性差异（H=2.78，df=2, p=0.250）。东方大苇莺卵识别的主要区域在卵钝端，说明钝端的斑点在鸟类卵识别中具有重要信号功能。但卵拒绝率还与涂卵的面积有关，全涂卵的识别率高于涂钝端和尖端。（5）标本识别巢寄生者和捕食者代表两种不同的天敌，东方大苇莺可以识别出二者的区别，并具有不同的巢防御策略（Enemy-specific defense strategies）。东方大苇莺对遇见率高的灰色型杜鹃的防御强度更高，而对熟悉的捕食者—喜鹊（Pica pica）则不敢攻击（GLLM: F4, 306 = 16.38, P < 0.001）。这一结果符合“动态风险评估”理论（Dynamic-risk assessment hypothesis）。另外，巢入侵者的遇见率对巢防御行为也有显著性影响。东方大苇莺是兼性一雄多雌制的鸟类，性别冲突也表现在巢防御策略上。一般地，雌性更容易发现标本；但在首次发动攻击上，二者没有显著区别（Mann-Whitney U: U = 10.5, P = 0.671）。在攻击强度上，雄性显著高于雌性（F1, 306 = 14.87, P < 0.001）。这说明雄性在巢防御方面的投入显著大于雌性。（6）雏鸟期两种寄主均缺乏对杜鹃雏鸟的识别。通过杜鹃转移实验（6只转移到震旦鸦雀巢内；8只转移到东方大苇莺巢内），两种寄主均能成功哺育出杜鹃雏鸟，而且杜鹃雏鸟的生长曲线和出飞体重（东方大苇莺巢：58.43±1.40g，n=3；震旦鸦雀巢：55.13±6.27g，n=2）均无显著性差异。杜鹃雏鸟在两种寄主中的繁殖成功率也无显著性差异（东方大苇莺37.3%；震旦鸦雀33.3%; Fisher Exact Test: p = 1.000）。繁殖失败主要原因是天敌捕食（东方大苇莺62.5%；震旦鸦雀50%）。从育雏努力和食物资源上看，尽管两种寄主育雏期的食物资源具有较大差异，但育雏努力没有区别（F1, 58 = 0.026, P = 0.876），东方大苇莺的育雏频次为8.64±7.00 次/小时（n=37），震旦鸦雀为7.26±4.25次/小时（n=22）。震旦鸦雀育雏的食物大小（0.54±0.44，n=210）小于东方大苇莺（0.74±0.67，n=362; T=3.701，df=570, p< 0.001），但每次育雏的食物数量（1.43±0.80，n=210）多于东方大苇莺（1.08±0.28，n=362; T=-7.704，df=558, p< 0.001），因此，震旦鸦雀可以通过增加每次喂食食物的数量来弥补食物大小的不足，以满足杜鹃雏鸟的正常生长。

The “arms races” between avian brood parasites and their hosts provides an excellent example to study biological coevolution. The counter-parts have evolved different parasitism and anti-parasitism strategies at different breeding stages. The Common cuckoo Cuculus canorus is a typical obligate brood parasite and widely distributes in large area of northern Palearctic region (both Europe and Asia). The cuckoos can parasitize more than 125 species of passerines, but only 20 species are regular hosts with 16 distinct cuckoo gentes, mimicking the eggs of one or a few host species in Europe. The host preference has been well confirmed in several gentes of Common cuckoo hosts. However, there were still two unsolved puzzles that how could the cuckoo find the “right” host and why some commonly sympatric potential hosts were absence of cuckoo parasitism?Oriental reed warbler Acrocephalus orientalis and Reed parrotbill Paradoxornis heudei were two reed-dependent breeding passerines with similar nest site requirements and nest architecture, and were locally sympatric distribution in their large distribution area. Oriental reed warbler was the common host of Common cuckoo, with heavily parasitism rate of 24.8% (61 of 246 nests), while Reed parrotbill was an accidental host with the parasitism rate lower than 1% (one of the 106 nests) . All the cuckoo eggs parasitized in the two hosts were perfectly mimicry to Oriental reed warbler eggs, but not to Reed parrotbill. This study didn’t completely support the habitat imprinting hypothesis, particularly in the respective of random searching hosts’ nests within the same habitat. The preferential parasitism on Oriental reed warbler might be determined by the following two main mechanisms: first, the initiative host choice of Common cuckoo, i.e. host imprinting and other signal clues associated with host identity; second, the limitation of some life-history characteristics and extraordinarily strong anti-parasitic abilities of the Reed parrotbill reventing cuckoo parasitism.(1) Population size and distributionThe population size of Oriental reed warbler is much bigger (250-300 pairs) than that of Reed parrotbill (50 pairs), and the distribution of the warbler is also larger than reed parrobills in our study site. They mainly locally sympatric bred in the same redbed, such as site A, where the two hosts’ nests were in randomly mosaic distribution, while in site B and C the dominant breeding host was Oriental reed warbler. All the Oriental reed warbler populations in three study plots have been parasitized by Common cuckoo with rough similar parasitism rate (20~30%) and most of the parasitized nests distributed closely to wires, which were important perch sites for Common cuckoo.(2) Isolation in the breeding timeReed parrotbills were resident birds and started to breed earlier (8 May) than Oriental reed warblers (25 May) and Common cuckoos (1 June), while there were no significant differences between that of Common cuckoo and Oriental reed warbler. The breeding time of Reed parrotbill was partly isolated from Common cuckoo, and ca. 55% of breeding attempts of Reed parrotbill have started before cuckoo parasitism. Furthermore, the parasitism risk of Oriental reed warbler was negative to the temporal availability of vulnerable nests during egg-laying stage. The increase of breeding synchronization will decrease the parasitism rate.(3) Nest site characteristics Both Oriental reed warbler and Reed parrotbill bred in the reedbed, but they chose different reed habitats. Oriental reed warbler preferred to nest on harvested and higher reed patch (reed height: >2.5m) with deeper water than Reed parrotbill (1.5-2.5m). but the micro-environmental difference between them were not considered to be the main reasons for the host preference of Common cuckoo. The distance to perch site of Reed parrotbill (67.93±7.64m) was larger than that of Oriental reed warbler (42.80±4.45m), which may affect on the cuckoo parasitism risk. Beside, the nest volume of Reed parrotbill was only half of that of the Oriental reed warbler, which was unfavorable in rearing the young Common cuckoo chicks. This study supported the perch approach hypothesis that the close distance to perch site was critically important for the cuckoo parasitism on Oriental reed warbler. But we didn’t find any significant differences in the distance between parasitized nests and unparasitized nests at site B and C, where most of the nests were within 30 m to the wires. This implied that the parasitism risk of Oriental reed warbler in the habitats with enough perch site may depend on other factors, such as temporal availability of nests.The study couldn’t completely support the nest exposure hypothesis. Even though the visibilities of parasitized nests were slight higher than unparasitized nests, the vegetation height and coverage of parasitized nests were much higher than unparasitized nests. Further investigations should pay attention to the trade-off between parasitism and predation by Common cuckoo as the nest exposure was also an important factor on the nest predation.(4) Egg mimicry and rejectionThe Common cuckoo eggs were perfectly mimetic to Oriental reed warbler eggs rather than Reed parrotbill eggs. The mimicry included both the background color and spots, and egg size. The egg sizes of our Common cuckoo (Egg length* breadth: 21.97mm*16.22mm, n=39) and Oriental reed warbler populations (21.21mm*15.49mm, n=104) were smaller than the corresponding Japanese populations (CC: 22.7mm*17.3mm；n=59; ORW：22.1mm*15.9mm, n=215) and European populations (CC: 22.26mm*16.57mm, n=91; GRW: 23.05mm*16.25mm; n=294).Reed parrotbills have a high egg rejection ability of Oriental reed warbler gen cuckoo eggs, particularly during egg-laying stage, and the rejection rate reached 100% (n=8), while the egg rejection rate of cuckoo eggs by Oriental reed warbler was only about 20%.Both Oriental reed warblers and Reed parrotbills have high egg rejection of non-mimetic eggs (model eggs, painted blue eggs and heterogeneous eggs) with egg rejection rate of Oriental reed warbler was slight higher (ca.70%) than Reed parrotbill(ca.60%). The Reed parrotbill may be an abandon host of Common cuckoo from the still-carrying egg recognition ability, but which have declined following the lower of the parasitism pressure. There were some obstacles for Reed parrotbill to eject the model eggs, but none for Oriental reed warbler. Most responses of egg recognition of Reed parrotbills were forced acceptance or nest desertion. The comparisons of grasp index and open angles of two hosts’ bills implied that it was impossible for Reed parrotbill to eject the model eggs, but a conflict result for Oriental reed warbler. The minimum open angle of Oriental reed warbler was about 30°, which make it possible to grasp the model egg directly, however the grasp indices of Oriental reed warbler were lower than the minimum grasp index value (200mm2) to eject cuckoo eggs. Existing video-recordings have showed that Reed parrotbill used puncture to reject the true non-mimetic eggs (our unpublished data), and great reed warbler also choose a similar puncture skill to eject model egg in Czech. Further video-recording should be conducted to inspect the egg rejection way of Oriental reed warbler. The egg recognition of Reed parrotbills will decreased following the egg hatching, with significant high rate at egg-laying stage (Wald=7.339, df=2, p=0.025). However, there was no such temporal changes for Oriental reed warbler. There were relative high costs to reject model eggs in two hosts, but almost no costs to reject true eggs.The egg recognition clues of Oriental reed warbler mainly placed at the blunt pole of the egg as the egg rejection rate of painted blunt eggs (45.5%) was significant higher than the painted sharp eggs (27.3%). However the egg rejection of maculation painted blue eggs (75%) was higher than other two groups, which implied that both spots and background were clues to egg recognition, while the importance of spots were higher than background.(5) Mounted dummy experimentsBrood parasites and predators represent different threats to passerines that may favor the evolution of enemy-specific defense strategies. Oriental reed warblers showed strong nest defense by frequent attacks in four dummy groups, with special highly attacks on the gray morph cuckoo and very low response (both less number of attacks and farer approach distance) to the common magpie (GLLM: F4, 306 = 16.38, P < 0.001). The results agree with the dynamic risk assessment hypothesis and that the nest defense of Oriental reed warblers was fine-tuned adjusted based on the local encounter rate of nest intruders.Oriental reed warbler was facultative polygamous birds. Potential sex-specific variation in parental investment may be manifested in sex-specific differences of nest defense behavior. Even though there were no clear sexual differences from the categorical defense variable value (response degree and the first attack), sex-special role were revealed from the attack number, with females responding more quickly, but making less attack behavior than males (F1, 306 = 14.87, P < 0.001).(6) Nestling stageNestling stage was another important co-evolution stage of brood parasitism. The cross-transferred experiments showed that both Oriental reed warbler and Reed parrotbill lacked nestling discrimination ability and the cuckoo chicks grew well provisioned by two hosts with similar growth curves and fledging weight of cuckoo chicks (ORW hosts: 58.43±1.40g, n=3; RP hosts: 55.13±6.27g, n=2). The breeding successes of cuckoo chicks in two hosts were also no significant difference (ORW hosts: 37.3%; RP hosts: 33.3%; Fisher Exact Test: P = 1.000) with most of nest failure resulting from nest predation (ORW hosts: 62.5%; RP hosts: 50%). There were no significant differences in the provisioning efforts between the two hosts (ORW hosts: 8.64±7.00 per hour, n=37; RP hosts: 7.26±4.25, n=22; F1, 58 = 0.026, P = 0.876), but significant differences in the prey composition. Reed parrotbills would increase the number of prey per feeding carriage (RP hosts: 1.43±0.80 preys versus ORW hosts: 1.08±0.28; T = -7.704，df = 558, p < 0.001) to make up the small prey size (RP hosts: 0.54±0.44 bill sizes versus ORW hosts: 0.74±0.67; T = 3.701，df = 570, p < 0.001), and ultimately to meet the nutrient requirements of cuckoo growth.