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Egg 'signatures' allow drongos to identify cuckoo 'forgeries' almost every time, study finds
African cuckoos may have met their match with the fork-tailed drongo, which scientists predict can detect and reject cuckoo eggs from their nest on almost every occasion, despite them on average looking almost identical to drongo eggs.
Fork-tailed drongos, belligerent birds from sub-Saharan Africa, lay eggs with a staggering diversity of colors and patterns. All these colors and patterns are forged by the African cuckoo.
African cuckoos lay their eggs in drongos' nests to avoid rearing their chick themselves (an example of so-called brood parasitism). By forging drongo egg colors and patterns, cuckoos trick drongos into thinking the cuckoo egg is one of their own.
But drongos use knowledge of their own personal egg "signatures"—their eggs' color and pattern –to identify cuckoo egg "forgeries" and reject them from their nests, say scientists. These "signatures" are like the signatures we use in our daily lives: unique to each individual and highly repeatable by the same individual.
Through natural selection, the African cuckoo's eggs have evolved to look almost-identical to drongo eggs—a rare example of high-fidelity mimicry in nature.
A team led by researchers at the University of Cambridge and the University of Cape Town, working in collaboration with a community in Zambia, set out to explore the effectiveness of "signatures" as a defense against highly accurate mimicry. The findings are published today in the journal, Proceedings of the Royal Society B.
They found that despite near-perfect mimicry of fork-tailed drongo eggs, African cuckoo eggs still have a high probability of being rejected.
Researchers carried out field work in the Choma district of Zambia during September to November across four years. The first step was to measure the differences in color and pattern of the fork-tailed drongo eggs and cuckoo eggs. The team found that the color and pattern of cuckoo eggs was, on average, almost identical to that of drongo eggs, and that all the broad types of drongo egg signatures were forged by the cuckoos.
"It is incredible how perfect the mimicry is. We have occasionally missed cuckoo eggs in the field because they looked exactly like the drongo clutch that they were found in," said lead researcher and Zoology Ph.D. student Jess Lund.
The second step involved "egg rejection" experiments in which the researchers simulated cuckoo visits by "parasitizing" drongo nests with foreign eggs from other drongo nests (as a proxy for African cuckoo eggs). They then checked the nest daily to see whether the drongo parents accepted the foreign egg as one of their own, or realized it was an imposter and rejected it by removing it from their nest. The team could then test what differences in color and pattern between the foreign egg and the drongo's own eggs best predicted whether or not the drongo parents were tricked.
By combining results from both steps of the study, the researchers were able to create a model that predicted how often, on average, an African cuckoo would have its eggs rejected by a fork-tailed drongo host. They found the predicted rate of rejection to be 93.7%.
"We were surprised to see that so many of the cuckoo eggs were predicted to be rejected," said Lund "Our additional simulations show this is likely due to drongos having evolved 'signatures' on their eggs. Even though cuckoos have evolved excellent 'forgeries,' individual cuckoos don't target individual drongo nests that match their own eggs. This means that for each cuckoo egg laid, the likelihood that it will be a good enough match to that drongo's 'signature' is very low."
Fork-tailed drongos have likely honed these signatures and detection abilities through natural selection, explains Lund. "It's very costly for drongo parents if they don't have these skills. If they can't tell a "forgery" from their "signature" their own chicks will be killed by the cuckoo when it hatches, and they will be stuck raising a ravenous cuckoo for a whole breeding season."
The results of the study suggest that a female cuckoo may only fledge two chicks in her lifetime—only just replacing herself and her mate. Researchers say that this would not amount to a sustainable population, which presents a puzzle because African cuckoos remain a common bird in many parts of Africa.
The researchers think that the fork-tailed drongos where the research took place could be particularly good at spotting "forgeries."
"Perhaps this part of Zambia is a hotspot for parasitism, where drongos have particularly fine-tuned defenses, and against which cuckoos stand little chance," said Lund.
注释：
belligerent: adj
表示" 好战的；交战的"，means "characteristic of an enemy or one eager to figh"，如：Tom always said some very belligerent things. 汤姆他老是说些十分挑衅性的话。
rear: v
表示"养育；饲养"，means ""，如：Her mother died of cancer and her aunt rears her. 她母亲因癌症去世,她姨妈将她抚养大。
brood parasitism: n
表示"孵育寄生"， 其中brood表示"孵"，如：The hen broods ten eggs this time. 这次母鸡孵十个蛋。
parasitism， 表示" 寄生状态；寄生病"，如：Add the suppression of parasitism, and it will be solved. 加上又消灭了各类寄生虫，问题将会得到解决。
forgery: n
表示" 伪造；"，如：The signature is proved to be a forgery. 签字被证明是非法模仿的或伪造的。
fidelity: n
表示"忠实；忠诚"，如：His fidelity and industry brought him speedy promotion. 他的忠于职守和勤于工作使他得到迅速晋级。
mimicry: n
表示"模仿；"，means "the act of mimicking"，如：Language learning usually necessitates conscious mimicry. 一般地说,学习语言就要进行有意识的模仿。
clutch: n
表示" 一窝鸡（蛋）；一群"，如：He's won a whole clutch of awards. 他获得一大堆奖。
parasitize: v
表示" 寄生于；寄生感染"，如：The larvae of flies parasitize earthworms. 蝇的幼虫寄生在蚯蚓上。
proxy: adj
1. adj表示"代理的"，如：How can I determine my proxy settings? 我怎么知道我的代理服务器的设置?
2. n表示" 代理人"，means "a person authorized to act for another"，如：You may appoint a proxy to vote for you.你可以委托他人代你投票。
ravenous: adj
表示" 贪婪的；极饿的"，means "extremely hungry"，如：The country has also become a ravenous consumer. 这个国家还成为一个贪婪的消费者。
中文简要说明：
杜鹃鸟是知名的「鸟中骗子」，牠们不筑巢也不养育，而是把蛋下在别种鸟类的巢里，由其他鸟类帮牠们育子，这种现象称为「巢寄生」。然而「天道循环」，这么自私的生存方式终于踢到铁板，科学家发现，叉尾卷尾(Fork-tailed common)能够分辨哪些是自己的蛋，哪些是骗子鸟的蛋，然后挑出来丢掉。
物理学网 (phys)报导，与其他杜鹃鸟相同，非洲杜鹃(African cuckoos)也是一种骗子鸟，而且牠们会选择蛋的外观相似的鸟类，在牠们的巢里下蛋，以免被发现。
不过，同样生活在非洲的叉尾卷尾(也称普通卷尾)可不会上当，科学家表示，牠们已发展出一套辨认自己孩子的办法，就是蛋上的斑点，这些班点只有自己认的出来，即使非洲杜鹃也演化出类似的斑点，但是逃不过叉尾卷尾的「法眼」。因为叉尾卷尾会记得自己蛋上的斑点形式，就像一种签名一样，而杜鹃蛋上的那些斑点，只是一种伪劣的「赝品」。
由剑桥大学和开普敦大学的鸟类研究团队，来到冈比亚进行为期4年的研究，第一步是测量叉尾卷尾和杜鹃蛋颜色和图案的差异。研究小组发现，杜鹃蛋的颜色和图案平均而言，与叉尾卷尾几乎相同，科学家很难分辨。
研究成员之一、博士候选人劳德(Jess Lund)说：「模仿的完美程度令人难以置信。我们偶尔会在野外也会错认两种蛋，除非确实标记过，不然看起来几乎一模一样。」
科学家做了一种实验，把A家叉尾卷尾鸟蛋，换到B家叉尾卷尾的巢里，也就是同种族鸟「生错」巢，会如何？结果发现，养到别人家的孩子无所谓，只要都是叉尾卷尾种族就没关系。然而，叉尾卷尾会针对性挑掉非洲杜鹃的蛋，尽管看起来很像。研究人员发现，叉尾卷尾的判断准确率达到93.7%。
劳德认为，可能这是环境使然，非洲杜鹃可能曾经诈骗了一段时间，在如此恶劣的环境下，叉尾卷尾自然磨练辨认自家孩子的能力。「如果父母不具备这种技能，那么他们自己的雏鸟在孵化时，就会被杜鹃杀死，而父母将会在整个繁殖季节里，一直在饲养一只贪婪的杜鹃。」
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