被巢寄生鸟当作目标的很多鸟类已学会拒绝寄生鸟的卵，但它们却很少有排斥寄生鸟的幼鸟的——如果这些幼鸟得以孵化出来的话，即便这些幼鸟与它们自己的幼鸟在大小上的差别显而易见。同一物种内的寄生是较为罕见的一种巢寄生形式，在其中幼鸟拒绝的确会发生。
研究人员对此进行了一系列幼鸟交叉养育实验，实验对象是美洲黑鸭。实验结果表明，父母利用一窝中的第一只作为一个模板来孵化，并以此作为依据来判断以后所孵化的是否有可能是潜在入侵者的后代。这种学习规则也许可解释为什么幼鸟识别作为针对鸟类巢寄生的一种寄主防卫手段令人迷惑不解地不存在：对大多数物种间寄生形式的寄主来说，幼鸟识别可能是起反作用的，因为入侵者通常孵化得早，有很好的机会成为同窝幼鸟的模板。（生物谷Bioon.com）
生物谷推荐原始出处：
Nature 463, 223-226 (14 January 2010) | doi:10.1038/nature08655
Coots use hatch order to learn to recognize and reject conspecific brood parasitic chicks
Daizaburo Shizuka1 & Bruce E. Lyon1
1 Department of Ecology and Evolutionary Biology, University of California, Santa Cruz, California 95064, USA
Correspondence to: Daizaburo Shizuka1 Correspondence and requests for materials should be addressed to D.S.
Avian brood parasites and their hosts provide model systems for investigating links between recognition, learning, and their fitness consequences1, 2, 3, 4. One major evolutionary puzzle has continued to capture the attention of naturalists for centuries: why do hosts of brood parasites generally fail to recognize parasitic offspring after they have hatched from the egg5, 6, 7, 8, 9, even when the host and parasitic chicks differ to almost comic degrees7? One prominent theory to explain this pattern proposes that the costs of mistakenly learning to recognize the wrong offspring make recognition maladaptive10. Here we show that American coots, Fulica americana, can recognize and reject parasitic chicks in their brood by using learned cues, despite the fact that the hosts and the brood parasites are of the same species. A series of chick cross-fostering experiments confirm that coots use first-hatched chicks in a brood as referents to learn to recognize their own chicks and then discriminate against later-hatched parasitic chicks in the same brood. When experimentally provided with the wrong reference chicks, coots can be induced to discriminate against their own offspring, confirming that the learning errors proposed by theory can exist10. However, learning based on hatching order is reliable in naturally parasitized coot nests because host eggs hatch predictably ahead of parasite eggs. Conversely, a lack of reliable information may help to explain why the evolution of chick recognition is not more common in hosts of most interspecific brood parasites.
