鸟类依赖于听觉、视觉和嗅觉通讯感知周围的生物和非生物环境，其中鸣唱和羽饰是鸟类最重要的两种通讯信号。鸣唱长而复杂，主要由雄鸟在繁殖季发出；羽饰具有丰富多彩的颜色。它们是鸟类第二性征的两种重要表型，受到性选择和自然选择等压力的作用，同一物种的鸣唱和羽饰在不同性别、不同地理种群、同一性别不同年龄段或者是不同个体之间发生分化。

本研究的研究对象为一种雀形目鸟类绿背姬鹟（green-backed flycatcher, Ficedula elisae）。绿背姬鹟存在鸣唱和羽饰延迟成熟现象，一龄雄鸟（second-year male, SY male）和二龄及以上雄鸟（after-second-year male, ASY male）的鸣唱和羽饰均有显著不同。本研究揭示了绿背姬鹟鸣唱和羽饰在雄鸟不同年龄段之间的分化，分析鸣唱在雄鸟不同个体之间的分化，探讨雄鸟的鸣唱复杂性是否能够真实地反映它们的身体质量。本研究的主要结果如下：

1. 绿背姬鹟的一个自发鸣唱句子由三个音节组组成：第一、第二音节组简短，第三音节组长而婉转、复杂多变。第三音节组具有一定的语法结构，其语法复杂性可以用线性指数来衡量。语法复杂性是鸣唱复杂性的一部分，鸣唱复杂性还包括音节组间和音节组内复杂性。研究结果表明，绿背姬鹟雄鸟鸣唱第三音节组的线性指数为0.217 ± 0.061 (0.140–0.353)，低于同类研究中孟加拉雀等鸟类的线性指数，表示绿背姬鹟的鸣唱语法更加复杂。比较绿背姬鹟ASY和SY雄鸟在第三音节组语法、音节组间和音节组内复杂性上的差异，发现ASY雄鸟第三音节组整体的复杂性显著高于SY雄鸟（p = 0.042)，其中的差异体现在第三音节组音节曲目上，而在语法和音节组内复杂性等指标上没有显著差异，表明绿背姬鹟SY雄鸟成长为ASY雄鸟后，发出了更多类型的第三音节组音节，但是并没有增加音节组时长，也没有增加每音节组包含的不同音节型数量和转换次数，更没有使音节组语法变得更加复杂。

2. 绿背姬鹟雄鸟复杂的鸣唱具有个体性，显示出明显的个体间差异与个体内稳定，可用于进行个体识别研究。利用判别分析和语图相似性分析对绿背姬鹟鸣唱的第一音节组进行个体识别分析，发现判别分析区分不同的第一音节组类型的正判率为94.5%，第一音节组的类型内语图相似性也显著大于类型间语图相似性。在一个繁殖季的时间范围内，判别分析区分不同的第一音节组类型的正判率为87.1%，第一音节组的类型内语图相似性也仍然显著大于类型间语图相似性。这些结果表明绿背姬鹟鸣唱第一音节组的类型间差异大于类型内差异，且同一类型在一个繁殖季的时间范围内保持稳定。绿背姬鹟复杂鸣唱的判别分析正判率与已有的利用简单鸣声开展的个体识别研究正判率相近甚至更高，综合语图相似性分析的结果，绿背姬鹟的复杂鸣唱可以用于非损伤性地区分与标记雄鸟个体，进而开展相关的鸟类研究工作。

3. 绿背姬鹟ASY雄鸟难以从羽色上区分SY雄鸟和雌鸟，视觉模型分析和领域入侵实验结果均支持模仿雌鸟假说。羽饰延迟成熟现象研究中，模仿雌鸟假说最受关注。过往验证模仿雌鸟假说的研究从人类视角来判断SY雄鸟与雌鸟是否相似，然而人类与鸟类视觉系统存在明显差异，导致模仿雌鸟假说往往验证失败。本研究基于鸟类视觉模型比较绿背姬鹟SY雄鸟与雌鸟的羽色差异，证实绿背姬鹟ASY雄鸟从头、背、腰和喉部的羽色上无法区分SY雄鸟和雌鸟（颜色距离 < 1 JND）；在视觉环境很理想的情况下，也难以从胸和腹部的羽色上区分SY雄鸟和雌鸟(1 JND < 颜色距离 < 3 JND)。野外领域入侵实验发现，ASY雄鸟对ASY雄鸟的攻击强度显著高于对SY雄鸟（p = 0.002）和对雌鸟（p < 0.001）的攻击强度，而对后两者的攻击强度之间不存在显著差异（p = 0.761）。这些结果都符合模仿雌鸟假说的预测，为羽饰延迟成熟现象的模仿雌鸟假说提供了有力的实证支撑。

4. 绿背姬鹟ASY雄鸟在体型上显著大于SY雄鸟，雄鸟的鸣唱复杂性无法反映雄鸟的身体质量。本研究对绿背姬鹟雄鸟测量常规形态参数（喙长、跗跖长、翼长、尾长、体长和体重）、头部体积（反映脑容量）、飞行能力、换羽速率、血液和体表寄生虫等反映身体质量的指标，避免了以往实证研究采用的身体质量指标单一的问题，比较了ASY与SY雄鸟身体质量的差异，分析雄鸟身体质量与鸣唱复杂性的关系。结果表明，ASY雄鸟在体型上显著大于SY雄鸟，但在诸如跗跖长与体重的线性回归残差、飞行能力、换羽速率和寄生虫感染率等主要身体质量指标上与SY雄鸟没有显著差异。不同于其他物种的类似研究，本研究未发现雄鸟的鸣唱复杂性与身体质量指标之间的显著相关性，表明绿背姬鹟雄鸟的鸣唱复杂性无法反映其身体质量。

本研究系统分析了具有鸣唱和羽饰延迟成熟现象的绿背姬鹟雄鸟的鸣唱和羽饰的分化，首次在鸣唱延迟成熟研究中关注鸣唱的语法等复杂性指标在年龄段之间的差异；首次在羽饰延迟成熟研究中利用鸟类视觉模型来验证假说，为模仿雌鸟假说提供了强有力的支撑；证实了绿背姬鹟雄鸟的复杂鸣唱具有很强的个体性，可以用于进行个体识别与标记，为基于复杂鸣唱的个体识别提供了首个实证；首次在延迟成熟研究中利用多种身体质量指标来全面评估ASY和SY雄鸟身体质量的差异，与其他物种的类似研究不同，本研究表明绿背姬鹟雄鸟的鸣唱复杂性不能反映其身体质量。

Birds rely on auditory, visual, and olfactory communication to perceive biotic and abiotic environments. Among all the conmmunication signals, song and plumage are the two most important in birds. Songs, which are long and complex, are mainly produced by male birds. Plumage is ornamented with beautiful color. As the two most important secondary sex characteristics in birds, song and plumage are affected by sexual and natural selections, thus exhibit great intraspecific divergence, including divergence between different sexes, different geographical populations, different ages, or different individuals.

In this article, intraspecific divergence of song and plumage was studied in a small passerine, green-backed flycatcher (Ficedula elisae). Both delayed song maturation (DSM) and delayed plumage maturation (DPM) are exhibited in this species, which indicated that ASY (after-second-year) and SY (second-year) males were significantly different in songs and plumage color. Specifically, this article focused on song and plumage divergence between different ages in males, and song divergence between different male individuals. Also the relationship between song complexity and male quality was included in this article.

1. A spontaneous song of green-backed flycatchers could be divided into first, second, and third phrases. Third phrases are long and melodious, showing certain syntax structure. Complexity of syntax structure could be measured by linearity index. Syntax complexity is a part of the song complexity, which also includes between- and within-song complexity. Results of this study showed that linearity index of third phrases were 0.217 ± 0.061 (0.140–0.353). Third-phrase complexity was significantly greater in ASY than in SY males (p = 0.042). However, this significant difference was only reflected in syllable repertoire, which meant that ASY males sang more syllable types in third phrases than SY males, but did not increase third-phrase length, different syllable types and transitions per third phrase, and syntax complexity of third phrases.

2. Complex songs of green-backed flycatchers show great among-individual difference and within-individual stability. Results of this study showed that discriminant function analysis (DFA) in classifying different third-phrase types reached a high correct rate of 94.5%, while third phrases showed significantly greater within- than among-type similarity in spectrographic cross-correlation analysis (SPCC). In a breeding season, DFA reached a high correct rate of 87.1%, and third phrases still showed significantly greater within- than among-type SPCC similarity. In conclusion, third phrases of songs were applicable in vocal individual identification in at least a breeding season in green-backed flycatchers.

3. Female mimicry hypothesis (FMH) is among one of the most concerned in DPM studies. However, little evidence is uncovered for FMH. This is probably because of the difference between human and avian visual systems, as SY males, which are female-like in human eyes, might not be female-like at all from avian perspective. With avian visual system, results of this study showed that it was very difficult for ASY males to distinguish SY males from females based on plumage color, even in ideal light environment (color distances < 3 JND). Results of intrusion experiments showed that ASY males attacked more to ASY males than to SY males (p = 0.002) or females (p < 0.001), while no difference was exhibited between the two latter (p = 0.761). These results were in consistence with predictions of FMH, giving valuable evidence for this hypothesis.

4. To comprehensively compare the differene between ASY and SY males on male quality, several indexes including morphological parameters, head volume, flght ability, moulting rate, blood parasite prevalence, and ectoparasite prevalence were measured. Results showed that, ASY males were greater than SY males in some body-size indexes, but this did not result in greater male quality for ASY than SY males in most male-quality indexes. Different form many other studies on the same topic, male quality was not correlated with song complexity, showing that song complexity could not reflect male quality in male green-backed flycatchers.

In conclusion, this article focused on intraspecific divergence of song and plumage in male green-backed flycatchers, a passerine in which both DSM and DPM are exhibited. Syntax complexity was firstly introduced in DSM studies. Avian visual model was firstly adopted in testing hypotheses in DPM studies. Evidence of vocal individual identification based on complex songs was firstly given by this study. Different from many other similar studies, song complexity could not reflect male quality in male green-backed flycatchers.