Many species throughout the animal kingdom construct nests for reproduction. A nest is an extended phenotype—a non-bodily attribute—of the individual building it. In some bird species, including our study population of blue tits (Cyanistes caeruleus), conspicuous feathers or other material are placed on top of the nest. These so-called nest ornaments do not contribute to nest insulation, but are hypothesised to have a signalling function. Here, we apply the concept of behavioural syndromes, with focus on between-individual variation (repeatability) and between-individual correlations, to the study of avian nest construction. We find that nest ornamentation is a moderately repeatable trait in female blue tits, which suggests it is an extended phenotype of the female. Furthermore, the tendency to ornament the nest covaries across females with another aspect of her extended phenotype, the composition of the nest lining material, and these two traits thus form an extended phenotype syndrome. Assuming the correlation is reflected on a genetic level, it implies that nest ornamentation and composition of the nest lining do not evolve in isolation; one aspect may be an evolutionary by-product of selection on the other aspect and their overall flexibility to respond to change is reduced. The avian nest is an extended phenotype (a non-bodily attribute) of its builder with potentially multiple functions in terms of insulation and signalling. In particular, many bird species’ nests contain nest ornaments, feathers or other materials that are placed on top of the nest and that stand out from the nest material due to their colour and/or size. We quantified between-individual variation (repeatability) of nest ornamentation behaviour in a wild population of blue tits and between-individual covariation (syndrome) of nest ornamentation to other features of nest construction. We find that nest ornamentation is a repeatable trait limited to females in our study population. The tendency to ornament the nest covaries across females with another aspect of her extended nest phenotype, the composition of the nest lining material. These correlated traits thus form an extended phenotype syndrome. It hence becomes crucial to recognise that a study of a single aspect of nest construction in isolation captures only a part of the complexity, as one aspect may have evolved as a correlated response of selection on the other aspect. Moreover, such a syndrome implies limited flexibility in the range of adaptive response.

中文翻译：

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巢饰和羽毛成分在野生鸟类中形成扩展表型综合征

整个动物界的许多物种都建造巢穴以进行繁殖。巢穴是构建它的个体的扩展表型——一种非身体属性。在一些鸟类中，包括我们研究的蓝山雀（Cyanistes caeruleus），显眼的羽毛或其他材料被放置在巢顶上。这些所谓的巢装饰品对巢绝缘没有贡献，但被假设具有信号功能。在这里，我们将行为综合征的概念，重点放在个体间变异（可重复性）和个体间相关性上，用于鸟类巢穴结构的研究。我们发现巢纹在雌性蓝山雀中是一种适度重复的特征，这表明它是雌性蓝山雀的扩展表型。此外，用其扩展表型的另一个方面、巢衬材料的组成以及这两个特征来装饰雌性的巢共变的趋势，因此形成了扩展的表型综合征。假设相关性反映在遗传水平上，这意味着巢的装饰和巢衬的组成不是孤立进化的；一方面可能是另一方面选择的进化副产品，并且它们响应变化的整体灵活性降低。鸟巢是其建造者的扩展表型（非身体属性），在绝缘和信号方面具有潜在的多种功能。尤其是很多鸟类的巢都含有巢饰，羽毛或其他放置在巢顶部的材料，由于它们的颜色和/或大小而从巢材料中脱颖而出。我们量化了野生蓝山雀种群中巢装饰行为的个体间变异（可重复性）以及巢装饰与巢构造的其他特征的个体间协变（综合征）。我们发现巢装饰是一种可重复的特征，仅限于我们研究人群中的女性。装饰巢穴的倾向与雌性的巢穴扩展表型的另一个方面有关，即巢穴衬里材料的成分。因此，这些相关性状形成了扩展的表型综合征。因此，认识到孤立地研究巢穴构造的单个方面仅捕获复杂性的一部分变得至关重要，因为一方面可能已经演变为另一方面的选择的相关反应。此外，这种综合症意味着适应性反应范围内的灵活性有限。