Data from: Shedding light on bird egg color: pigment as parasol and the dark car effect

The vibrant colors of many birds’ eggs, particularly those that are blue to blue-green, are extraordinary in that they are striking traits present in hundreds of species that have nevertheless eluded evolutionary functional explanation. We propose that egg pigmentation mediates a trade-off between two routes by which solar radiation can harm bird embryos: transmittance through the eggshell and overheating through absorbance. We quantitatively test four components of this hypothesis on variably colored eggs of the village weaverbird (Ploceus cucullatus) in a controlled light environment: (1) damaging ultraviolet radiation can transmit through bird eggshells, (2) infrared radiation at natural intensities can heat the interior of eggs, (3) more intense egg coloration decreases light transmittance (“pigment as parasol”), and (4) more intense egg coloration increases absorbance of light by the eggshell and heats the egg interior (“dark car effect”). Results support all of these predictions. Thus, in sunlit nesting environments, less pigmentation will increase the detrimental effect of transmittance, but more pigmentation will increase the detrimental effect of absorbance. The optimal pigmentation level for a bird egg in a given light environment, all other things being equal, will depend on the balance between light transmittance and absorbance in relation to embryo fitness.诸多鸟类的鸟卵色彩鲜亮饱满，尤以蓝至蓝绿色调的卵壳为甚，这一特征堪称独特：此类醒目的表型在数百个鸟类物种中均有分布，却长期未能获得演化功能层面的合理解释。我们提出，卵色素沉着（egg pigmentation）可介导两类太阳能辐射（solar radiation）伤害胚胎的途径间的权衡：一是辐射透过卵壳造成的损伤，二是辐射被卵壳吸收后引发的胚胎过热。我们在可控光照环境下，针对斑织雀（Ploceus cucullatus）色彩各异的鸟卵，对该假说的四个核心组分开展了定量检验：(1) 具有破坏性的紫外线辐射（ultraviolet radiation）可穿透鸟类卵壳；(2) 自然强度的红外辐射（infrared radiation）可加热卵内部环境；(3) 鸟卵颜色越深，光线透过率越低（"色素作为遮阳伞"）；(4) 鸟卵颜色越深，卵壳对光线的吸收越强，进而加热卵内部环境（"深色汽车效应"）。实验结果验证了上述全部预测。由此可见，在光照充足的筑巢环境中，色素沉着程度较低会加剧辐射透过卵壳带来的有害影响，而色素沉着程度较高则会加重辐射吸收引发的胚胎过热损伤。在其他条件均保持恒定的前提下，特定光照环境下鸟卵的最优色素沉着水平，将取决于光线透过率与吸收效率之间的平衡，以及该平衡与胚胎适合度（embryo fitness）的关联。