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艳丽，尤以蓝至蓝绿色调者为甚，这一特征颇为独特：这类鲜明的色彩在数百个鸟类物种中均有分布，却始终未能得到演化生物学层面的合理解释。本研究提出，鸟卵的色素沉着介导了太阳辐射伤害胚胎的两种途径之间的权衡：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卵内过热。本研究在可控光照环境下，以色彩各异的乡村织布鸟（Ploceus cucullatus）的鸟卵为实验材料，对该假说的四项核心内容开展了定量检验：(1) 具有破坏性的紫外线辐射可穿透鸟卵卵壳；(2) 自然强度的红外辐射可使鸟卵内部升温；(3) 鸟卵色彩越深，光线透过率越低（“色素遮阳伞”(pigment as parasol)）；(4) 鸟卵色彩越深，卵壳对光线的吸收越强，进而使卵内部升温（“深色汽车效应”(dark car effect)）。实验结果验证了全部四项预测。因此，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剧辐射吸收引发的有害影响。在其他条件均相同的前提下，特定光照环境下鸟卵的最优色素沉着水平，取决于光线透过率与吸收效率之间的平衡，而该平衡又与胚胎的适合度息息相关。