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.

诸多鸟类的卵（尤其是呈现蓝至蓝绿色调的卵）拥有鲜明色彩，这一在数百个鸟类物种中均存在的醒目特征，却长期未能得到演化功能层面的合理解释，堪称一项奇特现象。本研究提出，鸟类卵的色素沉着可介导太阳辐射对胚胎造成伤害的两条途径间的权衡：一是辐射透过卵壳直接照射胚胎，二是辐射被卵壳吸收后引发卵内过热。本研究在可控光照环境下，以色彩各异的村织雀（Village weaverbird, *Ploceus cucullatus*）的卵为研究对象，对该假说的四个核心命题进行了定量检验：(1) 具有损伤性的紫外辐射可穿透鸟类卵壳；(2) 自然强度的红外辐射可使卵内部升温；(3) 卵色越深，光线透过率越低（“色素作为遮阳伞”）；(4) 卵色越深，卵壳对光线的吸收越强，进而使卵内部温度升高（“深色汽车效应”）。实验结果全部支持上述预测。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过热危害。在其他条件均相同的前提下，特定光照环境下鸟类卵的最优色素沉着水平，取决于光线透过率与吸收量之间的平衡，且该平衡与胚胎适合度直接相关。