Experimentally impaired female condition does not affect biliverdin-based egg colour

It has been proposed that blue-green egg coloration is a condition-dependent female sexual trait that may modify paternal care in a post-mating sexual selection scenario. This pattern may arise because the pigment responsible for eggshell colouration (biliverdin) may be a costly and limited resource, whose availability is linked to female health state. Thus, it can be predicted that females whose condition is compromised should be constrained in their capacity to deposit biliverdin in the eggshell, thus producing paler clutches. To test this hypothesis, we performed a handicapping experiment by clipping some feathers of female spotless starlings before egg laying and measuring the colour of their clutches. We expected the handicapping treatment to increase flying costs, impairing female overall condition and resulting in paler clutches. Our experiment was successful in lowering the weight gain of handicapped with respect to control females. However, in contrast to our expectations, we found no effect of the treatment on eggshell colouration. Eggshell colour varied along the laying order, with initial eggs of the laying sequence being relatively paler than the rest of the clutch, but this pattern was not different between experimental groups. Despite a very similar methodology, our results differ from a previous study and offers no support to the post mating sexual selected hypothesis, questioning the general applicability of the sexual selection role of eggshell coloration.

有假说提出，蓝绿色卵色（blue-green egg coloration）是一种依赖于个体状态的雌性性征，在交配后性选择情境中，该特征可能会调控父本抚育行为。该模式的产生机制可能为：决定卵壳颜色的色素——胆绿素（biliverdin）——是一种代价高昂且有限的资源，其可获得性与雌性健康状态紧密相关。据此可预测，个体状态受损的雌性，其在卵壳中沉积胆绿素的能力将受到限制，进而产下颜色更浅的窝卵。

为验证这一假说，我们开展了剪羽损伤操控实验：在产卵前对雌性纯色椋鸟（spotless starlings）的部分羽毛进行修剪，并测量其产卵群的颜色。我们预期该处理会增加雌性的飞行成本，损害其整体个体状态，最终导致产卵群颜色更浅。

实验成功降低了处理组雌性相较于对照组的体重增量。然而，与预期相悖的是，我们未发现该处理对卵壳颜色存在任何影响。卵壳颜色随产卵顺序发生变化：产卵序列中的初始卵颜色相对较浅，但该模式在实验组与对照组之间并无差异。

尽管实验方法与此前研究高度相似，但我们的结果与前人研究存在分歧，且未为交配后性选择假说提供支持，这对卵壳颜色的性选择作用的普遍适用性提出了质疑。