Data from: A reliable technique to quantify the individual variability of iridescent coloration in birds

The study of iridescent coloration in birds emerged only recently, mainly due to the difficulty inherent in quantifying its directionality. Directionality restrains color perception to a limited angle and thereby causes drastic changes in brightness when an animal is in motion. Although a versatile goniometer for quantifying iridescent coloration has been developed recently (Meadows et al. 2011), so far, it has only been applied to measuring the highly directional iridescent coloration in a hummingbird species. Thus, the reliability of the goniometer for species displaying more common and less directional iridescent coloration has yet to be evaluated. Additionally, two important methodological aspects remain to be assessed before this apparatus can be used confidently: 1) whether directionality, which could be subject to sexual selection, can be quantified in a repeatable way; and 2) whether the apparatus gives more precise and accurate measurements than a less complex traditional method. Using feathers collected from 271 male tree swallows (Tachycineta bicolor) over two years, we found that the goniometer provided repeatable measurements of directionality across individuals and across three body regions, namely the crown, mantle and rump. The apparatus was also more repeatable than a traditional method involving a bifurcated probe and reduced a brightness bias associated with individual differences in barbule tilt. We strongly encourage researchers to invest in this methodological change considering the multiple advantages demonstrated and to quantify the directionality of iridescent coloration as to unveil its role in signaling and sexual selection.

鸟类虹彩色彩（iridescent coloration）的相关研究直至近年才逐步兴起，其核心阻碍在于难以量化其方向性。方向性会将色彩感知限制在有限角度范围内，进而使得动物移动时其体表亮度发生剧烈变化。尽管近年已有研究团队开发出可用于量化虹彩色彩的通用型测角仪（goniometer）（Meadows等，2011），但截至目前，该设备仅被应用于测量某蜂鸟物种的强方向性虹彩色彩。因此，该测角仪对于展现更为常见、方向性较弱的虹彩色彩的物种的可靠性，仍有待评估。此外，在能够放心使用该设备之前，尚有两项关键方法学问题有待验证：其一，可能受性选择（sexual selection）作用的色彩方向性，能否以可重复的方式进行量化；其二，相较于操作更简便的传统方法，该设备能否提供更为精准可靠的测量结果。本研究收集了两年间采集的271只雄性树燕（Tachycineta bicolor）的羽毛样本，结果显示该测角仪可对不同个体以及头顶、上背与臀区的色彩方向性进行可重复的测量。相较于采用分叉式探头的传统方法，该设备的测量重复性更优，同时还可消除因羽小枝倾斜程度的个体差异所导致的亮度偏差。鉴于本研究已证实该方法的多项优势，我们强烈呼吁研究者采用这一方法学改进方案，并通过量化虹彩色彩的方向性，以揭示其在信号传递与性选择过程中发挥的作用。