Do trace metals influence visual signals? Effects of trace metals on iridescent and melanic feather colouration in the feral pigeon

Trace metals are chemical pollutants of prime concern nowadays given their implication in several human diseases and their noxious effects on wildlife. Previous studies demonstrated their negative (e.g. lead, cadmium) or positive (e.g. zinc) effects on body condition, immunity and reproductive success in birds. Because of their effects on bird condition, trace metals are likely to influence the production of condition-dependent plumage colours, that may be used in mate choice. In the feral pigeon Columba livia, we investigated iridescent colouration in response to lead and zinc experimental (i.e. metal supplementation in standardized conditions) and natural exposure (i.e. metal concentrations in feathers of wild urban pigeons), and melanic feather colouration in response to experimental lead and zinc exposure. Both studies (i.e. experimental and correlative) consistently showed that lead exposure decreased iridescent neck feather brightness independently of colour morph. Moreover, lead, when provided alone, decreased melanic feather reflectance in the middle wavelengths while zinc supplementation increased melanic feather reflectance in the violet-wavelength. In conclusion, our study suggests that the colouration of iridescent and melanic feathers depends on the exposure to pollutants. Whether trace metal exposure affected the ability of birds to produce melanin pigments, to grow the microstructural feather elements required for maximum colour display, or to cope with bacteria that degrade feather microstuctures remains unclear. Future studies should investigate whether these metal-induced modifications of plumage colouration affect behaviours involved in sexual selection.

痕量金属（trace metals）作为当前备受关注的一类化学污染物，因其与多种人类疾病存在关联，且对野生生物具有有害影响而广受重视。既往研究已证实，痕量金属可对鸟类的身体状况、免疫功能及繁殖成功率产生负面影响（如铅、镉）或正面影响（如锌）。鉴于痕量金属会影响鸟类身体状况，其或可调控依赖于身体状态的羽色生成——这类羽色常被用于配偶选择。本研究以野生原鸽（Columba livia）为研究对象，一方面针对铅与锌的实验暴露（即在标准化条件下开展金属添加处理）与自然暴露（即野生城市鸽羽毛中的金属浓度水平），探究其虹彩羽色的变化；另一方面针对实验性铅、锌暴露，研究其黑色素羽色的变化情况。两项研究（即实验性研究与相关性研究）均一致显示：无论羽色形态如何，铅暴露均会降低虹彩颈羽的亮度。此外，单独施加铅时，会降低中波长范围内的羽毛黑色素反射率；而补充锌则可提升紫波长范围内的羽毛黑色素反射率。综上，本研究结果提示：虹彩羽与黑色素羽的着色情况均与污染物暴露水平密切相关。目前仍不明确的是：痕量金属暴露是否会影响鸟类合成黑色素的能力、发育出最大化羽色表现所需的羽毛微观结构组分，或是抵御降解羽毛微观结构的细菌。未来的研究可进一步探讨，这些由金属诱导的羽色改变是否会影响性选择相关的行为表现。