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.