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

Abstract

1 2Trace metals are chemical pollutants of prime concern nowadays given their implication in 3 several human diseases and their noxious effects on wildlife. Previous studies demonstrated 4 their negative (e.g. lead, cadmium) or positive (e.g. zinc) effects on body condition, immunity 5 and reproductive success in birds. Because of their effects on bird condition, trace metals are 6 likely to influence the production of condition-dependent plumage colours, that may be used 7 in mate choice. In the feral pigeon (Columba livia), we investigated iridescent colouration in 8 response to lead and zinc experimental (i.e. metal supplementation in standardized 9 conditions) and natural exposure (i.e. metal concentrations in feathers of wild urban 10 pigeons), and melanic feather colouration in response to experimental lead and zinc 11 exposure. Both studies (i.e. experimental and correlative) consistently showed that lead 12 exposure decreased iridescent neck feather brightness independently of colour morph. 13Moreover, lead, when provided alone, decreased melanic feather reflectance in the middle 14 wavelengths while zinc supplementation increased melanic feather reflectance in the violet-15 wavelength. In conclusion, our study suggests that the colouration of iridescent and melanic 16 feathers depends on the exposure to pollutants. Whether trace metal exposure affected the 17 ability of birds to produce melanin pigments, to grow the microstructural feather elements 18 required for maximum colour display, or to cope with bacteria that degrade feather 19 microstuctures remains unclear. Future studies should investigate whether these metal-20 induced modifications of plumage colouration affect behaviours involved in sexual selection. 21 22 23

Iridescent coloration of Tree Swallows relates to environmental metal pollution

“…Our results provided limited support for the hypothesis that environmental metals alter the feather color in Tree Swallows (McGraw et al 2002, Chatelain et al 2017), but with opposing effects among our three color metrics and with only partial congruence with other studies. Whereas adult swallows whose chicks exhibited higher concentrations of PC2 MET in feces had higher values of plumage hue (a shift from bluer to greener color), they also had higher values of plumage brightness.…”

“…Whereas adult swallows whose chicks exhibited higher concentrations of PC2 MET in feces had higher values of plumage hue (a shift from bluer to greener color), they also had higher values of plumage brightness. By contrast, feral pigeons (Columba livia) that were exposed to lead, exhibited declines in both blue iridescence and brightness but only in the absence of zinc (Chatelain et al 2017).…”

“…These contrasting results suggest that relationships among various metals and iridescent coloration may act on different metrics in opposing ways (Chatelain et al 2017).…”

“…Pollution can affect susceptibility to parasites in Tree Swallows (Gentes et al 2007b), which could extend to bacteria that alter the feather structure responsible for iridescence. Thus, while iridescent feathers may be revealing of environmental metals (Chatelain et al 2017), much work will be required to unravel the particular mechanisms and effects.…”

“…This link means that a localized reduction in ornament coloration could signal the first subtle decline in environmental quality, occurring when there is still time to identify and reverse the source of degradation (Hill 1995). Several studies have already explored the link between bird coloration and metal pollution by focusing on carotenoid-and melanin-based coloration (e.g., Eeva et al 1998, Dauwe and Eens 2008, Geens et al 2009, Giraudeau et al 2015, but the effects of metals on structural coloration, especially iridescence, remain largely unexplored (McCarty and Secord 2000, White and Cristol 2014, Chatelain et al 2017.…”