Rapid urbanization is a global phenomenon that is increasingly exposing organisms to novel stressors.  These novel stressors can affect diverse aspects of organismal function, including development of condition-dependent ornaments, which play critical roles in social and sexual selection.  We investigated the relationship between metal pollution, proximity to roads, and carotenoid- and melanin-based plumage coloration in a common songbird, the great tit (Parus major).  We studied populations located across a well-characterized metal pollution gradient and surrounded by roadway networks.  Metal exposure and road-associated pollution could reduce carotenoid-based pigmentation by inducing oxidative stress or affecting habitat quality, but metals could also enhance melanin-based pigmentation, through effects on melanogenesis and testosterone concentrations.  Using a large sample size (N >500), we found that birds residing close to a point source for metals had reduced ultraviolet chroma, a component of carotenoid-based pigmentation.  Moreover, birds with high feather metal concentrations had lower carotenoid chroma, hue, and ultraviolet chroma, with effects modified by age class.  Birds residing closer to roads also had lower carotenoid chroma and hue.  Melanin-based pigmentation showed high between-year repeatability, and no association with anthropogenic pollution.  Results suggest that carotenoid-, but not melanin-, based pigmentation is negatively affected by multiple anthropogenic stressors.  We are the first to demonstrate a negative association between roads and a plumage-based signaling trait, which could have important implications for sexual signaling dynamics in urban landscapes.

Blood and feather samples were collected from great tits (Parus major) breeding across a well-described metal pollution gradient in the vicinity of Antwerp, Belgium.  We measured concentrations of five metals (Pb, Cd, Cu, As, Zn) in blood and feather samples using a coupled plasma-mass spectrometer (7700× ICP-MS, Agilent Technologies, Santa Clara, CA, USA).  We measured carotenoid-based plumage coloration from breast feather samples using spectrophotometry.  Melanin-based coloration was measured by taking digital photographs of the breast and analyzing the images in ImageJ.  We also took standard body measurements (body mass, tarsus length) from birds. 

Outliers for the blood metal measurements were dropped prior to analysis.