Carotenoid- but not melanin-based plumage coloration is negatively related to metal exposure and proximity to the road in an urban songbird

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.