If humans aim to sustainably coexist with wildlife, we must understand how our activity impacts the communication systems of urban animal populations. We know much about the effects of anthropogenic noise on bird song, but relatively little about how avian visual signals are affected by urbanization. Such an effect may occur if urbanization alters the food available to species with color based on carotenoids, which they must obtain from their diet. Over 3 years, we compared a comprehensive suite of visual signals in male and female Northern Cardinals (Cardinalis cardinalis) in a rural and an urban population. We predicted that urban birds would have enhanced carotenoid-based signals as they likely have access to more carotenoids from invasive plants that thrive in cities. We used reflectance spectrometry, digital image analyses, and avian visual models to quantify hue, saturation, and brightness of chest (male), underwing (female), and bill (male and female) signals. Compared to rural males, urban males had redder chest feathers in 1 year and redder bills in every year. Urban females had more saturated underwing color than rural females in every year. These color differences were sufficient to be distinguished by the avian visual system. Female bill color did not vary between sites. Interestingly, urban birds had significantly reduced mass-related body condition compared to rural birds. Thus, urban males and females can display enhanced carotenoid-based signals despite being in relatively poor condition. We suggest that urban birds have more access to carotenoids from invasive honeysuckle (Lonicera spp.) than rural birds. This color enhancement could affect the information content of the signals and the dynamics of the social and mating systems. These results stand in stark contrast to the predominant trend of decreased color in urban birds and highlight the complex and varied potential effects of urbanization on animal communication.

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