Urbanization enhances ornament expression in a common waterbird

In birds, many components of visual communication (i.e. plumage and non-plumage ornaments) play an important role in signaling of individual quality. It is widely acknowledged that ornament expression may be modulated by environmental conditions, however, it remains relatively poorly explored whether and how urbanization affects the expression of non-plumage ornamentation in urban dwelling-individuals. Here, we investigated the effect of urbanization on the expression of bare-part (non-plumage) putative ornament (i.e., the frontal shield size) across eight populations of a common reed-nesting waterbird, the Eurasian coot Fulica atra. Most importantly, we found robust support for the positive effect of urbanization on shield size and its condition-dependent character in coots. Also, long-term monitoring of a single urban population revealed relationships between the ornament expression and nest site selection patterns, as coots with larger shields were bold enough to colonize more anthropogenically transformed urban sites with stronger human disturbance and better accessibility to anthropogenic food. At the same time, we found no support for associations between the shield size and either nest defense behavior or reproductive performance in coots, likely reflecting stochasticity of anthropogenic selective pressures. To our knowledge, this study provides the first evidence for the enhanced expression of an unpigmented non-plumage putative ornament in urban-dwelling birds. Our results show that the effects of urbanization on non-plumage components of quality signaling in birds may be complex and multifaceted, and reinforces a need for further investigation focusing on different types of ornamentation across divergent avian species.