Stream noise induces song plasticity and a shift to visual signals in a riverine songbird

Environmental noise can severely impair acoustic communication, thereby affecting key behaviors such as predator avoidance,¹^,² territory defense,³^,⁴ and reproduction.⁵^,⁶^,⁷ Persistent noise in some habitats is thought to have favored the emergence of multimodal communication systems.⁸^,⁹^,¹⁰ Multimodal signals, which integrate information across several sensory channels, can enhance signal detection and improve message clarity in challenging environments.¹¹ The capacity to flexibly adjust signaling strategies in response to noise is considered critical to the resilience and evolutionary success of communication systems.¹²^,¹³ However, direct evidence for noise-induced shifts between sensory modalities-termed "multimodal shift"-remains scarce.¹¹ Although river noise has been linked to the evolution of multimodal displays¹⁴^,¹⁵ and shifts¹⁶ in torrent frogs, examples from other taxa are lacking. Here, we investigate how the white-throated dipper (Cinclus cinclus), a riverine songbird, modulates both acoustic and visual signaling along noisy rivers. We find that the dippers adjust their songs to the ambient noise level. In addition, they use conspicuous blinking with white-feathered eyelids to compensate for acoustic masking in high-noise environments. Blinking rate was linked to local river noise, aggressive behavior, and conspecific presence. Calibrated field measurements revealed a negative correlation between song amplitude and blinking rate, consistent with a noise-driven multimodal shift. This indicates that song plasticity operates in tandem with visual signaling, showing that animals can dynamically reallocate investment across modalities in response to fluctuating environmental pressures. The fine-tuning of both signal performance and modality underscores the critical role of noise interference and signal flexibility in the evolution of complex communication systems.