Use of visual display in dark, cluttered environments.

This article investigates the presence and function of concealed white underwing patches in birds of the family Cettiidae, particularly within the genus Horornis, which inhabit dense, dimly lit understory environments where visual signaling is traditionally considered limited. Using playback experiments and high-speed video analysis in the Manali Wildlife Sanctuary, India, the study documents wing-flicking displays in Horornis fortipes (Brownish-flanked Bush Warbler) and Cettia castaneocoronata (Chestnut-headed Tesia), revealing that H. fortipes exhibits a conspicuous flashing effect due to its white underwing patches, while C. castaneocoronata lacks such patches. Examination of museum specimens and photographs across 23 species shows that white underwing patches are common in Horornis and present variably in a few other genera, suggesting these achromatic patches may facilitate visual communication in cluttered habitats by allowing selective conspicuousness during territorial interactions. The findings challenge assumptions about the limited role of visual signals in dense vegetation and highlight the need for further behavioral and experimental research on achromatic signaling in understory birds.