Use of visual display in dark, cluttered environments

Animals living in dense vegetation are limited in their use of visual signals due to the transmission constraints in these dim, cluttered environments. Birds in such habitats are often drab in appearance and thought to rely predominately on acoustic signals for conspecific communication. Here, we investigate the presence and use of a concealed underwing patch in the family Cettiidae. We find that this distinct white patch is widely present in the genus Horornis, with limited evidence for its presence in other genera. In response to simulated territorial intrusions, two species, Horornis fortipes and Cettia castaneocoronata perform wing-flicking displays which results in a flashing effect in Horornis fortipes. The presence of white underwings raises interesting possibilities about the role of hidden achromatic patches in facilitating visual communication in habitats traditionally thought to be unprofitable for this modality.