Data from: 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.