ANIMALS exhibit a diversity of colour patterns that are commonly used in interspecific and intraspecific communication1-3. Factors influencing the evolution of signals used in communication include the properties of the physical environment in which the signal is generated2-10, the perceptual systems of individuals (such as potential mates or predators) receiving the signal2,3,11-16, and the nature of the information signalled. In warblers of the genus Phylloscopus, species differences in colour patterns are correlated with light intensity of the habitat: brighter species live in darker habitats. I report here two observations that colour patterns function to increase conspicuousness, and are used in intraspecific communication. First, individuals make themselves temporarily more conspicuous by flashing the bright colour patterns in display, and are less conspicuous when not displaying. Second, experimentally increasing conspicuousness of males within a given habitat increases territory size, whereas experimentally reducing conspicuousness results in either a smaller territory or its total loss. Traits used in intraspecific communication are often thought to diverge as a result of variation in perceptual systems2,3,13,15-17. This study shows that variation in the physical environment can cause species divergence, and this will occur whether perceptual systems are variable or relatively constant.
