Referential signalling in birds: the past, present and future

Many species produce specific signals in response to environmental events. The specificity of these signals allows receivers to react appropriately to an event in the absence of other contextual cues. These functionally referential signals can be auditory, visual or multimodal and occur in antipredator, food and social cohesion contexts. In birds, acoustic signals used in antipredator defence are the most often studied. However, several species of birds produce functionally referential signals in other modalities, including gestures, and in other contexts, including food and social contact. The prevalence of functionally referential visual or multimodal signals may be underestimated. More research using innovative techniques is needed to test these signals. Food calls, particularly those produced during food provisioning, also require further study and may shed light on the ontogeny of food signals. Comparative studies across closely related species of birds may also reveal differences in the development of functionally referential versus motivational signals. Taken together, the type of research into functionally referential signals suggested herein will likely further our understanding of the ecological, social and physiological pressures that have shaped communication in birds.