"The near and far and big and small of avian communication"

Eugene S. Morton (National Zoological Park, Smithonian Institution)

Communication functions to manage the behavior of other animals without having to risk direct encounters. Communication thus often substitutes for fighting in contests over resources. It also is used by females to assess the qualities of potential mates. Communication is under different sources of natural selection depending upon how close the communicating animals are to one another. I will discuss the "near and far" aspects of communication separately in this talk. First, I discuss communication among animals near one another, where the likelihood fighting is highest and where environmental acoustics are not so important. The origins or roots of communication in animals near each other is still found in the first land animals, the amphibians. Frogs and Toads differ from birds and mammals in that they continue to grow after reaching sexual maturity. Therefore, both big and small individuals compete for places to call from. Larger individuals are able to produce lower pitched calls than smaller individuals. Larger individuals are also better fighters than smaller individuals, a general feature of all animals. Experiments with toads show that larger males have deeper voices than smaller ones and that the sound of a larger toad is sufficient to repel smaller male toads without fighting. Furthermore, females are attracted to the lower calls of larger males. Females, in other words, prefer to mate with larger individuals because they have proven they carry good genes by living longer. The general lesson from amphibians is that lower pitched calls are both more threatening to other males and attractive to females. How has the relation between body size and pitch of calls related to birds? In birds, and mammals, calls used by animals close to one another contain motivational information whose origin is found in the amphibian system just described. Lower pitched and harsh vocalizations are used by birds trying to retain or obtain some resource. The bird is trying to get another to move away and to do this, it "sounds large." On the other hand, when a bird uses a high pitched and tonal vocalization, it is trying to keep another individual close or to stay near it. The bird is appeasing another or it may be fearful of something÷it "sounds small." I call this the motivation-structural rules model and will discuss it with the aid of diagrams depicting these sounds and intermediates and discussing them in relation to how they function. It is hoped that the audience can be convinced that size and motivation are related through the pitch of calls and that these, in turn, are derived from the primative relation between body size and fighting ability. Vocalizations used over long distances, such as most bird song, are a different matter. Motivation is usually not as important as sounding close to others even when you may not be close. I will discuss a theory called Ranging Theory to describe how birds assess their distance from one another. Birds assess distance by using their excellent sense of hearing short time intervals, which is much better than ours. Birds can hear echoes from tree trunks that appear as fuzzy endings on spectrograms. So, to sound close to another, a song should be adapted to transmit as far as possible without "degrading". Degradation is any change in the song as it travels from the singer, such as from reverberation, changes in frequency components, or amplitude characteristics but not simply getting weaker in amplitude. The relation between bird song characteristics such as song learning in passerines, song repertoires, and dialects to avian distance perception and ranging will be discussed. I am most grateful to the organizers of the symposium for the invitation to speak to you.