"Conventional" signals in avian agonistic displays: Integrating theory, data and different levels of analysis

We present an integration of communication theory and data, drawing on exam ples from titmice (Aves: Paridae). We suggest how display actions such as l ifting the head, raising the nape feathers, crest erecting and spreading th e wings, act in agonistic communication when physical contact between oppon ents is rare. We propose that such displays largely act as strategic choice handicap signals. By giving these displays the signaller is believed to in cur costs which underwrite the reliability of the signals; it may strategic ally increase these costs (for example by display repetition or adding addi tional elements) to signal its condition, motivation and hence the subjecti ve value of a resource. It is shown that linking these ideas with earlier t heories on the causation of display components, leads to an explanation of why birds have a greater repertoire of signals associated with aggression/w inning, than with submission/losing. It is suggested that modellers of comm unication systems and those interested in the theory of signal design shoul d pay more attention to the evolutionary constraints imposed by signal orig in. (C) 1999 Academic Press.