Threat display in birds

The study of threat displays has long been an area in which theory and empi rical work have each spurred the other forward. Communication is currently the focus of great interest and effort on the part of modellers. A great de al that classical ethologists have accurately described about threat displa ys still lacks adequate explanation. Here we review the empirical literatur e on the use of threat displays by birds competing for small valued resourc es, both to refocus theoretical attention upon the key characteristics of t hreat and to assess the degree to which current theory explains these chara cteristics. We aim to demonstrate that threat displays communicate informat ion about aggressive motivation, but are not handicaps. Handicap models pre dict a single graded display, while the vast majority of studies report rep ertoires of about four to six discrete threats for any given species. These displays vary with motivational and strategic considerations, and may be d emonstrated to rank consistently on a scale of willingness to escalate, thu s providing information about aggressive motivation. We conclude by identif ying those features of avian threat displays that have not been adequately explained, in the hope that this reexamination of empirical data will help focus theoretical attention on these issues.