Acoustic exaggeration of size in birds via tracheal elongation: comparative and theoretical analyses

Abstract

At least 60 bird species possess an elongated trachea that typically forms loops or coils within the sternum or thorax. This peculiar trait has been known for centuries, and a wide variety of hypotheses have been proposed for its function. However, none of these hypotheses adequately accounts for its existence in all of the diverse bird species and habitats in which it is found. In this paper it is proposed that tracheal elongation serves to exaggerate the apparent size of a vocalizing bird. In normal birds, trachea length is correlated with body size, and thus the acoustic correlates of trachea length in bird calls could convey information about size. By manipulating such an acoustic cue (formant frequency dispersion), tracheal elongation allows a caller to duplicate sounds produced by a larger bird. Unlike previously-proposed hypotheses, this ‘size exaggeration’ hypothesis is found to be consistent with current theories of avian vocal production and a wide range of comparative ecological and behavioural data. A number of new predictions of the hypothesis are found to be upheld. Finally, tracheal elongation is considered as a possible example of ‘sensory exploitation’, and its ultimate functional significance in mate choice and/or territoriality is discussed.