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

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 k nown for centuries, and a wide variety of hypotheses have been proposed for its function. However, none of these hypotheses adequately accounts for it s 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 exag gerate the apparent size of a vocalizing bird. In normal birds, trachea len gth is correlated with body size, and thus the acoustic correlates of trach ea length in bird calls could convey information about size. By manipulatin g such an acoustic cue (formant frequency dispersion), tracheal elongation allows a caller to duplicate sounds produced by a larger bird. Unlike previ ously-proposed hypotheses, this 'size exaggeration' hypothesis is found to be consistent with current theories of avian vocal production and a wide ra nge of comparative ecological and behavioural data. A number of new predict ions of the hypothesis are found to be upheld. Finally, tracheal elongation is considered as a possible example of 'sensory exploitation', and its ult imate functional significance in mate choice and/or territoriality is discu ssed.