Credit: W. TECUMSEH FITCH

“Speak softly and carry a big stick”, said Theodore Roosevelt. But a Darwinian might regard this as bad advice, as demonstrated by W. T. Fitch's explanation of the unusually long tracheae of some birds (J. Zool. Lond. 248, 31-49; 1999).

In at least 60 bird species — including the trumpeter swan shown here — the trachea is thrown into coils or loops instead of taking a direct route between the throat and the lungs. The trait shows a puzzling diversity: it is found in six avian orders, and has probably evolved several times. It occurs in migratory cranes and swans, and in large, sedentary rainforest dwellers such as currassows. Closely related species show large variation in trachea length. In some species, only the males possess elongated tracheae, whereas in others both sexes do.

Fitch argues that the best explanation is that a long trachea allows a calling bird to give an exaggerated impression of its size. This works through an acoustic process known as ‘reduced formant dispersion’: a call produced by an elongated trachea has more closely spaced resonant frequencies, producing a deeper, more baritone sound.

A common feature of birds with tracheal elongation is that they nest in dense vegetation, where visibility is poor, and where duplicating the call of a larger bird will be useful in defending a territory, or possibly attracting a mate. The lack of visual cues reduces the possibility that their trick will be discovered, but one would expect natural selection to promote scepticism in listeners. This might not happen, however, if honest calls remained in a sufficiently large majority.

Faking bigness should go with selection for actual bigness, and Fitch found that birds with elongated tracheae are larger than related species without. Perhaps a modern Roosevelt should say: “Speak as loudly as you can get away with, but be prepared to have your bluff called.”