Bill opening and sound spectrum in barnacle goose loud calls: individuals with “wide mouths” have higher pitched voices

It is well known that vocal tract resonances are involved in sound production in mammals, but models of bird vocal production often assumed that their sounds are only source created (Greenwalt 1968). In large birds the source sound resonates off the sides of the long and curved trachea or within air sacs (Welty & Baptista 1988). Recently Nowicki (1987), who studied passerines with short rigid windpipes, showed that resonances are involved in bird song as well; however, the actual way resonances are produced is not yet well known. Several studies have shown that the tracheal length and diameter can influence the frequency spectrum of sounds (e.g. Warner 1972; Brakenbury 1978). As Nowicki & Marler (1988) emphasized, resonance i