SOUND PRODUCTION IN THE COLLARED DOVE - A TEST OF THE WHISTLE HYPOTHESIS

The mechanism of sound production in the collared dove Streptopelia de caocto was studied to test the validity of the 'whistle' model and to analyze the role of vocal tract resonances. In this study, the vocaliz ations of six male adult doves were recorded both in normal air and in a mixture of 80% helium and 20% oxygen (heliox), Depending on the way in which the syrinx operates, the spectral structure of the vocalizat ions is expected to show specific changes in heliox, The effects of he liox differed substantially depending on the type of vocal element. Ex cept for a significant decrease in amplitude, unmodulated elements, i. e. elements with a constant frequency, were relatively unaffected by h eliox, In contrast, modulated elements, i.e, elements with an abrupt i ncrease in frequency, showed a gradual increase in frequency with incr easing helium concentration, This specific increase in frequency stopp ed when the modulation frequency was 1.5 times the base frequency, eve n when the helium concentration was increased further. In some individ uals, a frequency band of half the base frequency was also observed. I n general, the proportion of modulated elements also showed a signific ant decrease, The lack or change in the fundamental frequency of the u nmodulated elements and in the base frequency of the modulated element s indicates that these vocalizations are not produced like a whistle. It is more likely that they are the result of vibration of the syringe al membranes. Generally, vocal tract resonances do not play an active role in the modification of vocalizations, although they might enhance the fundamental frequency passively. The results suggest that the alm ost pure tonal vocalizations are produced as such in the syrinx and un dergo limited modification when passing through the vocal tract. The e ffect of heliox on the modulation frequency suggests (1) that differen t sound-producing mechanisms may underlie different types of vocalizat ions, and (2) that resonance properties of the vocal system may be inv olved in the production or modification of the modulation frequency.