Two-Voice Phenomenon in Birds: Further Evidence

--Maternal calls of 11 wood duck (Aix sponsa) hens were recorded at nest sites in the field. Audiospectrographic analyses revealed that most of the hens uttered calls containing two (or more) temporally overlapping, nonharmonically related tones, thereby implicating two or more independent sound-producing sources in the syrinx. Evidence for the "two-voice" phenomenon has heretofore come mainly from songbirds. The present report presents the first evidence of this vocal phenomenon in waterfowl. --Psychology Laboratory, North Carolina Department of Mental Health, Dorothea Dix Hospital, Raleigh, North Carolina 27611. Present address: Department of Ethology, University of Bielefeld, P.O. Box 8640, D-4800 Bielefeld 1, West Germany. Accepted 22 December 1975. THE vocalizations of certain avian species contain two separate, nonharmonically-related tones. As these tones are uttered simultaneously and are not harmonically related, it is likely that two independent sound sources are operating simultaneously in the vocal tract of these species. The two-voice phenomenon was first described by Potter, Kopp, and Green (1947: 410-412). They noted a "double tone" in the sound spectrogram of a Brown Thrasher's (Toxostoma rufum) song. Subsequently, other investigators of bird vocalizations have found two voices in other species, almost always oscines (Borror and Reese 1956, Thorpe 1961: 112-113, Greenewalt 1968: 55-78, Stein 1968, Nottebohm 1971). Thorpe (1961: 112-113) interpreted sound spectrograms of songs of a Gouldian Finch (Chloebia gouldiae) and a Wood Thrush (Hylocichla mustelina) as portraying three and four simultaneous voices, respectively. The present report describes the first evidence suggesting the presence of multiple voices in waterfowl (nonoscines). The vocalizations of birds are generated by air passing across certain vibratile membranes in the syrinx, the vocal apparatus located at or near the tracheobronchial junction. Greenewalt (1968:55-78) and Stein (1968) independently presented models of avian sound production incorporating the capability of sounds being produced by two independent acoustical sources within the syrinx. In actuality, the syringes of many species have up to four membranes (two on each side of the syrinx) that could be involved in sound production. The best anatomical evidence for two independent acoustical sources in birds comes from Nottebohm (1971). He sectioned either the left or right root of the hypoglossus nerve, which innervates the muscles controlling the membranes on each side of the syrinx, and found that adult Chaffinches (Fringilla coelebs) no longer produced one of two nonharmonically-related tones in their species-typical song, depending on which root was sectioned. Nottebohm's results increase the validity of the two-voice phenomenon, for, prior to his research, the only evidence for this phenomenon was based solely on sound spectrograms of the voices of intact birds. The question remains how general the two-voice phenomenon is in nonoscine species. For example Greenewalt (1968: 56-78) found spectrographic evidence for two voices in 12 species of oscines but only in 7 nonoscine species: Eared Grebe (Podiceps nigricollis), American Bittern (Botaurus lentiginosus), Greater Yellowlegs (Tringa melanoleucus), Common (Redshafted) Flicker (Colaptes auratus cafer), Broad-winged Hawk (Buteo platypterus), American Golden Plover (Pluvialis dominica), and Bare-throated Bellbird (Procnias 567 The Auk 94: 567-572. July 1977 568 DAVID B: MILLER [Auk, Vol. 94