Short-Range and Long-Range Songs: Use of Two Acoustically Distinct Song Classes by Dark-Eyed Juncos

Abstract

-Communication signals can be divided into two functional classes: longrange signals and short-range signals. The study of bird song has concentrated almost exclusively on long-range songs. Because bird song often is used as a model system for studying learning, mate choice, and territoriality, this lack of attention to short-range songs may have misrepresented our understanding of communication systems. Short-range songs are expected to differ acoustically and functionally from those broadcast over long distances. Male Dark-eyed Juncos (Junco hyemalis) produce two classes of song that appear to function as long-range and short-range signals, respectively. These two classes differ markedly in frequency and syntax as well as in repertoire size. In terms of use, rates of short-range song production were highest in conjunction with courtship displays, when males were within close proximity to fertile mates, and during interactions with other males. In contrast, production of long-range song was not associated with courtship displays, did not vary significantly with the reproductive state of females, and was produced when males were relatively far from conspecifics. Received 5 May 1997, accepted 8 September 1997. THE TYPES OF SIGNALS USED in communication can be separated into two classes: those used over long distances and those used over short distances. These two classes represent ends of a continuum of signals that function over many different distances. Long-range signals are conspicuous, broadcast signals that often function as advertisements of territorial and reproductive status (Krebs et al. 1978, Clutton-Brock and Albon 1979). In contrast, shortrange signals often are inconspicuous and characteristically are directed at specific individuals that are within close proximity to the signaler (Wiley and Richards 1982, Smith 1991, McGregor and Dabelsteen 1996). The structure and function of avian vocalizations have been well studied. In particular, bird song has been categorized extensively and is thought to function mainly in territorial defense, mate attraction, and pair-bond maintenance (Kroodsma and Byers 1991). The study of bird song in turn has contributed greatly to our understanding of development and reproductive biology (Slater 1989, Marler 1990, Baptista and Gaunt 1994). A key factor in the success of these studies is the conspicuous nature of the signals involved. Some types of short' Present address: Department of Fisheries and Wildlife Sciences, Cheatham Hall, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA. E-mail: rtitus@indiana.edu distance vocalizations, such as those used in parent-offspring communication, have been well studied (Beecher 1990), but most of the work on avian vocalizations has focused on high-amplitude, long-range songs (LRS; Catchpole 1982, Wiley and Richards 1982, Smith 1996). Although many workers recognize that birds produce short-range songs (SRS) that function over shorter distances, the emphasis on LRS may under-represent the importance of vocal communication by only partially addressing the range of songs produced. The structure of LRS is constrained by environmental degradation, which limits the variety of sounds used to those that can be effectively transmitted across territories (Wiley and Richards 1982, Brown and Handford 1996, McGregor and Dabelsteen 1996). Songs transmitted over short distances should be less constrained by environmental degradation (Wiley and Richards 1982). The more complex structure of some SRS may facilitate their transmission specifically to intended receivers (McGregor and Dabelsteen 1996). A few studies have found components within the same songs that appear to be adapted for longand shortrange functions. Among Brown-headed Cowbirds (Molothrus ater), King et al. (1981) found that a small reduction (3 dB) in the signal-tonoise ratio decreased female response to playback by 50%. Other song components were