Quantitative Analysis of Prairie Grouse Vocalizations

Abstract

The structure of homologous vocalizations of Greater PrairieChickens (Tympanuchus cupido) and Sharp-tailed Grouse (T. [Pedioecetes] phasianellus) was studied during four mating seasons in northwestern Minnesota. In this region males of the two species form display grounds in similar areas and occasionally occur together in mixed grounds where they hold mutually exclusive territories. Hybrids constitute approximately 3% of the combined populations. This study tested the hypothesis that the structures of homologous vocalizations varied predictably with function to assist in reproductive isolation. Aggressive calls were more similar between the species than courtship calls. The dichotomous nature of the birds' vocal systems could promote heterospecific spacing and inhibit mating. The calls of two hybrid males varied in complex ways from those of either parental species. Although prairies are poor environments for transmission of acoustical signals, both species of grouse depend heavily on vocalizations in their communication. Sound transmission is enhanced in these species by low frequencies, use ofsound windows, and crepuscular displaying. Greater Prairie-Chickens (Tympanuchus cupido) and Sharp-tailed Grouse (T. phasianellus; A.O.U. 1982) are sympatric through a narrow zone in the midwestern and north central United States. In this zone hybridization typically occurs at a rate of 1-3% of the parental populations (Johnsgard and Wood 1968, Sparling 1980) although it has been recorded as high as 25% of the combined populations (Lumsden 1970). Ecological isolating mechanisms based on habitat preferences are weak, for males of both species frequently use the same areas for display grounds (Ammann 1957). Similarly, post-mating mechanisms are of little significance because hybrid females are fertile, their fecundity comparable to that in intraspecific matings (McEwen et al. 1969, Sparling 1980). Male hybrids however, may be unsuccessful in attracting mates. Several studies (e.g., Hamerstrom and Hamerstrom 1960, Lumsden 1965, Hjorth 1970, Johnsgard 1973) have suggested that displays may be very important in maintaining species integrity. Descriptions of visual displays, for example, indicate that courtship displays are very different between species while forms of agonistic behavior are similar. Through the use of playback experiments, I (Sparling 198 lb) showed that vocalizations are also important for maintaining species integrity because both Greater Prairie-Chicken and Sharp-tailed Grouse males responded to some of the other species' aggressive calls. The primary purpose of this paper is to further explain the role of vocalizations as isolating mechanisms between Greater PrairieChickens (GPC) and Sharp-tailed Grouse (STG). I test the hypothesis that structural differences among homologous calls vary predictably with function. An underlying premise of this hypothesis is that homologous epigamic vocalizations are more different between species than agonistic vocalizations. This relationship between epigamic and agonistic vocalizations could result if this dichotomy decreased transfer of information between potential heterospecific mates while promoting spacing between aggressive individuals. A second objective of this study is to examine relationships between the structure of calls and their functions. GPC and STG inhabit open grasslands where sound attenuation is especially high (Morton 1975, Marten and Marler 1977, Wiley and Richards 1978). Yet many of the vocalizations given by these birds, particularly the prairie-chicken boom, are noted for their long traveling distances. This acoustical property is probably related to the structure of the signals and to the way in which they are emitted.