Dialect Differences in the Song of Zonotrichia capensis in the Southern Pampas: A Test of the Acoustic Adaptation Hypothesis
| Autor: | Pablo Luis Tubaro, Enrique T. Segura |
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| Rok vydání: | 1994 |
| Předmět: | |
| Zdroj: | The Condor. 96:1084-1088 |
| ISSN: | 1938-5129 0010-5422 |
| DOI: | 10.2307/1369117 |
| Popis: | Although geographic variation in song is a relatively common phenomenon in songbirds (see Mundinger 1982), there are only a few cases in which the physical properties of song have been related to habitat structure. For example, studies of the White-throated Sparrow (Zonotrichia albicollis, Wasserman 1979, Waas 1988), the Summer Tanager (Piranga rubra, Shy 1983), the Northern Cardinal (Cardinalis cardinalis, Anderson and Conner 1985), and the Great Tit (Parus major, Hunter and Krebs 1979) indicate the use of lower frequencies and narrower bandwidth in forested than in more open habitats. The Rufous-collared Sparrow (Zonotrichia capensis) is unique in this regard because it shows higher frequency and broader bandwidth associated with more closed and mesic environments (Nottebohm 1975, Handford and Lougheed 1991). In addition, it has a complex system of dialects which are clearly distributed following vegetation structure (Nottebohm 1969, 1975; Handford andNottebohm 1976; Handford 1981, 1988; Lougheed et al. 1989; Handford and Lougheed 1991; Tubaro et al. 1993). In this species, dialects are defined according to the rate of note delivery in the final (trilled) part of the song. This "trill interval" is usually longer in closed (forested) areas than in open ones (for apparent exceptions in Handford 1988). At a community level of analysis, several studies have reported song differences between habitats. Chappuis (1971) and Morton (1975) found that the frequencies used by tropical forest birds were lower than those used by species living in open tropical habitats. In addition, birds living in open and edge habitats usually employ broad band and rapidly modulated sounds in relation to those from closed forested areas (Morton 1975, Wiley 1991). Different hypotheses have been proposed to account for these patterns of variation. Marler (1952) suggested th t the song of a local population of a species must differ significantly from that of other species sharing the same local habitat. Nottebohm (1969) suggested that populations adapted to different habitats will evolve markers as a reproductive isolation mechanism. In this case, dialect markers are arbitrary and not correlated with habitat structure. This hypothesis does not predict the observed convergence in song features among disjunct areas supporting similar habitats (Handford 1988). Finally, it has been proposed that variation in song structure may represent adaptations for long range communication (see for examples: Morton 1975, Nottebohm 1975, Marten and Marler 1977, Marten et al. 1977, Nottebohm 1985, Ryan and Brenowitz 1985, Handford 1988, Wiley 1991). According to this Acoustic Adaptation Hypothesis or AAH (Rothstein and Fleischer 1987) and the current ideas about habitat acoustics, slowly modulated signals are favored in forests, because they avoid the acoustic degradation generated by the accumulation of echos. In open fields, the main source of acoustic degradation is low rate amplitude fluctuation produced by moving cells of air with different temperature and humidity. This favors signals with high rates of repetition (Wiley and Richards 1978, Richards and Wiley 1980). Here, we report the existence ofa cline in song structure which partially contradicts naive predictions derived from the AAH. |
| Databáze: | OpenAIRE |
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