Microgeographic Variation in the Song of the Sage Sparrow

Abstract

-The Sage Sparrow breeds in large expanses of continuous sagebrush in southern Idaho. Analyses of the song syllables and shared syllable sequences of 40 males from five main study sites in Idaho revealed that individuals tended to be most similar to birds from the same population. Each male had one stereotyped song with only minor variations consisting of the omission of a few syllables from the end of the song. Three males apparently had the same song and same or nearly the same territory for three or four years. The most geographically distant population from the other four populations had the most distinct songs. However, neighbors on adjacent territories were not usually most similar to each other. Several syllables were widespread among males while uncommon syllables were often shared among males within a population. Stepwise discriminant analysis of ten variables from the same populations revealed that mean syllable duration and mean interval between syllables contained enough information to successfully assign 62.5% of the songs to the population to which they belonged. Six of an additional 10 songs recorded at five other sites in southern Idaho were classified into the main population geographically closest. Geographic variation in song and dialects have been described in sedentary avian populations (Lemon 1966, 1975, Kroodsma 1974, Baptista 1975, Bradley 1977, Payne 1978, Bowman 1979) and in migratory populations (Harris and Lemon 1972, Baker 1975, Orejuela and Morton 1975, Baptista 1977). Our understanding of dialects depends partly on recognizing the difference between true dialects and geographic variation in song. Differences in the songs of populations of sedentary species are readily explained in terms of the evolution of isolated populations (Bradley 1977, Bowman 1979) or in special social organizations (Jenkins 1977, Payne 1978). In migratory species where there is a greater opportunity for individuals to disperse among populations between breeding seasons and, thus, for population differences to be reduced, classical isolating mechanisms may also be important. Chief among the latter are geographical distance (Wright 1943) and variation in habitat (Nottebohm 1969, 1975, King 1972, Baptista 1977). To determine the nature of song variation in the absence of obvious isolating mechanisms it is necessary to examine populations of a migratory species separated by small distances in continuous homogeneous habitat. In this way geographic variation on the smallest scale might be described. In this report I describe differences in the songs of populations of the Sage Sparrow (Amphispiza belli). This species is migratory and occupies large expanses of continuous homogeneous sagebrush (Artemisia tridentata) habitat in southern Idaho. I recorded songs in populations separated by relatively less geographical distance than is customarily done in studies of this type. Even under these conditions where song variation seemed least likely to occur there were substantial differences in several song parameters among populations. I conclude that geographic variation in song among populations, most likely as a result of philopatry, occurs on a fine geographic scale in the Sage Sparrow. MATERIALS AND METHODS In 1976 and 1977 I recorded the songs and studied the behavior of four unmarked neighboring territorial males from March to July in Bingham Co., Idaho (Fig. 1, site SA). In May and June of 1978 I recorded 36 males at several localities in southern Idaho and in 1979 I recorded an additional 14 males. Forty males were recorded at five main sites (Fig. 1): SA, main study area; VC, a prominent volcanic cone; BB, Bingham-Bonneville county line; BT, Kettle Butte; MLF, Medicine Lodge Flats. The remaining 10 males were recorded at scattered locations: GR, C, E, JF and DF. I spent one morning at each site and traversed an area of about 2 km2 while recording songs. Vocalizations were recorded with a Dan Gibson Electronic Parabolic Microphone and an Audiotronics model 147A cassette recorder. Audiospectrograms were prepared on a Voice Identification, Inc. Series 700 sound spectrograph with linear scale and automatic frequency marker. I analyzed geographic variation with both a nonparametric and a parametric approach. For the nonparametric analysis, I defined and catalogued 40 different syllables based on frequency, duration and