Song Pattern Variation in the Sage Sparrow (Amphispiza Belli): Dialects Or Epiphenomena?
Total Page：16
File Type：pdf, Size：1020Kb
SONG PATTERN VARIATION IN THE SAGE SPARROW (AMPHISPIZA BELLI): DIALECTS OR EPIPHENOMENA? JOHN A. WIENS Departmentof Biology,University of New Mexico, Albuquerque,New Mexico87131 USA ABSTm•cT.--I investigatedvariation in the songsof Sage Sparrows (Amphispizabelli) breedingin the northwesternGreat Basin over a 3-yr period.Individuals sang a singlesong type, which did not changeduring the day, throughthe breedingseason, or betweenyears. Although different individuals and populationsdiffered in song parameterssuch as dura- tion, delivery rate, and notesand note typesper song,these variations exhibited no overall patternswith respectto geographyor habitat. Similarity matricesbased upon the sequential arrangementof note types in songswere used to draw comparisonswithin and between populations.Within-population vocal similarity of individuals varied considerably,from populationsin which most neighboringindividuals sang similar or identicalsongs to sit- uations in which most individuals sang quite different songs;in one population, song variationwas arrayedin "neighborhoods"of 3-5 individualssharing similar songs, which differed markedly from those of adjacentclusters of individuals. Song variation between populationswas significantlygreater than that within popula- tions. Somepopulations located close to one anothershared many songelements and pat- terns, but other nearby populationswere totally different and showedgreater similarity with populationslocated some distance away. The degreeof songsimilarity between pop- ulations was neither a simple function of the physicaldistance between the populations, nor was it related to the distribution of topographicbarriers in this region. Populations with greaterbetween-individual dissimilarity in songpatterns were alsomore variablefrom one year to the next and occurredin habitatsof lowervegetation stature and greaterpatch- iness. Generally, however, the patterns of vocal variation within and between populations were unrelatedto featuresof habitat structure,to the densitiesof SageSparrows or of other coexistingspecies, or to avian community attributes. I suggestthat SageSparrows may have a simple,single-song repertoire because the song servessimple functions, and selectionfavoring a more elaboratesong may be absent.The variety of patternsof within- and between-populationvocal similarity may be associated with differencesin populationturnover rates, song ontogeny, dominance relationships, and dispersal,all of which require documentationto establishthe foundationsof vocalvariation in this system(and others).The variety of populationpatterns, however, argues against a simplistic categorizationof this speciesas dialectal or nondialectal. Much of the variation that is expressedmay be a consequenceof chanceevents and represents"epiphenomena" that are random with respectto natural selection.Received 22 July 1981, accepted10 November 1981. THE songsof many bird speciesvary geo- Kroodsma 1980). Confronted with such diver- graphically.This variation may involve changes sity, we attempt to discern some consistent in the occurrence,structure, or sequencingof pattern in the geographical variation of a elementsin songs,in the types of songsused species'vocalizations. In severalspecies, song by individuals, or in the size of the song rep- variation is expressedin local dialects--suites ertoire of individuals and may be expressed of neighboring individuals sing quite similar within local populations, between popula- songs,which differ more or lessabruptly from tions, or over large geographicalareas (Thielcke those of more distant individuals. Song vari- 1969, Baptista and King 1980, Krebs and ation within a given local population is thus low, and variation between different local pop- • The editorialprocessing and reviewof this paper ulations may be substantial. were handledby JamesR. King. Unfortunately, "dialect" is defined some- 2O8 The Auk 99: 208-229. April 1982 April 1982] SageSparrow Song Variation 209 times in termsof boundariesseparating groups nia, and northern Baja California. At least in of individuals singing different songs, some- the northern portion of this range, breeding times simply in terms of song-sharingamong densitiesvary directlywith the groundcover- neighbors. Further, the spatial scaleon which age of big sagebrush(Arternisia tridentata), and song variation is considered to represent a di- habitatsdominated by spinescentshrubs such alect pattern may vary over severalorders of as cottonthom(Tetradyrnia spinosa) and grease- magnitude. Despite such imprecision, several wood (Sarcobatusverrniculatus) are generally alternativeexplanations of the origin or func- avoided (Wiens and Rotenberry1981). In areas tion of dialects have been proposed. Dialects of Oregon where we have studied their breed- may thus represent (1) epiphenomena of no ing biology intensively, populations do not adaptive or functional significance,resulting completely saturate the available habitat with from historical events unique to different lo- territories, the birds mate monogamously,and calities (e.g. founder effects);(2) adaptive fea- we have no evidence that any unmated birds tures that enhancelocal genetic specialization or "floaters" are presentin the areaafter breed- of individuals in relation to habitat or other ing has begun. Birds in Oregon often rear but environmental features; (3) socialadaptations, a single clutch, although some early-breeding which act in male-male competition or in mate individuals may complete two clutches if selectionand promote assortativemating; (4) weather conditions are benign. Foraging by shared responses of individuals to acoustic both sexes is concentrated within the area de- limitations of the physical environment that fined by the singing territory, but excursions may influence sound transmission and com- well beyond the territory boundariesinto areas munication;or (5) artifactsof samplingproce- occupied by other individuals are not infre- dures (Nottebohm 1972, Baker 1975, Payne quent (J. A. Wiens and J. T. Rotenberry,pers. 1981a). obs.). Adult birds that have previously bred Payne (1981a)proposes that one must frame successfullyin an area often return to the same suchexplanations as hypothesesyielding test- general location the following season[as Rich able predictionsin order to begin to sort out (1980) also observed in Idaho]. Our intensive the various alternatives.Before one can pursue banding of nestlings,however, has produced such hypothesis-testing,however, it is neces- no returns in subsequentyears, despite careful sary to determine exactly how vocalizations searchesof natal areasand their surroundings. vary in a given species.A boundedlocal dialect Males sing from the tops of shrubs within pattern is one possibility,but songsmight also their territories from their arrival in early vary more or less continuously, both within spring until after young are fledged. Singing and between populations--differences in tends to be interspersedwith boutsof foraging vocalizationsmight be a roughly linear func- (Rich 1980) and ceasesduring midday on hot tion of distance separating individuals (e.g. days. During singing bouts, songsare repeti- Bitterbaum and Baptista 1979). Alternatively, tively uttered at 10-20-s intervals. song variation might follow no discernible geographical pattern at all. Careful measure- METHODS ment and comparisonof songvariation within During the breeding seasonsof 1977-1979, I re- and between local populations of individuals cordedthe vocalizationsof 252 individual SageSpar- are necessaryto distinguish such alternative rows at 15 locations in the shrubsteppeof south- patterns and thus to determine whether tests eastern Oregon and northern Nevada (Table 1, Fig. of dialecthypotheses, or perhapsof someother 1). In 1977! recordeda minimum of 10 songsfor each hypothesesof geographicalvariation in song individual; analysisof these recordsindicated little structuring, are appropriate for a given within-individual variation in songfeatures (see be- species.Here I describeand analyze the geo- low). In 1978 and 1979 individuals were monitored graphical patterns of song variation in a mi- until at least two good-qualityrecordings were ob- tained. I recorded the individuals that I encountered gratory temperatefringillid, the SageSparrow while walking along a measuredline transect.This (Arnphispizabelli), a specieswell-suited to a allowed me to map the locations of individuals, en- study of such variation (see King 1972). suring that the same individual was not recorded Sage Sparrows are widely distributed as a more than once during a given year. All recording breeding speciesthroughout shrub desertsof was done during a single morning at each site, and the Great Basin, central and southern Califor~ all siteswere visited during the peak of the breeding 210 JOHNA. W•ENS [Auk, Vol. 99 season,in the latterhalf of Juneand earlyJuly. Color- Next, the sonogramtraces of eachnote type were marked individuals were availablefor study at the examinedvisually, and the notesgrouped according Cabin Lake locations.Analysis of the songsof these to their structuralsimilarity. In this manner, a den- birds permitted a determinationof the daily, sea- drogramdefining the hierarchicalpatterns of similar- sonal, and annual stability of song structureof in- ity among the note types was derived and used to dividuals. build a matrixof inter-notesimilarities. This stepis Songs were recorded on tape cassettesusing a importantto the analysisof songsimilarities, in that Sony TC-56 recorder,a