baker howard & sweet chickadee-ethology 2000

Biology Department, Colorado State University, Fort Collins

Microgeographic Variation and Sharing of the Gargle Vocalization

and Its Component Syllables in Black-Capped Chickadee (Aves,

Paridae, Poecile atricapillus) Populations

Myron C. Baker, Trina M. Howard & Paul W. Sweet

Baker, M. C., Howard, T. M. & Sweet. P. W. 2000:Microgeographic variation and sharing of the garglevocalization and its component syllables in black-capped chickadee (Aves, Paridae, Poecile atricapillus)populations. Ethology 106, 819Ð838.

Abstract

Throughout the year during agonistic encounters, black-capped chickadees(Poecile atricapillus) emit a vocal signal known as the gargle call. Each bird has arepertoire of structurally di�ering gargle calls; some are shared with others in thelocal area. As a basis for understanding the cultural evolution of this social signal,we initiated a study of gargle call repertoires of birds living in a narrow belt of con-tinuous riparian habitat occupied throughout by a resident population of chicka-dees. During two consecutive winter seasons, we sampled repertoires at threelocations over a distance of 8.4 km to quantify micro-geographical variation. Ana-lyses of vocal sharing and population di�erentiation were carried out on wholegargle calls and on the individual acoustic units (syllables) from which the wholecalls are constructed. We analysed 28 380 calls of 46 subjects in the two seasons ofstudy. Birds averaged 7.6 di�erent calls in their gargle repertoires. Calls were com-posed of about 10 syllables on average. Fifty-six di�erent syllables were used toconstruct the calls of all birds. Each study site had some gargle calls unique to thelocal birds and some that were shared with one or both of the other two sites.There was signi®cantly greater sharing of both calls and syllables among birdswithin sample sites than between sample sites. The frequencies of the di�erentkinds of gargles and syllables were signi®cantly correlated across the 2 yr of thestudy, but the correlation was stronger (r2� 0.93) for syllables than for whole gar-gle calls (r2� 0.61).

Corresponding author. M. C. Baker, Biology Department, Colorado StateUniversity, Fort Collins, CO 80523, USA. E-mail: [email protected]

U. S. Copyright Clearance Center Code Statement: 0179-1613/2000/1069±0819$15.00/0

Ethology 106, 819Ð838 (2000)

# 2000 Blackwell Wissenschafts-Verlag, Berlin

ISSN 0179±1613

Introduction

Studies of cultural evolution in animal societies attempt to describe changesin culturally transmitted traits (memes) with the passage of time and identifycauses of the observed change (Lynch 1996; Payne 1996). A host of questions andproblems is subsumed under this global perspective:

1. What are the physical or social boundaries delineating the populationwithin which one examines the temporal changes in the trait?

2. To what extent do individuals in the population share the trait?3. How similar is the form of the trait in di�erent individuals?4. Is the trait transmitted as a unitary feature or are there subunits of the trait

that are assembled into a ®nal form during ontogeny?5. When two populations di�er in the trait, what is the nature of the transi-

tion between them: abrupt and discrete, or gradual with quantitative di�erentia-tion?

6. What is the function of the trait in the social life of the organisms and howis this related to the forces of stasis or change in the frequency of the trait?

Vocal communication signals of birds have become a focus of this type ofresearch program (Laland et al. 1993) because of the marked dialect variation invocal features of many songbird species and because of the clear and frequentdemonstration of the learning processes by which acoustic signals are transmittedacross generations.

We regard the black-capped chickadee, Poecile atricapillus, as a model speciesfor studies of cultural transmission and evolution, and we have focused part of ourresearch program on one particular vocalization termed the gargle call (Ficken etal. 1978). Di�ering from territorial song, the gargle call is uttered year round, pri-marily in the context of agonistic encounters between individuals in close proxi-mity, such as contests over food (Baker et al. 1991). The gargle has a considerablymore complex acoustic structure than the more commonly heard `chick-a-dee'¯ock call and the whistle-like `fee-bee' territorial song of this species and di�ers infunction as well (Ficken et al. 1978; Hailman 1989). The call is about 0.5 s in dura-tion, composed of a sequence of discrete syllables, and may be delivered in con-junction with a display posture. Each bird has a repertoire of di�erent gargles, andthe size of the repertoire varies among individuals.

The present research was motivated by our observation, made ancillary toother studies, of the occurrence of a set of acoustically very similar gargle calls inthe same population sampled in 1984 and again in 1994, a time span correspondingto 4±5 generations of chickadees. We initiated a ®eld and laboratory study to pro-vide the level of detailed analysis we thought necessary to map the cultural evolu-tion of the signal, a level of detail lacking in most studies. An exception is the long-term and detailed study of indigo bunting (Passerina cyanea) song (summary inPayne 1996). Descriptive research published on the gargle call, while of consider-able use, was unsatisfactory in a number of ways. First, most previous recordingsof the gargle call have been made at bird feeders where calling sequences are shortand complete gargle repertoires of individuals are seldom obtained (Ficken et al.

820 M. C. Baker, T. M. Howard & P. W. Sweet

1987). In the few cases where repertoires apparently were more completelyrecorded, the sampling interval over which calls were recorded was unspeci®ed,leaving unanswered the question of rates of change in individual repertoires. Sec-ondly, study populations in prior research e�orts occurred in broken habitat, mak-ing it di�cult to evaluate the roles of physical vs. social isolation in contributing todi�erentiation of gargle call features (Ficken and Weise 1984). Thirdly, quantita-tive patterns of gargle call sharing within and between populations have not beenexamined in ways useful to understanding evolutionary changes within gargle calllineages or the extent of population di�erentiation. Fourthly, the time scale overwhich the gargles of a population have been compared is too coarse to gain anunderstanding of year-to-year addition or loss of memes (Ficken and Popp 1995).Thus, rates of cultural evolution in gargle calls are unavailable. Furthermore, wefelt that change and di�erentiation in the acoustic properties of the gargle vocali-zation could occur di�erently at the level of the whole gargle call and at the level ofthe individual syllable units from which gargle calls are composed. Therefore, weneeded to obtain concordant quantitative data on both calls and syllables to allowstatistical comparisons.

The present report summarizes two successive seasons of sampling of the gar-gle call in three populations. We examined complete repertoires of individualbirds, and described the repertoire set of whole gargle calls and of the complete syl-lable set in each of the three populations in the two consecutive seasons. Thesedata allowed quantitative analyses of the degree to which both whole gargle callsand their component syllables are shared among individuals within each popula-tion, and the extent of sharing between populations in close geographical proxi-mity.

Methods

Study Populations

Vocalizations of black-capped chickadees were sampled from three popula-tions along the Cache La Poudre River in Fort Collins, Colorado. The three popu-lations, Lee Martinez Park (LMP), Mulberry/Lemay (ML), and theEnvironmental Learning Center (ELC), are all connected by continuous riparianvegetation (cottonwood trees and shrub understory) occupied by a continuous dis-tribution of chickadees. LMP, the area farthest upstream, is 3.6 km from ML, themiddle area, which is 4.8 km from ELC, the area farthest downstream.

Sampling

Each year, ®eld work was initiated in Oct./Nov. with preparation of indivi-dual trap sites at ®xed locations within each of the three populations (three siteseach at LMP and ML, two sites at ELC), placement of traps (4-cell potter traps),and baiting of sites with sun¯ower seeds. All sites were baited weekly and each sitewas baited the day before trapping. A small number of chickadees was capturedduring a given trapping session in a given population and transported to a labora-

821Chickadee Call Sharing

tory at Colorado State University where they were placed into individual cageswith cardboard partitions between cages. Under these short-term housing condi-tions, it was rare that any gargle calling occurred in the common room. Thus, it isdoubtful that any bird's repertoire was in¯uenced by hearing gargle calls of otherswhile in captivity. Additionally, some individuals in our studies have beenrecorded several days following their ®rst recordings and no repertoire changeswere observed.

The birds were supplied with sun¯ower seeds, turkey starter, grit and water;cages were cleaned daily. The light cycle under which the birds were maintainedwas adjusted periodically to mimic the natural one. Birds from only one studypopulation at a time were present in the laboratory and were kept captive forabout 1wk before being released at the sites of capture. We did not know the ¯ockmembership of individuals. We trapped, recorded, and released birds from eachpopulation in succession, rotating through the three populations in this manner,and completing the ®eld season inMar.

Recording

Each bird in its home cage was placed individually into a sound proof cham-ber (Industrial Acoustics, Co.) for 1±2 h on one or more days. Recording chamberswere out®tted with a mirror located at one end of each of the two perches in thebird's cage and a Uher microphone (M157) at the center of the cage between thetwo mirrors. The microphone was connected to a Uher recorder (4200 ReportStereo IC) running at 9.5 cm/s. Gargle calls occurred in bouts of variable durationand were delivered by the subject to its mirror-image re¯ection (Censky andFicken 1982). Gargles were commonly given in strings of repetitions of a particulartype followed immediately with a string of an acoustically di�erent type, butswitching between types in the repertoire sometimes occurred with each utterance.

Recording complete repertoires of color-banded and free-ranging individualchickadees in the ®eld has never been accomplished because gargling interactionsare short-term, random in location, and the participants are often too closetogether to discern which calls are emitted by which bird. Population samples ofgargle calling of free-ranging chickadees have revealed no call types that are notalso found in the laboratory mirror image recordings (Malpede and Baker 2000).

Spectrographic Analyses

All recordings were examined with a DSP sonograph (Kay Elemetrics Model5500) using settings of DC-8 kHz frequency range, ¯at shaping, 300Hz transform,Hamming Window, and no averaging. Some calls were also examined with theDC-16 kHz range because they contained components above 8 kHz. We displayedthe ®rst call of a recording session on the upper half of the DSP monitor screenand examined each subsequent call on the lower half. When there was a change inthe structure of the call, that call was retained on the screen, and both calls weretransferred to the printer (Kay Elemetrics Model 5510). Thus, subsequent calls


could be compared to all previous call types, and new ones retained and trans-ferred to permanent copy.

Syllable Classi®cation

Each gargle call is composed of a sequence of individual syllables. These indi-vidual acoustic units were recognized by their temporal separation from oneanother and by their patterns of frequency changes in time (Ficken and Weise1984; Hailman and Griswold 1996; Miyasato and Baker 1999a). Most syllablesappeared as single continuous traces spectrographically (notes, Shiovitz 1975).Some syllables comprised more than one element, which always occurred togetheras a complex. However, even though two or more elements might often occurtogether in calls, if there were instances when each element occurred alone, thenthese elements were considered separate syllables. The di�erent kinds of syllableswere placed into a catalog of numbered types. Overall, syllables were stereotypedand most categories of syllables were readily recognized when occurring in di�er-ent call types of an individual or in di�erent birds' repertoires, as other studieshave noted (Ficken and Weise 1984; Hailman and Griswold 1996; Miyasato andBaker 1999a). Each syllable of a gargle call was assigned the number correspond-ing to its catalog number.

Gargle Classi®cation

As with syllables, the di�erent types of whole gargle calls in a bird's repertoirewere stereotyped and readily distinguished from one another. For the purpose ofdescribing the repertoires of individual birds, a gargle type was de®ned as a callthat di�ered consistently, by at least one syllable, from the other types in the reper-toire (Ficken andWeise 1984; Hailman and Griswold 1996). In most cases, the calltypes in a bird's repertoire di�ered in several syllables. Identi®cation of gargletypes was made easier by the delivery pattern of gargles during a gargling bout. Ina typical bout, a bird uttered a series of the same gargle type before switching toanother type (eventual variety, Hartshorne 1973). The new call was usually dis-tinctly di�erent in its syllable composition and therefore easily recognized as anadditional call type in the repertoire. Within a series of the same type of gargle call,however, minor variation may occur, such as an incomplete gargle with the term-inal syllable absent, or the deletion of the ®rst syllable. The inconsistency andinfrequency of these minor variants within gargle types made it easy to recognizethe more obvious variation between gargle types.

Once all the subjects' repertoires of gargle types were obtained and each gar-gle type was illustrated as a sonogram with numbered syllables corresponding tothe catalog of syllables, we made detailed comparisons among individual reper-toires to sort similar gargles into gargle groups. Sorting into gargle groups wasdone independently by two of us (MCB, TMH) with less than 5% of the assign-ments of gargles to groups in disagreement. These were resolved mostly by re-examination and discussion. In a small number of cases, the 75% rule (Miyasatoand Baker 1999a) was applied to decide on inclusion in a group. By this rule, simi-


larity (S) was calculated by:

S � s=�sa � sb � s�in which s is the number of syllables (in the same sequence) shared by a and b, sa isthe number of syllables unique (within the context of the two calls) to gargle a, andsb is the number of syllables unique to gargle b. Two gargles calculated to be 75%similar by this formula were placed in the same gargle group. As in the classi®ca-tion of gargles in a repertoire of an individual, however, the most common varia-tion within gargle groups was in the addition or deletion of syllables at the onset ortermination of a call. Thus, any particular individual could have its repertoire ofgargle types classi®ed into fewer gargle groups.

Population Repertoires

To evaluate the adequacy of sample size in each population, we plotted thecumulative number of gargle groups observed vs. the number of birds sampled(Kroodsma 1982). These `saturation' plots were constructed for each of the threepopulations by randomly ordering the subjects from a given population, countingthe number of gargle groups represented in the ®rst bird, the additional numberadded by considering the second bird, and so on for all the birds sampled. Five dif-ferent random orderings were carried out, thus providing ®ve `®rst bird' values,®ve `second bird' values, and so on. We then calculated and plotted the means andstandard errors for each set of ®ve values. A plateau in such a plot indicates thatadequate samples of birds and recordings were obtained and that recording addi-tional birds would not reveal new kinds of gargles in that population (Kroodsma1982).

Statistical Comparisons

Sharing of signals between individuals within populations and between popu-lations was carried out for individual gargle syllables and for whole gargle calls.Sharing was quanti®ed by use of an index developed by Hughes et al. (1998) fordescription of repertoire sharing in the songs of song sparrows (Melospiza melo-dia). The sharing index (H) is the proportion of the elements of a bird's repertoirethat are shared with any bird in the comparison group. Thus, for example, if a sub-ject has a repertoire of ®ve di�erent gargle groups, it might share one with bird A,two others with bird B, and have two not shared with any other bird in the refer-ence population. Thus, the subject shared 3/5 of its repertoire for a value of H�0.6. The same index was also used to describe sharing of gargle syllables, with theentire list of syllable types present in all gargle types of each bird forming the database for between-bird comparisons.

To compare values of H within and between populations, we carried out arc-sine (square root H) transformations (Sokal and Rohlf 1981) and expressed thequantities in degrees. Thus, values could range from 0 to 90 � (H� 0±1.0). Analysisof variance was performed on the transformed data, and if this resulted in a signi®-cant F-ratio, the patterns of sharing among birds within populations and between


populations were examined by Fisher's LSD pair-wise multiple comparisons test,which adjusts for experiment-wise error rate (Statview 5.0, SAS Institute 1998).

Results

Samples Obtained

In the 1996/97 season, we recorded 10 457 gargle calls from 22 subjects (LMP:3114 calls, seven birds; ML: 2358 calls, ®ve birds; ELC: 4985 calls, 10 birds). In the1997/98 season, we recorded 17 923 calls from 24 subjects (LMP: 8398 calls, ninebirds; ML: 4089 calls, six birds; ELC: 5436 calls, nine birds). The number of garglecalls recorded per bird did not di�er signi®cantly among the three populationssampled in either season (1996/97: F2� 0.05, p� 0.95; 1997/98: F2� 0.92, p�0.41; one-way ANOVA).

Adequacy of Sampling of Population Repertoires

The cumulative plots of the number of gargle groups found as a function ofthe number of birds examined in each population in both the 1996/97 and 1997/98seasons all tended to reach plateaus (Fig. 1). We ®tted second order polynomialfunctions to the data, all of which gave R2 values >0.90, and all but one were>0.97. For each polynomial equation we found the ®rst derivative and calculatedthe value on the abscissa at which it became zero. The ®tted functions for ®ve ofthe six plots reached maxima (slope� 0) at values of the abscissa less than actualsample sizes (Fig. 1). Only the function ®tted to the 1996/97 sample at ML had azero slope value greater than actual sample size (5.2 vs. 5.0). From these analyses,we judged the samples to be adequate for description of population repertoiresand for quantitative comparisons between populations.

Syllable Composition of Gargle Calls

The syllable composition of each gargle was examined, and all di�erent sylla-bles were collected into a catalog of 56 syllable categories (Fig. 2). Some syllablecategories were more variable than others, but boundaries between syllable cate-gories were usually distinct. Most syllables were stereotyped and easily recognizedwhen occurring in a newly examined gargle (e.g. Fig. 2: nos. 5, 11, 30, 34). Somedi�culty in assignment to categories was encountered with syllables that appearedas simple vertical lines (clicks or rapid frequency sweeps; e.g. Fig. 2: nos. 2, 3, 12,29). Closer examination of a number of these simple clicks, however, revealed con-sistent di�erences in mean frequency and frequency range.

Classi®cation of Gargle Types

Considering all birds in the two seasons of study, individual repertoires ofgargle types averaged 7.6 (SE� 0.6, range 3±18). A complete repertoire of an indi-vidual is illustrated in Fig. 3 and shows a typical range of variant gargle types.Close inspection shows that several syllable types are shared between gargle types,and that the consistent presence or absence of a single syllable (usually the terminal


one) can distinguish di�erent gargle types. The number of gargle types per bird didnot di�er signi®cantly among the three populations in either of the two seasons(1996/97: F2� 2.00, p� 0.16; 1997/98: F2� 0.15, p� 0.86; one-way ANOVA).

Classi®cation of Gargle Groups

We examined all the gargle repertoires of the birds from the three populationsin the 1996/97 season and sorted them into 34 gargle groups. For the 1997/98 sea-son, we identi®ed 26 gargle groups. Two common gargle groups, and their membersubjects, are illustrated in Figs 4 and 5 and indicate the kinds of variants that were

Fig. 1: Cumulative number of gargle groups found as a function of numbers of birds examined in eachpopulation in the 1996/97 and 1997/98 seasons. Standard errors were too small to illustrate. Equations

are best-®t second order polynomials; r2 values are for observed values vs. theoretical expectations


classi®ed as belonging to each gargle group. As can be seen, the more inclusive rule

used to classify gargle types into gargle groups ignored di�erences of one or two

syllables. Thus, the repertoires of most birds were reduced, because typically some

gargle types in a bird's repertoire di�ered by only one or two syllables. Individual

repertoires of gargle types (di�ering in at least one syllable) averaged 7.6, whereas

individual repertoires of gargle groups averaged 6.2.

Fig. 2: Catalog of 56 syllable types identi®ed over the 2 yr of the study. Multiple examples in a categoryare illustrated to indicate the range of variation within the category


Gargle Call Sharing among Birds and Populations

We used the sharing index (H) to provide a measure of the average amount ofsharing among birds within a given population and between that population andthe other two. In the 1996/97 season, gargle call sharing was greater among birdswithin each reference population than between that population and the other twopopulations for three out of six possible comparisons (Table 1). In the 1997/98 sea-son, call sharing was greater among birds within each reference population thanbetween that population and the other two for all six of the possible comparisons(Table 1). Thus, considering both study seasons, in nine out of 12 comparisons, thewithin-population sharing of gargles was signi®cantly greater than was sharingbetween populations.

Syllable Sharing among Birds and Populations

The sharing index H was also applied to the sets of syllables constituting thegargle repertoires of each bird in each population. This provides a measure of theaverage amount of sharing among birds within a given population compared tothe sharing between that population and the other two. In the 1996/97 season, syl-lable sharing was greater among birds within each reference population than

Fig. 3: Six gargle types in the repertoire of black-capped chickadee, WG


between that population and the other two in four out of six comparisons (Table2). With ML as the reference population, the pattern of sharing was anomalous:there was more sharing between ML and ELC than among birds within ML. Inthe 1997/98 season, syllable sharing was greater among birds within each referencepopulation than between that population and the other two in four out of six com-parisons (Table 2). Thus, considering both years, in eight out of 12 possible com-

parisons, the within-population sharing of syllables was signi®cantly greater thansharing between populations.

The general trends of the data on sharing indicate that population di�erentia-tion in syllable repertoires is only slightly less than di�erentiation in whole garglecall repertoires. The overall magnitude of the sharing index, however, averagedlarger for syllable sharing than for sharing of whole gargle calls. Within popula-tions, the sharing index for whole gargle calls did not di�er signi®cantly from thesharing index for syllables (U� 10, p� 0.20, Mann±Whitney U-test, Siegel 1956).Between populations, however, syllable sharing was signi®cantly greater than wassharing of whole gargle calls (U� 0, p� 0.0001, Mann±Whitney U-test).

Fig. 4: Gargle group designated BY4 from the LMP sample of 1996/97 showing the nine similar gargletypes of four birds


Gargle Call and Syllable Sharing between Populations

Our results can also be examined to determine whether any two of the threepopulations tend to have more vocal sharing than the other pairs. We were espe-cially interested to learn whether the geographically intermediate ML populationwas also intermediate in sharing of gargle calls or syllables with ELC and LMP, orif it tended to share more with one of the two adjacent populations. Consideringwhole gargle calls, the 1996/97 samples revealed a similar degree of sharing in thethree pairwise comparisons with no two populations sharing calls more than anyother two (all paired comparisons, ns). In 1997/98, ML shared signi®cantly more

Fig. 5: Gargle group designated YG4 from the ELC sample of 1996/97 showing the eight similar gargletypes of seven birds


calls with ELC than either did with LMP (ML±ELC vs. ML±LMP, p� 0.022;ELC±ML vs. ELC±LMP, p� 0.0002). Considering gargle syllables, the 1996/97samples revealed that ML shared signi®cantly more syllables with ELC than eitherof these two shared with LMP (ML±ELC vs. ML±LMP, p� 0.013; ELC±ML vs.ELC±LMP, p� 0.010). In 1997/98, once again ML shared more syllables withELC than either shared with LMP (ML±ELC vs. ML±LMP, p� 0.008; ELC±MLvs. ELC±LMP, p� 0.001).

Year to Year Changes

The frequency distribution of numbers of birds in each gargle group in eachof the 2 yr of the study shows a logarithmic decline when plotted from the most tothe least common gargle group (Fig. 6a, b). Of the 42 gargle groups found in the 2yr of sampling, 18 were present in both years, 16 were found in only the 1996/97sample, and eight in only the 1997/98 sample. The number of birds in each garglegroup was correlated between years (Fig. 7).

The number of occurrences of each syllable type in the set of repertoires fromall birds combined showed large variation and a logarithmic decline when plottedfrom the most common syllable to the rarest syllable (Fig. 8a, b). Of the 56 sylla-bles in the catalog representing both seasons combined, 54 were found in the 1996/

Table 1: Comparisons of gargle call sharing within and between populations. Sharing index(H) values are arcsine (square root proportion calls shared). Pairwise comparisons employ

experiment-wise error rates (Fisher's LSD)

Studyseason

Referencepopulation

Samplescompared

Sharing indexH (�SE)

Overallheterogeneityamong samples

Pairwisecomparison

1996/97 LMP LMP-LMP 77 (13) F2,18� 3.71,LMP-ML 44 (7) p� 0.043 p� 0.020LMP-ELC 60 (5) ns

ML ML-ML 52 (14) F2,12� 0.27,ML-LMP 40 (10) p� 0.76 nsML-ELC 53 (14) ns

ELC ELC-ELC 81 (5) F2,27� 21.00,ELC-ML 36 (6) p<0.0001 p<0.001ELC-LMP 44 (3) p<0.001

1997/98 LMP LMP-LMP 80 (10) F2,24� 3.78,LMP-ML 51 (7) p� 0.037 p� 0.026LMP-ELC 51 (7) p� 0.024

ML ML-ML 80 (7) F2,15� 11.34,ML-LMP 33 (7) p� 0.001 p� 0.0003ML-ELC 58 (6) P� 0.043



97 sample and 51 in the 1997/98 sample. The frequency of occurrence of syllabletypes in the two seasons of the study was correlated (six birds sampled in bothyears were removed from the 1997/98 data before correlation was calculated; Fig.9).

Discussion

We conclude that the gargle call of black-capped chickadees exhibits signi®-cant di�erentiation at the three sampling locations along a corridor of riparianhabitat occupied by an uninterrupted population of the species. Quantitativevalues of an index of sharing revealed signi®cantly higher levels of sharing amongbirds within sites than between sites. From the point of view of a resident bird at agiven site engaged in a vocal exchange, there is a greater probability of sharing agargle call with another bird from the same site than of sharing a call with a birdfrom either of the other two sites.

In examining the geographical patterning of the acoustic units from which thewhole gargle calls are constructed, the syllables, we again found, for all three loca-tions, signi®cantly greater sharing within a sample area than between that samplearea and either of the other two. This occurred even though the overall magnitude

Table 2: Comparisons of syllable sharing within and between populations. Sharing index(H) values are arcsine (square root proportion syllables shared). Pairwise comparisons

employ experiment-wise error rates (Fisher's LSD)

Studyseason

Referencepopulation

Samplescompared

Sharing indexH (�SE)

Overallheterogeneityamong samples

pairwisecomparison

1996/97 LMP LMP-LMP 82 (4) F2,18� 6.76,LMP-ML 69 (1) p� 0.006 p� 0.002LMP-ELC 75 (1) p� 0.045

ML ML-ML 76 (4) F2,12� 4.59,ML-LMP 73 (1) p� 0.033 nsML-ELC 85 (3) p� 0.047


1997/98 LMP LMP-LMP 82 (2) F2,24� 4.05,LMP-ML 77 (3) p� 0.031 nsLMP-ELC 72 (2) p� 0.009

ML ML-ML 82 (3) F2,15� 10.36,ML-LMP 65 (2) p� 0.001 p� 0.0005ML-ELC 77 (2) ns

ELC ELC-ELC 88 (2) F2,24� 20.58,ELC-ML 79 (2) p<0.0001 p� 0.011ELC-LMP 68 (2) p<0.0001


of sharing of syllables was larger than the sharing of whole gargle calls. These pat-

terns of sharing and di�erentiation of both gargle calls and syllables occurred over

a total distance of 8.4 km with the intermediate site only 3.6 km from the most

upstream site and 4.8 km from the downstream site.

An earlier study examined smaller samples from the same two more distant

sites of the present study (LMP, ELC) and reported signi®cant di�erences in call

sharing (Miyasato and Baker 1999a). The present project sampled an intermediate

area (ML) to determine whether the di�erences between the more distant locations

gradually grade through the intervening population or if they meet in a more dis-

crete border zone. The present samples address this issue.

With the exception of the 1996/97 sample of whole gargle calls, which

revealed no signi®cant di�erences in sharing between pairs of the three popula-

Fig. 6: Number of birds with gargle calls in each gargle group in (a) 1996/97 and (b) 1997/98. Solid linesthrough data points are ®tted logarithmic functions given by the equations; r2 values are for observed

values vs. theoretical expectations


tions, the 1997/98 samples of whole gargle calls, as well as both years of data on

syllable sharing, all consistently grouped the ML population with that of ELC.

Even though ML is physically closer to LMP (3.6 km) than to ELC (4.8 km), the

gargle calls and syllables suggest there is a steeper gradient of change, or a more

discrete border, between ML and LMP. On the basis of our data, we hypothesize

gradual clinal change from ELC to ML and a steep cline between ML and LMP.

Considering the nature of the variation and sharing of calls and syllables over

these linear and narrowly con®ned small populations, a more re®ned description

of the geographic changes may not be possible.

Ficken and Weise (1984) described dialects in gargle types and syllables at

locations in Ozaukee County, Wisconsin. At three sites within 650m of each

other, there were very similar sets of syllables and call types, but there was consid-

erable di�erentiation at sites 5.7 and 9.8 km apart. For these greater inter-site dis-

tances, the landscape was interrupted by areas of habitat unsuitable for

chickadees. Such unsuitable habitat could act as a partial barrier to dispersal and

social contact between di�erent gargle dialects, and contribute to the maintenance

of di�erences. Our results indicate that di�erent gargle call traditions are main-

tained even within a continuous distribution of chickadees.

At a location in Dane County, Wisconsin, 130 km from the Ozaukee County

site, Hailman and Griswold (1996) found no vocal di�erentiation over a distance

of 2 km. Compared to the Ozaukee County gargles, however, they found nearly

complete di�erentiation in the population syllable set, and therefore in call types.

We ®nd no overlap between our syllable catalog and those representing the two

Wisconsin sites.

Fig. 7: Correlation between numbers of birds in each gargle call group in 1996/97 vs. 1997/98 seasons.Equation is best-®t straight line; r2 value is for observed values vs. theoretical expectations


Examination of both whole gargle calls and of the constituent syllablesshowed similar patterns of commonness and rarity, and did so in both years of thestudy. Best-®t functions of the empirical frequency distributions of gargle callgroups and syllable types were logarithmic in form and the numbers of birds pos-sessing each of the gargle call groups was similar in the 2 yr of the study, suggestinga substantial degree of cultural stasis. This stasis was more pronounced in the cor-relation of the frequency of use of each syllable type between the 2 yr of the study(r2� 0.93) than in the correlation of gargle groups between years (r2� 0.61). It isworth remembering that all the birds in the second year of the study, that hadalready been sampled in the ®rst year, were removed from the second year's databefore correlations were made. Thus, the constancy of syllable and gargle callpopularity in vocal signaling of these populations is substantial.

Fig. 8: Frequency of use of 56 syllable types constituting gargle call repertoires of all birds in (a) 1996/97 and (b) 1997/98


Long-term data are available from Ficken and Popp (1995) who grouped

2295 gargle calls recorded from 1970 to 1981 as one sample (their `Older Period')

and compared the call types and syllables to those found in a second sample of 795

calls recorded from 1988 to 1989 (their `Newer Period'). The frequencies of the 14

most common kinds of syllables were very similar in the Older Period and Newer

Period samples, and most of the more common types of gargles recorded in the

Older Period were still present in the Newer Period. The grouping of numerous

years in the Older Period for comparison to two more recent years, with a 7 yr hia-

tus, makes it di�cult to determine rates of cultural evolution in these traits.

Why some gargle calls or syllable types are consistently common whilst others

are consistently rare is not obvious. Possibilities range from intrinsic di�erences in

the di�culty of producing certain syllables or calls, to functional explanations in

the communication system itself. How particular calls or syllables might be

deployed in di�erent contexts has not been explored, although in a laboratory

playback experiment, a larger repertoire of calls was somewhat more aversive than

a small repertoire (Baker et al. 1996). An important issue is to determine to what

extent the abundance patterns of syllables and gargles are independent. The sylla-

bles constituting the gargles in the most common gargle groups are not necessarily

the most common syllables overall. Instead, the most frequent syllables tend to be

those found in a wide variety of di�erent calls of many individual birds. This is

re¯ected in our results on sharing patterns of syllables and gargles. Although the

within-population values of the sharing index for whole gargle calls was not signi®-

cantly di�erent from sharing of syllables, the frequency of sharing syllables

Fig. 9: Correlation plot of frequency of use of each syllable in the 1996/97 sample vs. the 1997/98 sam-ple. Data from six birds sampled in both years were removed from the 1997/98 data before correlation

was calculated


between populations was signi®cantly greater than was sharing of whole garglecalls. Syllables can combine in a great many ways to form a large variety of garglecalls, although it appears that there are syntax rules that do not allow total free-dom of combination and sequential order within a call (Ficken and Popp 1992). Itwould, however, be theoretically possible to have an absolutely even frequency dis-tribution of syllable types but a skewed frequency distribution of the kinds of gar-gle calls formed from them.

We argue that it is important to examine variation in both syllables and callsas units of cultural stasis or change. It would be possible, for example, to see littleor no temporal change in syllable repertoires in a population but rapid turnover ofgargle call structures, something like the evolution of a new written language fromthe same alphabet. An alternative would be that both syllables and whole garglecalls follow the same rate of change. However, if gargle calls were stable over time,then of necessity the syllable pool would also remain unchanging. Although thedata we report here span only 2 yr, they do indeed suggest that the frequency distri-bution of the syllables changed less than that of the gargle groups.

Finding persistent patterns that change little or not at all over time begs thequestion of whether the birds ®nd particular communicative signi®cance to, forinstance, more common gargle calls or syllables. A previous playback study hasshown that the di�erences in gargle calls among populations are discriminated(Miyasato and Baker 1999b): familiar local gargle calls were more aversive thanwere unfamiliar calls of distant populations. To the extent that local calls are moree�ective `keep away' signals in contests, an immigrant lacking the local calls wouldbe at a disadvantage because others would tend to ignore its signal. Such an asym-metry in the e�ectiveness of vocal signals sets up the kind of selective social pres-sure that could explain the development of larger repertoires in individuals(Andersson 1980; Craig and Jenkins 1982).

Acknowledgements

Thanks to Todd Tracy, Lori Miyasato, Chandra Malpede, and Bob Black for their contributionsto this study. Birds were obtained under permit (Federal: MB694924, Colorado: TR036) and monitoredby the ACUC of Colorado State University (Protocol 256A).

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Received: October 12, 1999

Initial acceptance: February 5, 2000

Final acceptance: March 13, 2000 (J. Brockmann)


baker howard & sweet chickadee-ethology 2000

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