The Auk 113(2):370-380, 1996

A COMPARATIVE ANALYSIS OF PASSERINE MOBBING CALLS

MILLICENT SIGLERFICKEN TM AND JAMESPOPP 2 •Departmentof BiologicalSciences; and 2FieldStation, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin53201, USA

ABSTRACT.--Weexamined the acousticstructure of mobbingcalls of 52 speciesof passerines representingan array of taxa.Marler suggestedthat callsgiven during mobbinghave char- acteristicsenhancing locatability that include abrupt onsetsand a wide frequencyrange. Only aboutone-half of the specieswe studiedproduced calls during mobbingthat had these characteristics.However, clicks(probably produced by bill snapping)sometimes were given during divesat predators.Vocalizations given during mobbingwere quite diversein acoustic structure.Two trendswere noted:phylogenetic patterns; and possibleconvergence in certain speciesengaged in interspecificflocking. Received4 May 1995,accepted 7 September1995.

ATTENTION CONCERNING the evolution of harassmentin causingthe predatorto leave the avian vocalizations has focused on song (e.g. vicinity, hasa considerableamount of empirical Kroodsmaand Miller 1982).However, nonsong support (Curio 1978, 1994). vocalizationsmay offer even better possibilities In a seminal work, Marler (1955) demonstrat- for understanding the evolution of acoustic ed the antitheticalacoustic structure of two types structure. Marlet (1957) pointed out that some of vocalizationselicited by predators.Calls giv- avian vocalizationssuch as song showed marked en when a hawk is flying overheadoften are specific distinctivehess, while others lacked high pitched, cover a narrow frequency range, these features and often were similar among and lack abrupt onsetsor terminations, while unrelated species,indicating the influence of those given during mobbing cover a wide fre- different selection pressureson diverse vocal quency range and show abrupt onsets.Calls in categories.Marlet (1955, 1957) and Marlet and the first category may have features making Hamilton (1966) pioneered studies of evolu- them difficult to localize, while mobbing calls tionary forcesshaping acoustic structure of avi- have features enhancing locatability (Marlet an calls, and it is surprisingthat little research 1955, 1959,Marlet and Hamilton 1966),and may has followed this lead. facilitate the recruitment of other individuals Our comparativestudy focusedon the acous- (Marlet 1959). While few experimental studies tic structureof soundsgiven during mobbing. demonstratedetectability of callsby the species A variety of behavior patterns with different emitting them, the "churr" mobbing call of the structuresand functions may be elicited by GreatTit (Parusmajor) has frequency character- predators,including distractiondisplays ("bro- isticswithin the bestdetectability range for the ken wing") near the nest or young, "distress" speciesand transmitsover longer distancesthan calls by captured individuals, and "alerting" the species'seeet calls given to a flying hawk calls when a predator is sighted. The term (Klump and Shalter 1984). When birds mob a "alarm" is sometimes used in the literature, but predator, they call and their actionsoften are its meaning is ambiguous and could include very conspicuous,including approachingand severalcategories of vocalizations.Klump and retreating. If the main function of mobbing is Shalter (1984), recognizing the need for distin- to induce the predator to move on, the partic- guishing among categoriesof antipredator be- ipation of numerousindividuals engagedin ha- havior, provided a comprehensive classifica- rassing the predator would be advantageous. tion. The distinguishing feature of mobbing vo- Sohogramsof the mobbing calls of seven spe- calizations is that they are given while ap- cies of British passetinesrepresenting several proaching the predator, with the callers different families are remarkably similar (Mar- frequently changing positions (Klump and let 1959:fig.16), suggestingconvergent evolu- Shalter 1984). Proposed functions of mobbing tion. are numerous(reviewed by Curio 1978),but the We examine both the structure of mobbing "move-on hypothesis,"stressing the role of the callsand a number of different hypothesescon-

37O April 1996] MobbingCalls 371 cerning their evolution: (1) Do mobbing calls corded.The expectationis, however, that with of passerinesbelonging to a wide variety of taxa a large and diverse assemblageof speciessam- confirm Marler's predictions of convergent pied, trends will allow tentative conclusions. acousticstructure in diverse taxa?Convergence We hope that the findings will encouragefur- would be indicatedby abrupt onsets(clicklike ther study of a complexphenomenon. patterns) and a wide frequency range (Marler 1959, Marler and Hamilton 1966). It is not clear METHODS whether theseattributes must always co-occur We recordedbirds at severalsites, both during the for easeof localization.By definition, a click breeding and nonbreeding seasons,and in response coversa wide frequencyrange and hasa sharp to a variety of stimuli (Table 1). In all cases,the birds' onset,but a call could cover a wide frequency responsesmet our criterion of mobbing (i.e. an ap- range and not be a click. In other words, some proach toward the stimulus, usually within about 10 calls might have one feature that would sup- m and often closer).In a few cases,especially with posedlyenhance locatability, but not necessar- owl playback,several species participated in the same ily both. While mobbing calls are often clicks mobbing event. Individuals were not color-banded, (Marler 1959),their most important feature is a but in many cases recordings were obtained from wide frequencyrange, and the soundsof dif- three or more individuals. ferent speciesmay show considerablevariation We useda SonyProfessional Walkman cassettetape recorderand a Nakamichi directional microphone ex- in other attributeswhile exhibiting this salient cept for calls obtained at nestswhere a Realisticultra- feature (Marler and Hamilton 1966). We ex- thin microphone was placed within I m of the nest. amineMarler's hypothesis not only by analysis Sonogramswere producedusing a Kay Digital 7800 of the characteristicsof mobbingcalls, but also Sona-graphat the 150 Hz band width setting (to pro- by comparisonof this categoryof calls with vide a compromisefor temporal and frequencymea- passerinenestling calls, where different selec- surements). Measurements were made from SOhO- tion pressuresare presumablyoperating (Popp gramsusing a ruler. We then obtainedmeasurements and Ficken 1991). (2) Are there also phyloge- of the following variableson five callsfor eachspecies netic trends in the acoustic structure of the calls? and calculatedmean values: (1) maximumfrequency Are thesephylogenetic patterns superimposed (kHz); (2) minimum frequency;(3) frequency range (maximum-minimumfrequency); and (4) call dura- on the predictedstructural convergence due to tions (measurementsgiven for individual units, al- selectionfor certaintypes of acousticstructure? though some were given in seriesof repetitions). (3) Doesthe acousticstructure of mobbingcalls In addition to measurements,calls were categorized provide insights concerning the functions of by their general configuration on SOhograms,a re- mobbing?Features promoting locatability might flectionof both temporaland frequencyaspects (Ta- supportthe move-onhypothesis, indicating the ble 2). This analysis was done blindly, as the cate- main function is recruitment of other individ- gorizer did not have information on the identity of uals in harassingthe predator. However, are the species.The categoriesare somewhatarbitrary, additional selection pressuresoperating? (4) but supplement the data obtained from measure- While callsimilarity among unrelated taxa could mentsin giving an overall view of what speciesshare certaincall similarities.The followingcategories were be due to natural selectionfavoring a narrow used:(I) Thin verticalline. These are very short-dura- range of "best" acousticdesign for mobbing tion soundscovering a wide frequency range. Some effectiveness,could convergent evolution of sound like a click, others like a sharp chip. (II) Thick calls of unrelated taxa in some cases be due to verticalbar. These calls are similar to the previous direct selectionto facilitate interspecificcom- category,but are longer in duration and the quality munication? differs. They often have a harsh sound and do not We recorded calls in a variety of situations sound like clicks. (III) Horizontalband. These calls are including natural mobbingof predators(and a longer in duration than those in the previous two few nest-holecompetitors), presentation of owl categories.The frequencyrange varies. In somecases, mounts,and playbacks,as well ascalls induced only a single band is given, but in others, stacksof bandsoccur (i.e. Black-cappedChickadee). (IV) Chev- by our presencenear nests.Recordings were ron. Calls are usually of short duration and have a made during both the breeding and nonbreed- chevron shape,with an ascendingfrequency, a peak, ing seasons.We probably have not sampledthe and a descendingfrequency. (V) Diversegroup. All of entire repertoire of mobbing calls from certain these callsare acousticallymore complexthan those species,and differentpredators might elicit calls in other categories,although they do not necessarily in some speciesthat differ from those we re- share common patterns. 372 F•CKENAND POPP [Auk, Vol. 113

TABLE1. Characteristicsof mobbingcalls. For specieswith two kinds of calls,shorter call given first.

Con- Frequency(kHz) Duration Species Sitea textb Highest Lowest Range (s) Tyrannidae Willow Flycatcher(Empidonax traillii) W HN 6.16 2.17 3.99 0.020 Say'sPhoebe (Sayornissaya) A HN 5.35 2.15 3.20 1.092 Dusky-cappedFlycatcher (Myiarchus tuberculifer) A PT 3.10 2.21 0.89 0.162 Ash-throated Flycatcher(M. cinerascens) A FT 5.10 2.70 2.40 0.073 Great CrestedFlycatcher (M. crinitus) W CT 5.45 1.88 3.56 0.238 Sulphur-bellied Flycatcher(Myiodynastes luteiventris) A O • 6.85 1.91 4.94 0.008 8.26 2.35 5.91 0.350 EasternKingbird (Tyrannustyrannus) W HN 7.67 5.05 2.63 0.113 Hirundinidae Tree Swallow (Tachycinetabicolor) W HN 5.69 3.00 2.69 0.010 5.44 3.58 1.86 0.133 Bank Swallow (Ripariariparia) W HN 7.97 0.24 7.73 0.008 5.46 2.53 2.93 0.092 Barn Swallow (Hirundorustica) W HN 5.92 3.97 1.95 0.170 Corvidae Steller's Jay (Cyanocittastelleri) A O 2 5.53 3.18 2.35 0.589 AmericanCrow (Corvusbrachyrhynchos) W O 3 3.81 1.22 1.53 1.817 Paridae Black-cappedChickadee (Parus atricapillus) A CT 5.18 2.97 2.21 0.124 Mexican Chickadee (P. sclateri) W PT 7.66 3.34 4.33 0.008 Bridled Titmouse (P. wollweberi) W HN 7.08 2.59 4.49 0.028 Aegithalidae Bushtit (Psaltriparusminimus) A PT 7.53 4.92 2.61 0.081 Sittidae Red-breasted Nuthatch (Sitta canadensis) A PT 4.89 3.95 0.94 0.030 White-breasted Nuthatch (S. carolinensis) A PT 3.30 1.86 1.44 0.036 Pygmy Nuthatch (S. pygmaea) A PT 4.07 3.21 0.86 0.031 Certhiidae Brown Creeper (Certhiaamericana) A PT 8.06 6.97 1.09 0.143 Troglodytidae Bewick'sWren (Thryomanesbewickii) A HN 6.23 2.67 3.55 0.026 House Wren (Troglodytesaedon) W HN 6.89 3.30 3.58 0.023 Muscicapidae Golden-crownedKinglet (Regulussatrapa) A PT 8.46 7.44 1.02 0.114 Ruby-crownedKinglet (R. calendula) A PT 7.48 2.84 4.65 0.017 Eastern Bluebird (Sialia sialis) W HN 5.95 2.04 3.90 0.017 3.02 2.38 0.65 0.120 Hermit Thrush (Catharusguttatus) A PT 4.58 2.03 2.55 0.029 4.48 2.08 2.40 0.143 Wood Thrush (Hylocichlamustelina) W HN 6.03 2.69 3.34 0.016 American Robin (Turdusmigratorius) W HN 3.55 1.43 2.12 0.060 Mimidae Gray Catbird (Dumetellacarolinensis) W HN 3.12 1.24 1.88 0.029 6.10 2.35 3.75 0.458 Brown Thrasher(Toxostoma rufum) W HN 7.45 3.22 4.23 0.048 Vireonidae Solitary Vireo (Vireosolitarius) A O 6 5.93 2.58 3.35 0.159 Hutton's Vireo (V. huttoni) A 04 5.75 2.73 3.02 0.289 Warbling Vireo (V. gilvus) -- O s 6.56 3.20 3.36 0.250 Red-eyed Vireo (V. olivaceus) -- O s 5.06 2.55 2.51 0.218 April 1996] MobbingCalls 373

T^n•,E 1. Continued.

Con- Frequency(kHz) Duration Species Sitea textb HighestLowest Range (s)

Emberizidae YellowWarbler (Dendroica petechia) W HN 7.39 3.74 3.64 0.016 Black-throatedGray Warbler (D. nigrescens) A PT 7.11 3.60 3.52 0.008 Grace'sWarbler (D. graciae) A PT 7.05 3.97 3.08 0.008 AmericanRedstart (Setophaga ruticilla) W HN 8.21 4.00 4.21 0.016 CommonYellowthroat (Geothlypis trichas) W HN 7.47 3.82 3.65 0.031 Red-facedWarbler (Cardellina rubrifrons) A PT 6.64 3.60 3.05 0.011 PaintedRedstart (Myioborus pictus) A HN 6.28 2.88 3.40 0.248 Olive Warbler(Peucedramus taeniatus) A PT 3.66 2.77 0.89 0.114 NorthernCardinal (Cardinalis cardinalis) W I-IN 7.39 5.67 1.72 0.031 IndigoBunting (Passerina cyanea) W GM 8.14 4.94 3.20 0.051 ChippingSparrow (Spizella passerina) W HN 11.10 7.48 6.62 0.041 FieldSparrow (S. pusilla) W HN 12.28 5.06 7.21 0.056 SavannahSparrow (Passerculus sandwichensis) W GM 7.99 4.50 3.50 0.008 SongSparrow (Melospiza melodia) W HN 4.44 3.42 1.02 0.039 Yellow-eyedJunco (Junco phaeonotus) A PT 7.31 5.05 2.27 0.016 Red-wingedBlackbird (Agelaius phoeniceus) W HN 5.37 2.63 2.74 0.023 EasternMeadowlark (Sturnella magna) W GM 5.35 3.39 1.96 0.087 CommonGrackle (Quiscalus quiscula) W HN 3.77 1.81 1.96 0.034 ß (W) Wisconsin; (A) Arizona. • (GM)Great Horned Owl (Bubovirginianus) plastic model; (PT) playback of NorthernPygmy-Owl (Glaucidiura gnoraa) calls; (FT) playback of FlammulatedOwl (Otis fiamraeolus) calls; (CT) playback ofBlack-capped Chickadee mobbing calls; and (HN) human near nest with young. Other (O) contexts,which included interactions with: (1) AcornWoodpeckers (Melanerpes formicivorus; (2) Cooper's Hawk (Accipiter cooperi); (3) Osprey (Pandionhaliaetus); and (4) Gray-breasted Jays (Aphelocoma ultramarina) near nest; as well as(5) sonogramsbased on recordings from "A FieldGuide to Bird Songs,"Houghton Mifflin Co.

RESULTS catability.This call has a frequencyrange of about3 kHz. Callsof the sevenspecies in Marler We examineour data in relation to the hy- (1959)are of short duration. Five speciescover pothesisstated earlier concerningthe acoustic at least 7 kHz, with two covering a narrower structureof avian mobbingcalls. Sohograms are frequencyrange of about3 kHz. If we use3 givenin Figure1. In somecases we includetwo kHz asthe minimum frequencyrange for a cri- calls for a single specieswhen the soundsare terionof a wide frequencyrange, 21 of 29 calls quite different. The sampleincludes 52 passer- (72.4%)in our categoriesI and II would be in- ine species:7 suboscines(Tyrannidae); and 45 cluded. oscinesfrom a varietyof differentfamilies. The The callsincluded in categoriesIII, IV and V largestsample (18 species)is of membersof the are even more diverse in acoustic structure than Emberizidae. those in I and II. Of 29 calls in these categories, Generalacoustic patterns.--The SOhograms (Fig. 8 (33.3%)covered a wide frequency range by 1), call measurements(Table 1), and categori- our criterion.These calls usually lacked abrupt zations(Table 2) indicate considerablediversity onsets. of acoustic structure. Twenty-nine species or AlthoughMarler (1955)predicted a wide fre- 55.8%of the total species(n = 52) have at least quencyrange and short duration, no predic- one mobbingcall in their repertoirethat fails tions were made about the absolute nature of into categoriesI and II (thin verticallines and highestor lowestfrequency. However, a wide thick vertical bars). However, if clicks used in frequencyrange would lead to the expectation attackdives are excluded,only 48.1%of species of a high maximumfrequency and a relatively arein thesecategories. Thus, about one-half the low minimum frequency.Considerable varia- speciesdo not give brief, wide-frequency-band tion occursin all aspectsof frequency(Table 1). callsin mobbing.Marler (1955)used the "chink" The maximumfrequencies of the Field Sparrow of the Chaffinch(Fringilla coelebs) as an exemplar andChipping Sparrow are above 10 kHz, while of a mobbingcall demonstratingfeatures of lo- thoseof mostother species are much lower. The 374 FICK•NAND POPP [Auk,Vol. 113

TAnLœ2. Categorizationof mobbingcalls. For species having more than onetype of call,shorter call des- ignated as 1 and longer as 2 (in parentheses).

I. Thin vertical line.--Eastern Bluebird (1), Tree Swallow (1), American Redstart, Yellow Warbler, Grace's Warbler,Sulphur-bellied Flycatcher (1), Red-faced Warbler, Black-throated Gray Warbler, Yellow-eyed Junco, SavannahSparrow, Indigo Bunting,Bank Swallow (1) II. Thickvertical bar.--Willow Flycatcher,Wood Thrush,Ruby-crowned Kinglet, CommonYellowthroat, BrownThrasher, Red-winged Blackbird, Hermit Thrush (1), Bewick's Wren, Ash-throated Flycatcher, Common Grackle, Gray Catbird (1), Bridled Titmouse,Northern Cardinal, House Wren, Mexican Chickadee,Field Sparrow,Chipping Sparrow III. Horizontalband.--Gray Catbird (2), Steller'sJay, American Crow, Warbling Vireo, Red-eyed Vireo, Hut- ton's Vireo, SolitaryVireo, Black-cappedChickadee, Olive Warbler, EasternBluebird (2) IV. Chevron.--SongSparrow, White-breasted Nuthatch, American Robin, Pygmy Nuthatch, Red-breasted Nuthatch,Dusky-capped Flycatcher V. Diversegroup.--All more complexthan other groups:Bank Swallow (2), EasternMeadowlark, Brown Creeper,Golden-crowned Kinglet, Tree Swallow (2), EasternKingbird, Barn Swallow, Painted Redstart, Great CrestedFlycatcher, Say's Phoebe, Hermit Thrush (2), Bushtit,Sulphur-bellied Flycatcher (2)

lowest frequencyalso varies. Some sounds are iants occurring in different contextsand pre- extremelybrief clicks,while othersare very long sumably encoding somewhat different mes- (e.g. somecorvids). However, duration and ab- sages(Brown 1985). solute-frequencyparameters may not be asim- Somespecies have both shortclicklike sounds portantfor the postulatedquality of locatability and longer calls.We were unable to determine as the other qualities(i.e. rapidity of onsetand the specificcontexts of both calls in all cases, frequency range). but a trend is clear. In all swallows studied, Call repetitions.--Callsof somespecies usually Sulphur-belliedFlycatchers, and EasternBlue- occurin a series.These repetitive calls are shown birds, the click was given during a dive on the as doublets in Figure 1, although they were predator.The othercalls of thesespecies are not partsof longerseries. For example,Bridled Tit- given in attack,but in approachesand harass- mice producedstrings of 20 or more note rep- ment. For exampleM.S.F. observedseveral Pur- etitions. Rapidly repeated calls tend to be of ple Martins (Prognesubis; no recordingsob- short duration, but somespecies such as wood- tained) approach a perched Sharp-shinned warblersthat haveshort chips usually give only Hawk (Accipiterstriatus). On initial approach, singlets. typicalswallow "gurgling" sounds were made, Specieswith more than one type of mobbingcall.- but a click occurred in a dive on the hawk. We undoubtedly did not samplethe total mob- Phylogenetictrends.--The sonogramsin Fig- bing-callrepertoires of many of thesespecies. ure 1 are arrangedaccording to families and are Somespecies have more than one type of mob- in AOU Check-list(1983) order. Here we produce bing call. Sometimesthe differencesbetween an overall appraisalof the similaritiesand dif- calls are relatively minor (see dd and ee for ferencesof certaingroups. In somecases, a suf- Hermit Thrush in Fig. 1). In other cases,how- ficient number of representativesfrom within ever, the differencesare more dramatic(e.g. hh a family is availablefor comparisons;in other and ii for Gray Catbird). American Crows have cases,the numberof speciessampled is toosmall. a complex graded seriesof mobbing calls,vat- The only suboscinesfor which we had re-

Fig. 1. Sonogramsof mobbingcalls: (a) Willow Flycatcher;(b) Say'sPhoebe; (c) Dusky-cappedFlycather; (d) Ash-throatedFlycatcher; (e) Great CrestedFlycatcher; (f and g) Sulphur-belliedFlycather; (h) Eastern Kingbird; (i and j) Tree Swallow; (k and 1) Bank Swallow; (m) Barn Swallow; (n) Steller'sJay; (o) American Crow; (p) Black-cappedChickadee; (q) MexicanChickadee; (r) Bridled Titmouse;(s) Bushtit;(t) Red-breasted Nuthatch;(u) White-breastedNuthatch; (v) Pygmy Nuthatch;(w) Brown Creeper;(x) Bewick'sWren; (y) House Wren; (z) Golden-crownedKinglet; (aa) Ruby-crownedKinglet; (bb and cc) EasternBluebird; (dd and ee) Hermit Thrush;(ff) Wood Thrush;(gg) AmericanRobin; (hh and ii) Gray Catbird;(jj) BrownThrasher; April1996] MobbingCalls 375

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k m n 0.0I 015 Time (s) (kk)Solitary Vireo; (It) Hutton'sVireo; (mm) Warbling Vireo; (nn) Red-eyed Vireo; (oo) Yellow Warbler; (pp) Black-throatedGray Warbler; (qq) Grace's Warbler; (rr) AmericanRedstart; (ss) Common Yellowthroat; (tt) Red-facedWarbler; (uu) PaintedRedstart; (vv) Olive Warbler;(ww) Northern Cardinal;(xx) Indigo Bunting; (yy)Chipping Sparrow; (zz) Field Sparrow; (A) SavanahSparrow; (B) Song Sparrow; (C) Yellow-eyed Junco; (D) Red-wingedBlackbird; (E) EasternMeadowlark; (F) CommonGrackle. A 0.0-to 0.5-sscale used for all calls,except for yy and zz, which have 0.00-to 0.25-sscale. 376 FICKENAND POPP [Auk,Vol. 113

7- 6

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3- 2- 1 0 ff hh ii jj kk i i o.o 0.5 Time (s) Fig. 1 (continued). cordingswere sevenspecies of flycatchers.Con- lied Flycatcherand Say'sPhoebe) are complex siderablediversity occurswithin membersof with notes of varied structures. The oscines show the group.Calls of somespecies are acoustically diversityas well, and similaritiesthat are prob- simple;others (especially those of Sulphur-bel- ably dueto commonancestry are apparent only April1996] MobbingCalls 377

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Fig. 1 (continued). in certain groups.The two corvid speciespro- four speciesof vireoswere analyzed(two from duce similar long-bandedcalls. The three nut- recordingsdescribed as "scold" calls). Despite hatchesbasically have chevron-shapedcalls, obviousspecies differences, calls of three spe- with speciesdifferences in the details. Calls of ciesare very muchalike, with the other species 378 FICKENAND POPP [Auk, Vol. 113

(SolitaryVireo) showingsome structural resem- 1957), ancestral patterns may be retained. In blance. All basicallyproduce a harsh snarl. special cases,selection for heterospecificcom- The family for which we have the largest munication in mixed-speciesflocks may lead to number of representativesis the Emberizidae. convergence. Formerly, this family was split into a number Problemsoccur in a large-scalecomparative of different families, some of which are now approach such as ours. In most cases,the total relegatedto subfamilystatus (AOU 1983).Most acousticrepertoire for these species is un- wood-warblers(Parulinae) and sparrows(Em- known, and somecalls may be nonspecific,oc- berizinae) have mobbingcalls that are very sim- curring in a wide variety of situations.For ex- ilar, typically a short chip coveringa wide fre- ample,the "tp" of the EasternPhoebe (Sayornis quencyrange. Among the wood-warblers,how- phoebe)occurs in many situations including ever, two notable exceptionsoccur: the Painted mobbing,and the "twh-t" is alsogiven in mob- Redstart, and the Olive Warbler. The Painted bing and many other contextscharacterized by Redstart probably has closestaffinities with indecisive behavior (Smith 1977). An interest- Central and South American congeners(no oth- ing questionwould be whether trends in mes- er representativesin our sample).The call of sage specificationsare different in flycatchers the Olive Warbler is very different from that of and oscines. Another unanswered question other members of the Parulinae. In fact, a re- concernswhether special calls, not given in oth- markable resemblance occurs between its call er contexts,occur in mobbing. For the wood- (Fig. lw) and that of the EasternBluebird (Fig. warblers, the same calls seem to occur in mild lbb). The resemblanceis unlikely to be con- alarm and, therefore, are not specificto mob- vergentbecause this kind of slurredwhistle was bing (M.S.F. pers. obs.).This trend may be true not found in any other speciessampled. The for other species.If many calls encodegeneral phylogeneticrelationship of the Olive Warbler messages,little interspecificcall convergence is uncertain,and its closestrelatives may be the would be expected, perhaps explaining our Muscicapidaeto which bluebirds also belong, findings for only a slight trend for calls with rather than the wood-warblers (AOU 1983). abrupt onsetsand wide frequencyrange. Generalsimilarities are sharedamong the other Someof the speciesstudied form mixed-spe- emberizids,except the buzzy call of the mead- ciesflocks. Possibly, some resemblances among owlark is markedly different from the others. distantly related speciesmay be related to se- A few groups are very heterogeneous.For lection for interspecificcommunication as sug- example, the speciesof the Muscicapidaeare gestedby Moynihan(1962). For example,Brown very different from each other. The mobbing Creepersand Golden-crownedKinglets are fre- callsof the three paridsare different despitethe quent flock associates(M.S.F. pets. obs.) and general acousticsimilarity and apparent ho- give calls during mobbing that are strikingly mologiesof many callswithin Parus(Hailman similar. Convergencemay result from selection and Ficken in press). for interspecificcommunication rather than for calls that are easily locatable. In fact, if flock DISCUSSION associatesare usuallyin closeproximity during most activities,locatability would probably not Acoustic structure.--Acoustic structures of be important. The unusual nature of the very- mobbingcalls of different passerinespecies are high-pitchedbuzzy callsof the Golden-crowned diverse, contrary to predictions of convergent Kinglet and Brown Creeper supportsthe idea evolution. In this relatively large comparative of call convergencerelated to mixed-species study,we found that fewer than 50%of species' flocking. Perhaps the striking differencesbe- callsconformed to Marler's prediction of sharp tween the mobbingcalls of the congenericRuby- onsetsand wide frequency ranges.While some crownedand Golden-crownedkinglets are due call convergencemay occur, probably related to selectionfor convergencewith different flock to easeof locatabilityas postulatedby Marlet associates. (1955),other selectionpressures shape mobbing Mobbing calls of the two speciesof chicka- calls in somespecies. Phylogenetic constraints dees for which we have information (Mexican operatein somegroups and, in the absenceof and Black-cappedchickadees) are quite differ- selection for specific distinctiveness (Marler ent, probablydue to a usageshift despitethe April 1996] MobbingCalls 379 overall similarity of the repertoiresof the two mobbingcalls of diversetaxa, another approach species.During closeapproaches to a potential to the problem of whether convergent evolu- predator,Black-capped Chickadees usually give tion is occurringin mobbingcalls is to compare Chick-a-deecalls consisting of stringsof D notes, them with the resultsof a comparativestudy of sometimesbut not always, preceded by intro- nestling calls.Contrary to expectations,calls of ductory notes (A, B, or C; see Hailman et al. nestlingswere not related to obviousfeatures, 1985). However, Mexican Chickadees give suchas open versuscavity nesting,or locata- stringsof C noteswhen actuallyharassing the bility (Popp and Ficken 1991).Rather, phylo- predator, the A and D notes and combinations genetic constraintswere of major importance, thereofbeing used more often during the initial as well as the relationship of nestling calls as approach(Ficken et al. 1994). Both chickadees precursorsof somefledgling and adult calls.A have A, B, C, and D note types that seem ho- comparisonof the sonogramsof nesting calls mologous(the B is very rare in MexicanChick- of 71 speciesof passerineswith our sample of adee),but usagediffers and the callsgiven dur- mobbing calls indicates greater diversity in ing the closestapproach to a predatorare dif- acousticstructure of nestlingcalls. Nestling calls ferent in the two congeners. included the following categories:repeated, Function.--Insome cases two typesof sounds tonal, complex, multiple banded, arched and are given, with a click occurringduring a dive, noisy. This classificationwas not appropriate while a totally different soundis made during for mobbing calls, many of which have sharp approachand mobbingwhile perched.The two onsetsand covera wide frequencyrange. These soundsprobably have very different functions findings suggestsome convergence due to se- and are directed at a different set of individuals. lectionfor locatabilityin mobbingcalls, but not The callsgiven on approachmay be directedat nestling calls. conspecifics,or even heterospecificsin casesof Generalconclusions.--Our findings lend only mixed flocks, and aid in recruitment of others. weak supportto the suggestion(Marler 1959, In contrast,the click is probably directed at the Marler and Hamilton 1966) that mobbing calls predatorrather than a conspecific,and mayhave have acousticdesign featuresfor easeof loca- an acousticdesign that startlesand possiblydis- tion that include abrupt onsetsand wide fre- tracts the predator. Both types of calls could quencybands. Support for Marler'sview comes induce predators to leave the vicinity (sup- from analysisof experimentalresults showing porting the move-onhypothesis of mobbing), that broad-bandsignals are more easily locat- one through recruitmentand subsequentha- able than narrow band signals (Klump and rassment, and the other due to an attack. As Shalter 1984). However, our resultsindicate that Leger and Carroll (1981) pointed out the selectionpressures other than locatabilitymay "smoothupswept vocalization" given in dives be operative. by the Phainopepla(Phainopepla nitens) is very Despitesome of the obvioussources of error unlike the harsh repetitive mobbing calls of in a broadstudy of mobbingcalls such as ours, many species,but attack callswould not be ex- the general validity of the comparativeap- pected to possessfeatures that aid in localiza- proach is substantiated.Examination of a di- tion. verseassemblage of both closelyand distantly In contrastto othersounds made during mob- related specieshelps provide insightsconcern- bing, clicksseem to be nonvocaland probably ing the selectivepressures shaping call struc- producedby mandiblesnapping. These snaps ture. Also needed are intensive repertoire anal- or clicks are made only by a few species.All ysesof singlespecies; such studies would allow thespecies noted clicking feed on flying insects, analysisof the fine-grainedaspects of message and suchmandible snapping is probablya com- categorizationsin antipredatorcalls. mon part of their prey-capture behavior that has become ritualized as attack behavior. Per- hapsother species, lacking the preadaptationof ACKNOWLEDGMENTS this feeding responsewere unable to evolve similar sounds. M.S.F. is grateful for the use of the facilities at the Comparisonwith nestlingcalls.--In addition to Southwestern Research Station of the American Mu- assessingsimilarities and differencesamong seum of Natural History, and thanks Elizabeth and 380 FICKENAND POPP [Auk, Vol. 113

Jack P. Hailman, John Ficken and Paula Nania for comparedwith written English.Semiotica 56:191- assistance in the field. 224. KLUMP, G. M., AND M.D. $HALTER. 1984. Acoustic

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