DUAL SINGING BY NEW GUINEA

JAREDM. DIAMONDAND Jomv W. TERBORGI-I A relativelyuncommon but intriguingform of song,the significance of which is imperfectlyunderstood, is synchronizedduetting betweena mated male and female. Most reportsof birds engagingin this comefrom the tropics,with examplesfrom Central America (Skutch, 1940), (Hayerschmidt, 1947), Africa (Moreau, 1941; Thorpe, 1963; Grimes,1966), Madagascar(van Someren,1947), and Malaya (Young, 1942). A few examplesfrom the temperatezone have alsocome to light among owls, Himalayan (Osmaston,1941) and Australian (Robinson, 1947: 14-17) ,and the CanadaGoose (Branta canadensis)(Col- lias and Jahn, 1959), CarolinaWren (Thryothorusludovicianus), and three speciesof quail (Stokesand Williams, 1968) of North America. A selectedlist of speciesmay be foundin Van Tyne and Berger(1959: 140). The presentpaper is an analysisof all instancesknown to us of duetting by New Guineabirds, based largely on our previouslyunpublished field observations.As it has been suggestedthat th'eincidence of duetting may correlatewith type of habitat,possibilities for visualcontact between sing- ers,or sexualdimorphism of plumage,these points are consideredin each speciesdiscussed. Duetsmay be dividedconveniently into threecategories. (1) The leader (generallythe male) singsone phrase,and at its conclusionthe second bird (generallythe female)sings another phrase. Often the followerbe- ginsso precisely upon cessation of the leader'sphrase that the resultmay easilypass for a conventionalsingle song, unless the observercan seethe singersor is doseenough to hear that differentnotes come from different places.For example,the MarbledWood-Quail (Odontophorus gujanensis) callsa rapidlyrepeated "corcorovado," in which"corcoro" is alwayssung by onemember of a pair and "vado"by the other (Chapman,1929: 275; Armstrong,1963). The term"antiphonal singing" has been used for this type of duetting,in whichmale and femalesing alternately rather than simultaneously.In this categorybelong the Central Americanwrens de- scribedby Skutch(1940) andAmaurornis olivaceus, Pitohui kirhocephalus, and Philemonnovaeguineae (see below) of New Guinea. (2) Membersof a pair singdifferent phrases simultaneously. The synchronizationbetween the twophrases may nevertheless be very exact;the femalemay beginonly a fraction of a secondafter the male. In New Guinea the duets of Mega- podiusfreycinet, Campochaera sloetii, and Coracinamontana are of this type. (3) The maleand femalesing virtually identical phrases in unison. Suchunison duets are practicedby Centropustoulou (Cuculidae)of Mad- agascar(van Someren,1947), Aspathagularis (Momotidae) of Central 62 The Auk, t]5: 62-82. January, 1968 Jan.1968 ] DIAlVI:ONDAI•DT•RBORCa, DualSinging 63

America (Skutch, 1945: 495), and other specieslisted by Power (1966) but not, to our knowledge,by any New Guinea species. The physiologicalproblems posed by synchronizedduetting are well il- lustratedby Thorpe's (1963) studieson the shrikeLaniarius erythrogaster of Africa. In this speciesthe duettersare up to 20. yards apart in dense brush and out of visual contact,but their calls are neverthelessdelivered almost simultaneously: a yoick-like sound from the leader and a "tearing hiss-like" soundfrom the follower. Sound recordingshowed that the sec- ond bird may begin as little as 72 millisecondsafter the first, the exact duration of the interval being characteristicof a given pair. This is several times shorterthan the fastestacoustical response time for man. For any given pair the variation of this interval is very slight, the standard devia- tion being 5 to 12 milliseconds,or severaltimes less than the comparable figure for man (20-55 milliseconds). Similar acousticalresponse times havebeen determined from duetsfor the relatedspecies Laniarius barbarus (Grimes,1965) and for the Orange-chinnedParakeet (Brotogeris jugularis) (Power, 1966). Thus, synchronizedduetting demandsan ability both to react rapidly to an acousticalstimulus and to time an interval accurately. Duetting betweena paired male and female resemblesacoustically, but hasa very differentsignificance from, anotherclass of dual singing,namely the severalkinds of countersingingbetween males. When the pausesbe- tween successivephrases of a territorial songare longer than the phrases of the songitself, two maleson adjacentterritories may tend to singalter- nately, thus giving each the opportunity to hear the other's song during the pause. Such countersingingoccurs frequently in the European (Troglodytestroglodytes) (Armstrong, 1944) and, in our experience,in many speciesof South American antbirds (Formicariidae). Males that meet at the bordersof their territories may both sing loudly and simulta- neouslyto threaten each other. In somespecies, when one male beginsto singother males in territorieswithin hearingdistance begin to singat more or lessthe sametime. A bizarre instanceof a male-male dual songthat is identical in form to the third (unison) type of male-femaleduet is pro- vided by the Blue-backedManakin (Chiroxiphiapareola) of South'Amer- ica; displayin front of a femaleis routinely carriedout by two maleswhich perch side-by-side,almost touching each other, and call in unison(Snow, 1956: 90). Apart from this specialcase, male-male countersongcan gen- erally be distinguishedfrom male-femaleduetting by the fact that in the former,coordination is generallyimprecise and the two singershave iden- tical phrases,whereas male-female duetting is usuallyprecisely coordinated and the partners often sing very different phrases. However, somemale- femaleduets involve no moreprecise coordination than doessome counter- singing,and in someduets the male and femalegive identicalphrases. For 64 D•A•40•DA>/D TERBORGH, DualSinging [ Vol.Auk 85 these reasonsthe distinction between duetting and countersongcannot al- ways be made with certainty in monomorphicspecies if one has not col- lected both singers. We have therefore discussed,in addition to all pre- sumedinstances of duetting in New Guinea, five casesthat we assumeat present to representcountersong, but of which one or two may possibly turn out to be male-female duets.

METHODS AND LOCALITIES

The observationsdescribed here were made during three separate expeditionsto New Guinea. One of the purposesof these expeditionswas to study altitudinal distribution of bird speciesby taking censusesat different elevations above sea level. These cen- suseswere based to a large extent on song, and it thus became necessaryto track down and observe repeatedly the songsof as many speciesas possible. Field work was car- ried out by both of us from June to Septemberof 1964, and by one of us (J. M.D.) from June to September of 1965 and 1966. Observationswere made at the following localities, all in the eastern (Australian-mandated) half of the island: and vicinity on the southeast coast; the area from to Bulolo on the Huon Gulf, and the lower Markham Valley; the eastern highlands between , Okapa, Karimui, and Goroka; on the north coast; the North Coastal Range (Prince Alexander, Torricelli, and Bewani Mountains); and the northern edge of the Basin. All major types of habitat and a range of elevations from sea level to 12,300 feet were covered. In addition we have included for comparative purposes some ob- servations on Australian birds made in Queensland and New South Wales, and on South American birds made in the Department of Ayacucho, . The identification of all New Guinea speciesfor which we observed dual singing is supported by speci- mens depositedin the American Museum of Natural History and the Harvard Mu- seum of Comparative Zoology. Tape recording was not undertaken, and the descrip- tions are based on field notes.

RESULTS During thesestudies we identified the songsof 217 New Guinea species (146 passetines,71 nonpasserines).Of this total 3 were found to practice regularly somekind of dual singingthat might have been duetting, 4 oc- casionally,1 rarely, and 209 not at all (this doesnot includesome obvious casesof countersongthat could not possiblyhave been confusedwith duet- ting). In addition•E. T. Gilllard onceobserved a duet in Philemonnova½• guineae,a commonspecies that we encounteredmany times but never foundengaging in dual singing,and in a megapodewhose vocalizations we never heard (Gilllard and LeCroy, 1966). Details of these instancesof dual singingfollow under the individual speciesconcerned. No other pos- sibleexamples of duettingin New Guineahave cometo our attention. Megapodius/reycinet. Common Scrub-hem--This is a speciesof the floor, occurringmost commonlyin the lowlandsbut rangingocca- sionallyup to 6,000 feet. It is well known for constructinghuge mounds of rotting vegetationin which'the eggsare laid and incubatedby the heat of decay. The male and femaleare identicalin appearance.While we have 1968Jan. ] Dm•o•)^•13 TERBORG•, DualSinging 65 had no first-handexperience of its vocalizations,E. T. Gilliard (Gilliard and LeCroy, 1966: 254) observedit to engagein dual singingin the area of the middle Sepik River and describedthe performanceas follows: Male (?), "kok, nyacal,nailleue" (ascending to a high screech).Female (?), a stutter delivered in duet with the peal of the male and beginningjust after the male began and continuing throughout the call "nu-nu-nu-nu-nu-." These duetted calls were heard at all hours of the day and night, but were most prevalent on moonlit nights. Often at night the birds called within 200 feet of my tent (which was just outsidethe forest and about 150 feet from an occupied mound) and from sound orientation I determined that the birds kept closely together in pairs. In fact, it was only rarely that the call of the female seemed to emanate from a distance of more than 10-20 feet from that of the male. Most times, the sound seemedto come from birds standing almost side by side. From this descriptionit seemslikely that the male and female are in visualcontact during many of the diurnalduets, though probably not for the nocturnal ones. Amaurornis olivaceus. Rufous-tailed Moorhen.--We encountered this in tall, dense grass borderingnative gardens, roads, and fields; oth'erobservers have also found it in swamps(Mayr, 1941). It sleepsin low treesbut otherwiseremains on the ground. The male and female are identical in appearance. Most vocalizations of Amaurornis olivaceus are duets between two in- dividualsconcealed in the grass. The first bird givesa catlike wail that remainson nearly the samepitch, and as this wail starts to drop in vol- ume, the secondbird beginsa similar wail at about the samepitch. Each bird then alternatelywails from a few to a dozentimes, each wail lasting betweenone-half second and a secondbut overlappingboth the preceding and followingwails of the othersinger, so that the effectis of a continuous sound. Sometimesthe seriesappears to be on the verge of dying away when the duet is suddenlypursued with renewedvigor for severalmore alternations.The seriesmay be introducedby hen-likeclucks which acceler- ate and crescendobefore breaking into the wails. Becauseof the dense grassin which the performancetakes place, it is rarely possibleto watch the singers,but they appearto be from 4 to 10 feet apart and to move about as they sing. They are surely out of visual contact throughoutthe performance,which may be heard at any time of day and infrequentlyat night as well. The duets given by birds from the Karimui Basin (A. o. ruficrissum)and from the north coast (A. o. molucannus)are indistin- guishableto our ears. Althoughthe duet is the commonestkind of vocalizationin this species, solitary individualsmay also give a seriesof clucks. The wails, however, are never heard outside of a duet. On one occasion we heard a call bk-bk-bkfrom the grasswhich we assumedto be a native chicken,until we 66 D•^•ro•D ^•) TERBORGn,Dual Singing [ Vol.Auk 85 spotted an adult and a juvenile Amaurornis. The adult was probing for food and cluckedwhen it found a morsel,whereupon the juvenile would come up and receive it. Dacelo gaudichaud. Gaudichaud'sKingfisher.--This small kookaburra is noisy and commonin the middle story of second-growthand sometimes in the forest from the lowlandsup to 2,000 or 3,000 feet. The male and female differ only in the color of the tail. The commonestvocalizations are a short bark repeated at one-second intervalson the samepitch, and a shorttrilled bark repeatedat one-second intervalson successivelylower pitches. The barking of one individual fre- quently stimulatesanother at somedistance (generally well out of visual range) to begin barking at the sametime. On one occasionwe observed two individualsbarking simultaneouslyin an isolatedtree standingin a garden. There is no synchronizationin suchcases. One singermay cease well before the other, or the two may bark at different rates, so that the two seriesbecome progressively out of phase. These are generally con- sidered typical features of male-male countersong. The Blue-winged Kookaburra(Dacelo leachii) of the southNew Guineasavannah has a very similar trilled bark, and Rand (Mayr and Rand, 1937) describedseveral individuals calling simultaneously,much as with Dacelo gaudichaud. In the remainingmember of the genusas well, the Laughing Kookaburra (Dacelo gigas) of Australia,several individuals perched next to eachother on the samelimb may call simultaneously. Campochaerasloetii. Golden Cuckoo-shrike.--The isolated, brilliantly colored,and monotypicgenus Campochaera is endemicto mainland New Guinea,where it is distributederratically in the lowlandsand hills up to about 3,600 feet and is nowherecommon. Its habitat is the crownsof tall forest trees. The usual vocalizationis a duet carried out by groupsof two to four individuals. The leader delivers a rapid series of half-a-dozen musical spitted notes,all on the samepitch. Simultaneouslywith the last several notes,a secondindividual gives a whistledslur which may be a simpleup- slur, a downslur,or up to two or three slurs. Somecommon patterns are depictedin Figure 1. The duet is given repeatedlyas the birds chaseeach other in flight just above the crown of a tree, or else as they perch in the treetops. The notesare high and melodious,and the effect is most attrac- tive. After perhapsone-half minute in one tree the group flies off to an-

Figure 1. Patterns of duetting by Campochaerasloetii. The repeated notes (lower line) are given by one bird, while the slurs (upper line) are given by a different bird. 1968Jail. ] DIAlVrONDA•I•T•RBORCn, DualSinging 67

other tree 100 to 500 feet away, duettingas they go or elseuttering rapid repeatednotes on one pitch similar to the leader'spart in the duet. The performanceis then repeatedin the next tree. A groupcan readilybe fol- lowedby soundas they progressdown the ridge of a mountainor gradually describea wide circle throughthe forest.The singersremain in closevisual contact throughoutthe whole performance. The male and female differ slightly in plumage,the throat being black in the male but gray in the female. Singinggroups contain individuals in botk plumages,but we have not beenable to determinewhether the leader is a male and the followera female,because the singersare in the forest crownand often in rapid motion. The performancegenerally takes place in the morning(never at night or as part of the dawn chorus)at any time from an hour after sunrise until noon. The birds then become silent and settledown to feeding. As describedbelow, two other New Guineacampe- phagids(Coracina schisticepsand Coracinamontana) conductstrikingly similar communaldisplays, and that of C. mo,ntanaalso involvesduetting. A purposeof the displaysmay be to assertand maintain a territory held communallyby the group (seep. 69). Coracina montana. Mountain Cuckoo-shrike.--Thisis a conspicuous, fairly commonspecies of mid-montaneforest from about 3,500 to 8,000 feet, usually found in the upper story but occasionallydescending to the middle story to feed. In the male the plumageis blue-gray above, while the wings,tail, and underpartsfrom chin to tail are black. The femaleand immaturemale differ in that while the lores,chin, wings, and tail are black, the underpartsare blue-grayas the upperparts. The song of Coracina montana is a loud duet that carries considerable distancesand formspart of a stereotypeddisplay similar to that of Campo- chaerasloetii. Th'e display is carried out by a group of usually three or four, sometimesonly two, birds, and the groupseems always to includeat least one female and one adult male. The groupalights in a conspicuous positionin the crown of a tall tree, either a dead one or one with a com- mandingview. The memberstypically perch within a few feet of each other, often immediatelynext to eachother on the samebranch, and al- ways in unhinderedvisual contact. In a typical pattern (Figure 2,A) the male givesan upslurredwhistle succeededimmediately by a downslurred whistle, followedat short intervalsby severalmore such pairs, then at longerintervals by singleupslurs. Immediately after the beginningof each slurthe femalecommences a series of eitherthree or four unmusical,harsh noteson the samepitch (kek or chuck), which are completedwithin the time spanof the male'sslur. Commonvariations are that the performance may begin with singleslurs rather than pairs, and that the concluding seriesmay be downslursrather than upslursor elsepairs of slurs. In the 68 D•A•O>•])AND TE]mO]•C,•{, Dual Singing [ Auk I_Vol. 85

(d) -,-/ k,._ ...f' k,._ x,..k... k,._k,._ x,..

Figure 2. Patterns of duetting by Coraclna montana. The slurs (upper line) are given by the male, while the repeated notes (lower line) are given by the female. Pat- terns (a) and (b) are the usual ones in the eastern highlands, while (c), (d), and (e) are characteristic of the North Coastal Ranges.

North CoastalRange the initial slursmay be virtually disyllabic,whereas in the easternhighlands the units are alwayssimple slurs. Figure 2 gives someof the commonpatterns. Among New Guinea dual singersthe song of this speciesinvolves the finest synchronizationand would offer interest- ing material for an analysisof time relations,similar to that carriedout by Thorpe (1963) for Laniariuserythrogaster. It shouldbe stressedthat al- thoughwe have heard the duet on severalhundred different occasionsin sevendifferent areas, we havenot encountereda singleinstance of the male singingalone. The singersare always perchedwhen the Coracinamontana duet is given,whereas in Campochaerathe duettinggroup. may either be perched or elsewhirling above th'ecrown. The duet may be followedby someun- coordinatedgroups of keks similar to the femaleelement of the duet but softer. Groupsusually remain only a minuteor two in a tree beforeflying off together to another tree 200 to 1,000 feet away. In flight the group uttersloud groups of unmusicalkeks but no slurs. When they alight in the next tree, the duet is repeated,and so on in successivetrees, with a burst of keksin flight betweenthe trees. The progressis easily followedby ear, and it is our impressionthat a given groupuses the sameduetting trees and followsa prescribedroute each day. Usually the route will take a groupalong a mountainridge for a verticaldistance of up to 1,000 feet, then into the adjacentvalley and perhapsacross to the next ridge if it is sufficientlyclose. On one occasionwe observeda group followinga circu- lar route a few hundred feet in radius and returning to the same tree within one-half hour. When not engagedin this performance,the birds are in- 1968Jan. ] DIAMONDA•IDTERBORGIt, DualSinging 69 conspicuousand remainsilent while feeding. The duettingis confinedto daylighthours beginning well after sunrise,most commonly in the morn- ing but occasionallyin the afternoon. We have never heard it either at night or as part of the dawn chorus. As noted above, the displayinggroup usually consistsof three or four individuals,and all appear to contributeto the keks in flight, although only one male and one femalecarry out the duet. Regardingthe composi- tion of the groups,we succeededonce in collectinga completegroup of four, and foundit to consistof two adult males,one adult female,and one immaturemale moltingfrom the female-liketo the male plumage. On five occasionsour native collectorsbrought in three birds that had been to- geth•er,but may havehad a fourth partner that escaped,with the following composition:one adult male and two adult females;one adult male, one adult female,and one unsexedimmature bird in female-likeplumage; one adult male, one adult female,and one immaturemale in female-likeplu- mage; two adult malesand one adult female; and three adult males. On eight occasionsthe native collectorsobtained two birds together,possibly from larger groups,and sevenof thesepairs consistedof one adult male and one adult female,while the eighthwas an adult male and an unsexed bird in female-likeplumage. In one pair both the male and female had enlargedgonads, but this was generallynot the case. All our specimens were obtainedbetween June and September,and we suspectfrom the re- portsof nativesthat this speciesgenerally breeds from Novemberto Feb- ruary where we collectedin New Guinea. It is possiblethat in Coracinamontana and in Campochaerasloetii the duettingbehavior not only strengthensbonds between the participantsbut also representsa communalterritorial display. Each group may jointly hold a feedingterritory, to which it maintainsits claimsby circling the territoryonce or twicea day while displaying.While we havenot observed definiteterritorial confrontations between adjacent groups, this interpreta- tion is suggestedby the selectionof conspicuousperches and by the useof establishedand more or lesscircular routes. Functionsof duetting in ter- ritorial behaviorhave previouslybeen indicated for Laniarius barbarus (Grimes,1966) and Brotogerisjugularis (Power, 1966). Th'efollowing ac- countof the displayof Coracinaschisticeps suggests an additionalsignifi- canceof duettingby Coracinamontana. Coracinaschisticeps. Grey-headed cuckoo-shrike.--This species does not engagein dual singingand is discussedhere for comparisonwith Coracina montana. The male is largely blue-gray,with the lores and most of the wingsand tail blackish,thus resemblingfemale or immaturemale C. mon- tana in pattern. The female(and immaturemale) is largelybrown, with the headand muchof the tail gray and the wingsblack. In addition,this 70 DmvtO•l)A•I) TEI•BOImIX,Dual Singing [ Auk 1_Vol. 85 speciesis smaller than C. montana (wing, 108-120 mm, vs. 123-135 mm for C. montanain easternNew Guinea). It occursfrom sea-levelto about 3,000-4,000 feet in the upper story (sometimesthe middle story) of the forest,and apparentlyreplaces C. montanaat lower altitudes. In all, 11 superficially rather similar membersof the Coracina occur in New Guinea (5, includingmontana and schisticeps,are sometimesseparated un- der the genericname Edolisoma). From our own familiarity with the be- havior of nine of thesespecies, and from publishedaccounts of the other two, it appearsthat schisticepsand montanaare the only onesthat prac- tice the distinctive group tree-top displays describedin this paper. The vocalizationsof the other speciesare also rather different, rangingfrom a crescendoingseries of buzzes,progressively increasing in volume, in C. morio, and a parrot-likewhee-chew in C. papuensis,to harsh chirpsand screamsin C. caeruleogrisea. The displayand songof C. schisticepsare similar to thoseof C. mon- tana exceptthat the female doesnot reply to the slurredwhistles of the maleand thusthere is no duet. In the display,groups of two to four birds, consistingof both malesand females,occupy conspicuous perches close to- gether in the crown of a tree and remain there a few minutesbefore flying on to the next tree a few hundred feet away. When perched,the male de- livers a seriesof slurs,of which somerepresentative patterns are given in Figure 3. The similarity to the patternsof Coracinamontana (Figure 2) is obvious,and the quality is also similar, varying from a musicalwhistle to a whistle more nasal than that of C. montana. Some male vocalizations of C. schisticepsare practicallyindistinguishable from thoseof C. montana exceptfor the absenceof a femalereply (compareFigures 2,B and 3,C).

(o) .,,/-"x. .,,/-"x. x,_

(c) ../x.._ x._

(d) J-J- • x..._ J-J-•

(f) j-_j- j-_j- Figure 3. Song patterns of Coracinaschisticeps. The whole song is given by the male, and no duet is involved. Patterns (a) and (b) are from the North Coastal Ranges;(d), (e), and (f) from the easternhighlands; and (c) from both the North Coastal Rangesand the easternhighlands. 1968Jan. ] DIAIV•OIq'DAm)T•RSORC•, Dual Singing 71

At the end of the seriesof male slurs,several birds join in with a rapid, low, harsh,and lessloud chuck-chuck-chuck.Finally the groupflies off to the next tree, with' severalbirds simultaneouslyand noisily calling chuck- chuck-chuckin flight. We collectedone group of four in to•o and foundit to consistof one adult male, two adult females,and one immature male moltingfrom the female-liketo male plumage. On four occasionswe ob- tained threebirds that may or may not have had a missingpartner: two malesand a femaletwice, one male and two females,and three males. The gonadswere generally not enlarged. Thus the display is in all respects similar to that of C. montanaexcept for the slurredwhistles of the male being unaccompanied. Among New Guinea birds a number of congenericspecies inhabit alti- tudinal bandsnearly or completelyexclusive of eachoth.er (e.g., Cratero- scelismurina and C. robusta[Timaliinae], Peltopsblainvillii and P. mon- tanus [Muscicapinae],Epimachus fastosus and E. meyeri [Paradisaeidae], A mblyornismacgregoriae and A. subalaris[Ptilonorhynchidae], and Ptilo- prora perstriataand P. guisei[Meliphagidae] ). Presumablythe members of suchpairs have rather similar ecologicalrequirements, so that they can- not sharethe samearea any morethan can the membersof a superspecies group. Whereas the membersof a superspeciesare separatedby a geo- graphicalboundary, the membersof an altitudinalpair meet at a contour line on many or all th'emountain ranges of New Guinea. Coracinaschisti- cepsand C. montanaprovide an additionalexample of two specieswith mutually exclusivealtitudinal preferences,such that the presenceof each speciesseems to offer the principalbarrier to the other'saltitudinal range. On different mountainsthe transitiontakes place at variousaltitudes be- tween 2,700 and 4,200 feet. For exampleon the northwestridge of Mt. Karimui in the easternhighlands, a flat volcanicplain on the north sideof the mountainpermits many tropical speciesto spreadupwards to higher altitudesthan usual elsewherein New Guinea. Consequentlythe highest altitude at which C. schisticepswas observedwas 4,030 feet, and the lowest altitude for C. montana was 4,115 feet. In the North Coastal Range the transitionwas studiedon Mt. Somoro,Mt. Nibo, Mt. Menawa, and Mt. Turu. On Mt. SomoroC. montanaextended regularly down to 2,990 feet, and C. schisticepswas encounteredonly below this altitude exceptfor a pair collectedat 3,100 feet. On Mt. Nibo the transition occurredat some altitude between2,700 and 3,000 feet. On Mt. Menawa, C. montana was found downto 2,650 feet, and C. schisticepsoccurred only below this alti- tude exceptfor onespecimen at 2,775 feet. On the fourth and lowestmoun- tain, Mt. Turu, only 3,750 feet high, C. montanawas missingentirely and C. schisticeps,apparently freed from the competitionwith montana,oc- curredall the way up to the summit,800 to 1,000 feet higher than on the 72 DIAMONDANDTERBOR6I{, DualSinging [ Vol.Auk 85 other mountainsof the North Coastal Range. Thus, the altitudinal ranges of thesetwo speciesare complimentarywith only marginal overlap, i.e., the lowest occurringgroup of montana and the highest occurringgroup of schisticepsappear to invade eachother's areas by little more than 100 feet. Considerationof presentdistributions of other New Guinea altitudinal pairs suggeststhat thesepairs originatewhen one population of a polytypic speciesor superspeciesgroup reinvadesthe range of a related population that has developeda slightly different altitudinal preferenceduring geo- graphical isolation. Many of these altitudinal pairs are rather similar morphologicallybut havevery differentsongs, suggesting an importantrole of voice in the isolatingmechanisms (e.g., Peltops blainvillii and P. mon- tanus,Epimachus fastosus and E. meyeri). Thus, it can be suggestedspec- ulatively that the fine synchronizationbetween male and female in the duet of C. montanamay functionin additionto the plumagedifferences as one of the isolatingmechanisms between it and C. schisticeps.The duet could readily have evolvedfrom a schisticeps-likesong, as the female ele- ment of the duet is already presentin the songof C. schisticepsas the group notes delivered after the male slurs and in flight. In addition the exampleof duettingby Campochaeraas well may suggestsome predisposi- tion towardsduetting in New Guineacampephagids, unless these two simi- lar duetsevolved quite independently. Pitohui kirhocephalus.Greater Wood-shrike.--This commonspecies of lowlandforest ranges up to about 3,500 feet and is found in the lower and middle story of the forest and in second-growth,particularly among dense tanglesof vines and other vegetation. No other New Guinea bird shows suchgeographical variation in plumage,its subspecificcolor variation en- compassingthe whole range of patterns found among the other five spe- ciesof the genus. The male and female are identical in the raceswe have studied,though this is not true of someother races. Because of its loud voice and preference for the interior of thickets, Pitohui kirhocephalusis much more often heard than seen. The vocaliza- tions are rather varied, commonelements being upslurred whistles, con- nectedpairs of noteson differentpitches, a dry seriesof rattled notespro- ceedingupscale, and clear, whistled,staccato notes. The quality varies from mellowto harsh. A typical songwill be introducedby a whistledpair of notes,followed after a short pauseby two more pairs in quick succes- sion, and then by a disorganizedjumble of notes and slurs in irregular rhythm. The first singermay be answeredby oneor two otherbirds within a distanceof 20 feet, deliveringthe samekinds of chattersat the sometime but without any detectableorganization or synchronization.The singers are certainly out of visual contact. Finally one may hear a loud whirring of wings and observeseveral birds fly out of the thicket and chaseeach 1968Jan. ] D•^zvrom•^•) TERBORGH, DualSinging 73

(b) _•/ ./-_

Figure 4. ?atterns of duetting b7 ?itohui kixhocephalus. In pattern (a) one gives the upslur, and a different bird gives the single note tup which follows it after a variable de]a7 or sometimesis omitted. In pattern (b), which is given separatedb7 pauses,one bird gives the upsJutand another the secondnote, but it is uncertain which bkd gives the last two notes or whether they are given b7

other into a nearby thicket. The fact that all the singersuse the same kind of "song,"and that there is no fine synchronizationbut simply sev- eral chattersgoing on simultaneously,suggests that this commonperform- ance is an exampleof countersingingbetween rival males, not of duetting betweena mated male and female. We have heard this kind of singing both in the easternhigh]ands (P. k. brunneiceps)and in the North Coastal Range ( P. k. brunneicaudus) . In addition an entirely different kind of dual singing,which we have heard only on the north coast,is surely a male-female duet. The leader deliversa singleupslurred whistle that is generallyanswered at an interval of one to three secondsby a mellow staccatotup from another bird 10 to 30 feet away. The upslur and tup are then repeatedseveral seconds later with the singersapparently not having changedposition (Figure 4, A). The interval betweenthe slur and the tup varies, however,on successive repetitions. Sometimesthe tup is omitted entirely in responseto one slur but is then given after the next. (Another instanceof an "optional" female reply is providedby the duet of the AustralianWhip-bird (Psophodes olivaceus),in which a long whistle of the male, that increasesexplosively in volume,may or may not be followedby chew-chewfrom the female.) In a more complicatedvariant of three elements,the slur and tup are fol- lowedat an approximatelyequal interval by a pair of notes (Figure 4, B). We were unable to determinewhich of the singersgives this pair of notes or whetherit is givenby a third individual,in analogyto the "trio-singing" of Laniariusaethiopicus and Cossyphaheuglini (Thorpe and North, 1965). The three-elementperformance is most often heard as part of the dawn chorus,when it may be repeateda dozen times or more. On one occasion we heard two groupscarrying on the three-elementduets simultaneously from adjacent thickets; the two duets were not synchronizedwith each 74 DIAMONDAm>TER•ORO•, DualSinging [ Vol.Auk 85

Figure 5. Dual singing by Melidectes rufocrissalis,heard on only one occasion. One bird (upper line) gave a seriesof caws, and a secondbird (lower line) gave a similar seriessuch that each note appeared in the pausesbetween the notes of the first bird. other. The two-elementduet (slur plus tup) may be heard at any time in th'emorning or afternoon. The speciesdoes not sing at night. Thus, Pitohui kirhocephalusapparently engages in both countersinging and, in at least one race, duetting. The densethickets the speciesinhabits ensurethat the singersare not ordinarilyin visualcontact. The duet also differs from that of Campochaerasloctii or Coracinamontana in that the time relationsbetween the first and secondsinger are variable and in that most of this species'vocalizations do not involve duetting. Melidectesrufocrissalis. Yellow-browed Honey-eater.--Th'is honey-eater is a commonand noisy inhabitant of the forest interior and forest edge from 4,500 to 8,500 feet, and is foundin the middleand lowerstory. It has attracted interest becauseit hybridizesreadily with its congenerM. bel- fordi in areas of disturbedecology (Mayr and Gilliard, 1952; Gilllard, 1959; Diamond,1967). The maleand femaleare identicalin appearance. A commoncall is a seriesof nasal and piercing, or else hoarse and rau- cous,caws, either on the samepitch or slightly ascendingor descending. Another call is a seriesof bugled notes which are alternately on two dif- ferent pitches. We observeddual singingin this specieson only one oc- casion,by two birds perchedon the samelimb two feet apart and facing eachother. Each individualgave a descendingseries of cawsat the usual rate (two or threeper second),but the noteof onebird occupiedthe pause betweenthe precedingand followingnote of the other bird (see Figure 5). The performancewas not repeated. While a distinct possibilityexists that this wasa duet betweenmale and female,competitive singing between two rival males appearsto us the more probable interpretation. The fact that th'enotes did not get out of phaseon this one occasionmay have been due to chanceand neednot imply fine synchronization. Oreornis sub•renatus. Sub-bridledHoney-eater.--A speciesof erratic distribution,this honey-eateris locally commonfrom 4,500 to 8,500 feet in the forestmiddle story but is absentin many apparentlysuitable areas. The male and femaleare identicalin appearance. The songis a rapid, loud, bubbling,and cheerfulseries of noteswhich first risesand then falls in pitch, progressivelydecelerating and sometimes D•^•rot•i) ^t•i) TE•BORO•X,Dual Singing 75 1968

(b) •---' ......

Figure 6. Two typical song patterns of Oreornis subfrena•us. Dual singing, which is the exception rather than the rule, consistsof two birds giving the same pattern at the sametime, the secondbird starting a few notes behind the first bird. concludingin threeslurs on the samepitch, after which the pattern is im- mediatelyrepeated one or two times (Figure 6). About one-quarterof the renditionsinvolve dual singing. A secondbird perchedin the sametree as the first, 3 to 10 feet from it, and generallyin sightof it, deliversan iden- tical songbeginning a shorttime (i.e., severalnotes) after the leaderstarts. Both the singleand dual songsare heard frequently in fruit trees where several individuals have come to feed. The fact that both individuals de- liver the samesong, which is moreoften heardas a solo,suggests that com- petitive singingbetween males is involved,though male-femaleduetting remainsa possibility. Xanthotis chrysotis. Brown Xanthotis.--This is one of the most abun- dant birds of the New Guinea lowlands,becoming progressively less com- monwith increasingaltitude, until it dropsout around5,000 feet. It may be seenat any heightin a tree, thoughmore often in the lowerand middle storiesthan in the treetops. The male and female are identical in appear- ance. In the easternhighlands (X. c. rubiensis),the vicinity of Port Moresby (X. c. guilanettii),and the vicinity of Lae (X. c. madaraszi)the songis basedon a pattern lastingabout two secondsand consistingof three notes separatedby pauses.The pattern is immediatelyrepeated without change a half-dozentimes in succession.On the north coast (X. c. philemon) the songdiffers by being deliveredat least twice as fast. Dual singingoccurs occasionally.After one singerhas given the three-notepattern once,an- other singer may join him for the seconddelivery of the pattern on the samepitch and in approximatesynchrony, and the two continueon for severaldeliveries, though one may stop beforeth'e other. Sometimesthree birds may sing at the same time. The singersare never closetogether, generally50 to 200 feet apart, and of courseout of visualcontact. It seems a reasonableassumption that the singersare males maintaining adjacent territories. Philemonnovaeguineae. New GuineaFriar-bird.--This largehoney-eater is commonin the lowlands,ranging occasionally up to 2,000 or 3,000 feet. It is found in the middle and upper story of trees, much more often in 76 D•A•O•D A•D TEaBOaGH,Dual Singing [ Auk / Vol. 85 second-growthand aroundvillages than in the forest. The sexesare iden- tical. The songis basedon a repeatedpattern of three or four loud nasal slurs, and theepattern is repeateda few to a dozentimes in immediatesuccession. We never noted dual singingin this species,but Gilliard (Gilllard and Le- Croy, 1966: 273) observedthat on at least one occasionthe pattern was divided between two birds:

The leatherhead (Philemon novaeguineae) has a repertory of bugled notes that are so clownish they almost defy description. These are delivered very often from semi- concealedperches high up and commandingan extensiveview. "Stick-ta-ba-co," rap- idly repeated, is a common call; "ka-kek-ka-ke-kek-ku" is another loud call, as is "yes-joe-joe-jup-kip-jup." Behind Bogadjim I confirmed what I had long suspected, that this speciessings antiphonally; as I watched, one called "ki-kor-rik," to which another, directly overhead, added "queww." They did this a number of times. I would have taken the call for that of one bird had I not been standing where I was. After learningof Gilliard's observation,we paid particular attention to this speciesand had the opportunity to observeit at Port Moresby, Lae, and Wewak, and at many localities on the southernslopes of the North Coastal Range. In every instancewhen we tracked down the origin of a song,the whole of the songcame from a singleindividual. Thus duetting in Philemon novaeguineaemust be regardedas a relatively uncommonor perhapsa geographicallylocalized practice. On the basis of Gilliard's de- scriptionand of the kinds of treeswhich Philemonfrequents, it is highly probable that the pair observedby Gilliard were in visual contact. Basically, Gilliard's observationwas that a given pattern which is nor- mally delivered as a solo by one bird may also be divided between two singersas a duet. At least two similar casesexist among Australian birds. First, the chew-chewthat followsthe male'sexplosive whistle in Psophodes olivaceus(see earlier) may be given eith'erby the male or by the female. Second,the songpee-o-wit--te-he of the Magpie-lark(Grallina cyanoleuca) may be deliveredas a solo,or elsepee-o-wit may come from the male and te-he from the female,or vice versa (Robinson,1947: 14-17). Thorpe and North (1965; 1966) cited additionalexamples.

DISCUSSION Most of the 10 New Guinea speciesthat have come to our attention as dual singersdo not exhibit sexualdimorphism of plumage. Hence in only one instancecan we be positiveabout the sexof the singers(Coracina mon- tana, where a male leads and a female follows). The followingreasoning makes it probable,however, that male-female duetting is occurringin six speciesand mal•male countersingingin five: In th'e casesof Megapodiusfreycinet, Campochaera sloetii, Coracina 1968Jan. ] DIAMONDANDTERBORGIt, DualSinging 77 montana,and Philemonnovaeguineae both distinguishingcharacteristics of male-female duetting are present; the vocalizationsof the leader and fol- lower differ, and their time relationsare rigidly defined. In the Pitohui kirhocephalus"duet" the singershave somewhatflexible time relationsbut entirely different notes,while in Amaurornisolivaceus the leader and fol- lower sing similar phrasesbut in a strict time relation. These six vocal performancesalso share in commonthe fact that they form an unbreakable unit, i.e., half of the duet is not given as a solo. Thus presumedduetting betweenmale and female occursregularly in four species(Megapodius freycinet, Amaurornisolivaceus, Campochaera sloetii, and Coracinamon- tana), occasionallyin one species(Pitohui kirhocephalus),and rarely in one species(Philemon novaeguineae). The remainingcases (Dacelo gaudichaud,the doublechatter of Pitohui kirhocephalus,Me'lidectes rufo.crissalis, Oreornis sub7renatus,and Xan- thotischrysotis) resemble each otker and differ from the male-femaleduets in that both singersgive the samephrase and in that theseidentical phrases are much more often heard as solos. Countersongis the only reasonable interpretationfor the dual singingin Dacelo gaudichaudand Xanthotis chrysotis. The degreeof synchronizationin the casesof the Pitohui kirho- cephalusdouble chatter, Oreornis sub7renatus, and Melidectesru7ocrissalis leavesduetting as a distinct,though still a lesslikely, possibility. The incidenceof duetting amongNew Guinea birds is thereforeabout 3 per cent (6 out of 217 species,assuming these 217 to be a representative sample). The songsof most of these specieswere observedsufficiently often and closely to make it unlikely that undetectedduetting was oc- curring and that the true incidencediffers much from this 3 per cent fig- ure. Among the 217 species,the songsof 141 were keard more than 10 times (hundredsof times in many cases),the songsof 173 more than 4 times, and of 196 more than once. At best among this group a few more casesmay turn up of birds that duet infrequentlyor only in certain geo- graphicalareas, as Pitohui kirhocephalusand Philemonnovaeguineae. The 6 New Guinea duetters belong to 6 different genera, 5 different families, and 3 different orders, so that duetting must have evolved in- dependentlymany times. Why did duetting evolve, and what is its bio- logical significance?Probably there is more than one explanation,as the New Guinea duettersshare little in common. Their habitats may be tall grass,the forest floor, densevegetation of the lower and middle story, second-growthtrees, the forestmiddle and upper story, and the crownsof tall trees. Some live in the lowlands and others in the mid-montane zone. One of the duetstakes place mostoften on moonlitnights, another during the dawn chorus,two during presumedterritorial displays,and two at any daylight hour. The singersare always in immediatevisual contact in two 78 DIA3/rOI•DAI•D TERBORGtt; Dual Singing [ Vol.Auk 85 species,generally or sometimesin visual contactin two or more species, and entirely out of sight of each other in the other two species. Two of the speciesshow sexualplumage dimorphism,four do not. Five species have dull plumage,one (Campochaerasloetii) is brilliantly colored. Per- haps the only commondenominator is that all are more or lesssocial birds. Coracina montana and Campochaeraslo.etii are always encounteredin groupsof two to four; MegapodiusJreycinet and Amaurornisolivaceus fre- quently,but by no meansalways, in pairs; and Pitohui kirhocephalusand Philemonnovaeguineae in abundant,loosely organized populations of fre- quently interacting individuals. The significanceof somecases of duetting is suggestedby the facts that duettingis much commonerin the tropicsthan in the temperatezone, and that many tropical duetters are birds of dense vegetation, in which the singersremain out of visual contact. This correlation has been well ex- pressedby Thorpe (1961: 50):

The explanation suggestsitself that birds living in the dark and tangled under- growth of tropical , conditions which are likely to hinder the development of mutual visual displays,are the more likely to replace these by the elaboration of vocal displays. It is perhaps also significant that in Europe the only group which seems to employ antiphonal vocalization persistently is the owls which are, of course, mainly nocturnal. Among the speciesthat sing duets out of sight in tropical undergrowth are Laniarius erythrogaster (ThOrpe, 1963) and Laniarius barbarus (Grimes, 1966) of Africa, several Central American speciesdiscussed by Skutch (1940), and the Coraya Wren (Thryothorus coraya) of South America. The duets may have a specific role in maintaining contact be- tweenmembers of a pair as they move about. For example,the male and female of Saltator intermedius sing a simple duet throughout the year, while the male alone sings a very different song in the nesting season (Skutch, 1940). At leasttwo temperate-zoneduetters, Psophodes olivaceus of Eastern Australia and Thryothorusludovicianus of North America, also live in densethickets where the singerscannot see each other. Two of the six New Guinea duetters fit very well into this pattern: Amaurornis olivaceus(in tall grass)and Pitohui kirhocephalus (in denseundergrowth). Whether Megapodiusfreycinet belongsin the samecategory is more doubt- ful; the singersmay be out of sightwhen they duet on th'e forest floor on moonlitnights, but this may not be true for their daytime duets. The other three New Guinea duets cannot be explained on this basis, for the singersdefinitely maintain closevisual contact in Ca•npochaera sloetii and Coracinamontana and probably do so in Philemonnovaeguineae. This finding also appliesto someduetters from other parts of the world, suchas the CanadaGoose of North'America (Collias and Jahn, 1959) and Jan.1968 ] DIArYtONI)A•I>T•RBORC•, Dual Singing 79

Brotogerisjugularis (Power, 1966) and Monassanigrifrons (Bucconidae) of SouthAmerica. In the last-namedspecies, an inhabitant of clearings and second-growth,members of a pair engagein a prolongedand raucous antiphonalduet while perchedon the sameor nearby branches.Similarly someAustralian duetters, such as Grallina cyanoleucaand severalof the Cracticidae,are birds of open country and in visual contactas they sing. Armstrong(1963) has pointedout that theseAustralian species gear their breedingseasons to the onset of the rains, which come at unpredictable times of year. In order to begin nesting as soon as the rains arrive, the birdsmust already be pairedand may in fact remainin pairs and maintain territoriesthroughout the year. Thus duetsmay be onepossible behavioral deviceinvolved in maintainingpair-bonds for prolongedperiods in antici- pation of a favorabletime for nesting. Sucha role of duettingis also sug- gestedby evidencethat details of the duet enable membersof a pair to recognizeeach other as individuals. For example,Thorpe (1963) found that the interval betweenthe beginningof the male'sand the female'snote in Laniariuserythrogaster is characteristicof eachpair, and differentpairs of Laniariusbarbarus (Grimes, 1966) and Grallina cyanoleuca(Robinson, 1947) can be distinguishedby a characteristicrepertoire of patterns.Stud- ies on captivebirds have madeit clear that a pair of birds learnsto duet in perfect synchronyonly after many monthsof practice (Thorpe and North, 1966; Power, 1966). Thus the great majority of duettingspecies, regardlessof habitatand geographicalrange, prove to be specieswith stable pair-bondsthat remainintact outsideof the breedingseason. This and the previous explanation of duetting are not mutually exclusive--both may apply simultaneouslyto birds of tropical thickets in areas of somewhat unpredictableseasons. Skutch (1940) noted that the wrensand other spe- ciesof CentralAmerican thickets that practicedduetting were alsospecies that remain in pairs throughoutthe year, and the samecan be said from our experienceof the South American Coraya Wren. Too little is known of breedingseasons in New Guinea birds to make a final assessmentof this hypothesis,viz. that duets are characteristicof speciesthat must remainpaired for muchof the year becausetheir nesting dependson unpredictableclimatic changes.However the available infor- mation is compatiblewith the hypothesis.In the easternhighlands in 1964 and 1965 June, July, and Augustwere the driest months,and December and Januarythe wettestmonths, but this variesgreatly from year to year. A preliminaryanalysis of gonadsize in specimenswe collected,plus infor- mationgathered from natives and localEuropeans in New Guinea,suggests that many birdsof the grasslandsand forestlower story breedin dry pe- riods,many fruit-eatingbirds breed in rainy periods,and somespecies 80 DIA1V[ONDAND TERBORGI:I, Dual Singing [ Auk k Vol. 85 breedmore or lessat randomthroughout the year. Ripley (1964) also ob- servedseasonal breeding patterns in western New Guinea. All specimens of Campochaerasloetii we collected,and almostall Coracinamontana, had the gonadsin nonbreedingcondition. This was to be expected,as our col- lectionswere made in the dry seasonand both speciesfeed to a large extent on fruit (Mayr and Rand, 1937: 95; Rand, 1938: 2; 1942: 463; Gilliard and LeCroy, 1961: 46; Terborgh'and Diamond, MS). Neverthelessboth speciesshowed markedly territorial behavior and went about strictly in pairsor smallgroups containing both sexes.Thus the stereotypedduetting displaysof thesecampephagids may well have played a role in preserving pair bondsin readinessfor the rains and the nestingseason. A third possiblerole of duettingis the one discussedfor Coracinamon- tana, whoseduet may serve as an isolating mechanismagainst its non- duetting altitudinal representativeCoracina schisticeps. This can best be regardedas an additional or incidental role of duetting in C. mon.tana,as Camp.ochaerasloetii performsa very similar duetting display despiteits positionas a distinctmonotypic genus, while all other New Guineaspecies with altitudinal representativesmaintain reproductiveisolation by vocal or other meanswithout resort to duetting. On the one hand duetting cannot be assigneda unique explanationnor, on the other hand, can it be regardedas a unique behavioralresponse to one or more setsof conditions.Far more New Guineabirds inhabiting just as dense vegetation as Amaurornis o'livaceusand Pitohui kirhocephalus, and whosenesting is just as dependentupon unpredictable seasonal changes as that of Campochaerasloetii and Coracinamontana, maintain contact or remain mated without engagingin duets. The evidencein most caseswhere duetting doesoccur suggests it to be one meansthat may evolve to cement permanentor semipermanentpair-bonds, and that this particular form of intra-pair communicationand recognition is more likely to arise in habi- tats where visual displayswould have much lessvalue for thesepurposes.

mc KNOWLEDG1V[ENTS

It is a pleasure to acknowledge our debt to Ernst Mayr for advice and discussion about New Guinea birds; to Dean Areadon and R. A. Paynter for making available for study the bird collectionsat the American Museum of Natural History and the Harvard Museum of Comparative Zoology, respectively;to Nicholas Collias, Thomas Howell, Alan Grinnell, and Jack Hailman for suggestionsconcerning the manuscript; and to the many residentsof New Guinea whose cooperation and hospitality made the field work possible.These studieswere supportedby the Frank M. Chapman Fund of the American Museum of Natural History, the National Geographic Society, the Explorers Club, the American PhilosophicalSociety, and Sigma Xi. Part of the work was carried out during tenure of a Junior Fellowship from the Society of Fellows of Harvard University by one of us (J. M.D.). 1968Jan. ] DIAMONDANDTERBORGH, DualSinging 81

SUMMARY Synchronizedvocal duetsbetween a mated male and female, such as have been reportedfor speciesfrom most other parts of the tropics,are describedand analyzedfor New Guineabirds. Four New Guineaspecies (Megapodiusfreycinet, Amaurornis olivaceus, Campochaera sloetii, and Coracinamontana) duet regularly,one (Pitohui kirhocephalus)occasion- ally, and one (Philemonnovaeguineae) rarely. In addition,five examples of dual singingare described(Dacelo gaudichaud,Pitohui kirhocephalus, Melidectesrufocrissalis, Oreornis sub/renatus, and Xanthotis chrysotis) which appearto representcompetitive countersinging between males, but of which one or two may prove to be male-femaleduets. The incidenceof duettersamong New Guineaspecies is about 3 per cent. The most interestingexamples involve the campephagidsCampochaera sloetiiand Coracinamontana, which duet in the courseof similar stereo- typed displays,possibly of a communalterritorial nature. A closerelative of Coracinamontana, C. schisticeps,has an altitudinalrange just below that of C. montanaand exclusiveof it. The songof C. schisticepsis very similar to that of C. montanaexcept for lacking the femaleelement of the duet, so that the duet might act as an isolatingmechanism between these closely related species. The singersare out of sight of each other in the casesof Amaurornis olivaceusand Pitohui kirhocephalus,sometimes or generallyin sight in Megapodius/reycinetand Philemonnovaeguineae, and alwaysin immedi- ate visual contact in Campochaera sloetii and Coracina montana. It is suggestedthat duetting may arise in responseto two different situations: the need for birds living in densevegetation where visual contactis diffi- cult to evolveintricate vocal displaysrather than visual displays,and the importancefor birdswhose nesting season depends upon unpredictable cli- matic changes,such as the start of the rainy season,to becomeand remain paired long in advanceof breeding. LITERATURE CITED ARMSTRONG,E.A. 1944. Some notes on the song of the wren. British Birds, 38: 7O-72. AR•STRONO,E.A. 1963. A study of bird song. London, Oxford Univ. Press. CHAPMAN,F.M. 1929. My tropical air castle. New York, Appleton. Cot•]•AS,N. E., ANDL. JAHN. 1959. Socialbehavior and breedingsuccess in Canada Geese(Branta canadensis)confined under semi-natural conditions. Auk, 76: 478- 509. D•AMO•O, J.M. 1967. New subspeciesand recordsof birds from the Karimui Basin, New Guinea. Amer. Mus. Novitates, no. 2284. GrL]•ARD, E. T. 1959. The ecology of hybridization in New Guinea honeyeaters. Amer. Mus. Novitates, no. 1937. GIt,]Z•ARD,E. T., AND M. LECRo¾. 1961. Birds of the Victor Emanuel and Hinden- burg mountains, New Guinea. Bull. Amer. Mus. Nat. Hist., 123: 1-86. 82 D•A•OX• ANDTERBORGIt• Dual Singing l[ Vol.Auk 85

Gr•;•Am), E. T., Am) M. LECRo¾. 1966. Birds of the middle Sepik region, New Guinea. Bull. Amer. Mus. Nat. Hist., 132: 245-276. Gm•Es, L. 1965. Antiphonal singingin Laniarius barbarus barbarus and the audi- tory reaction time. Ibis, 107: 101-104. GRI•Es, L. G. 1966. Antiphonal singing and call notes of Laniarius barbarus bar- barus. Ibis, 108: 122-126. I-IAvERscHMmT,FR. 1947. Duetting in birds. Ibis, 89: 357-358. MAYR, E. 1941. List of New Guinea birds. New York, American Museum of Natu- ral History. MAYR, E., Am) E. T. Gi•;•;•Am). 1952. Altitudinal hybridization in New Guinea honey- eaters. Condor, 54,: 325-337. MAYR, E., ANDA. L. RAND. 1937. Resultsof the Archbold Expeditions. No. 14. The birds of the 1933-34 Papuan expedition. Bull. Amer. Mus. Nat. Hist., 73: 1-248. MOREArt,R.E. 1941. "Duetting" in birds. Ibis, 5: 176-177. OS•ASTON,B.B. 1941. "Duetting" in birds. Ibis, 5: 310-311. POWER,D. M. 1966. Antiphonal duetting and evidence for auditory reaction time in the Orange-chinnedParakeet. Auk, 83: 314-319. RAND,A.L. 1938. Resultsof the Archboldexpeditions. No. 20. On somepasserine New Guinea birds. Amer. Mus. Novitates, no. 991. RAND,A.L. 1942. Resultsof the Archbold expeditions.No. 43. Birds of the 1938- 1939 New Guinea expedition. Bull. Amer. Mus. Nat. Hist., 79: 425-516. R•m;E¾,S.D. 1964. A systematicand ecologicalstudy of birds of New Guinea. Bull. Peabody Mus. Nat. Hist., no. 19. ROBINSON,A. 1947. Magpie-larks--a study in behavior. Part 3. Song. Emu, 4,7: 11-28. SKrrTCH,A. F. 1940. Social and sleepinghabits of Central American . Auk, 57: 293-312. SKrrTCH,A. F. 1945. Life history of the Blue-throated Green Motmot. Auk, 62: 489-517. SNow, D.W. 1956. Courtship ritual: the dance of the manakins. Kingdom, 59: 86-91. STOKES,A. W., ANDH. W. WI•XA•(s. 1968. Antiphonal calling in quail. Auk, 85 (this issue). THORPE,W.H. 1961. Bird Song. Cambridge University Press, 143 pp. TaORP•, W. H. 1963. Antiphonal singingin birds as evidencefor avian auditory reaction time. Nature, 197: 774-776. THORPE,W. H., ANDM. E. W. NORTH. 1965. Origin and significanceof the power of vocal imitation: with specialreference to the antiphonal singing of birds. Nature, 208: 219-222. THORPE,W. H., AND M. E. W. NORTH. 1966. Vocal imitation in the tropical bou- bou shrike Laniarius aethiopicusmajor as a means of establishingand maintaining social bonds. Ibis, 108: 432-435. VANSO•EREN, V. D. 1947. Field noteson someMadagascar birds. Ibis, 89: 235-267. VAN TYm•, J., ANDA. J. BERGER.1959. Fundamentalsof ornithology. New York, Wiley. YOUNG,C.G. 1942. "Duetting" in birds. Ibis, 6: 110-111.

Departmentof Physiology,University of CaliforniaMedical Center,Los Angeles,California (J. M.D.), and Department of Botany, University of Maryland, CollegePark, Maryland (J. W. T.).