The Auk 111(1):178-184, 1994 LOSS OF FUNCTION IN TERRITORIAL SONG: COMPARISON OF ISLAND AND MAINLAND POPULATIONS OF THE SINGING HONEYEATER (MELIPHAGA VIRESCENS) MYRON CHARLES BAKER • ZoologyDepartment, University of WesternAustralia, Nedlands, Perth, 6009 Australia ABSTRACT.--Istudied vocal recognitionin populationsof Singing Honeyeaters(Meliphaga virescens)on Rottnest Island and the adjacent mainland of Western Australia through song- playbackexperiments. Morphological evidence suggestedthe island population had differ- entiated. ! addressedthe question of whether there was divergence in the vocal signaling system.! found significant loss of function of the territorial advertising song by comparing responsesof the Rottnest subjectsand those of subjectsfrom the mainland Nedlands popu- lation. The low level of responseby Rottnest birds to songsfrom the mainland was similar to the responseby Rottnestbirds to songsof the co-occurringGolden Whistler (Pachycephala pectoralis).On the mainland, Nedlands birds responded to the songs of another mainland population,located the samedistance away as RottnestIsland, at an elevatedlevel similar to their responseto songsof their own Nedlands population. I conclude that there are popu- lation-specificvocal featuresin the songsof RottnestSinging Honeyeatersthat result from isolation. The isolation causedby the water barrier between RottnestIsland and the mainland is greater than isolation by distance through continuous populations on the mainland. I speculate that vocal recognition was probably lost as a result of an early colonization of Rottnestand the subsequentestablishment of a new vocaltradition in the island birds. Received 31 July 1992, accepted24 April 1993. THE STUDY OF the differentiation of island of a new vocal tradition through founder effect forms from mainland source populations has (Thielcke 1973, Baker and Cunningham 1985). contributed much to our understanding of evo- This could have consequencesfor the isolation lution in birds (Lack 1947, Bond 1948, Amadon of gene pools (Carson 1971, Baker and Marler 1950, Raikow 1977). Numerous traits have been 1980, Kaneshiro 1980). shown to diverge as a result of isolation on Previous work indicated that Singing Hon- islands. Traits most commonly documented as eyeaters(Meliphaga virescens) of the Rottnest Is- diverging are morphological,and include beak land (WesternAustralia) population on average length, body size, wing length, and plumage have a mass that is about 20% more than the coloration (Murphy and Chapin 1929, Amadon adjacent mainland form (Wooller et al. 1985). 1953, Van Valen 1965, Grant 1965, 1968). Experienced birders also have noted that Rott- Rarely have isolated island populations of nestSinging Honeyeatersare somewhatdarker, birds been examined for divergence in com- with more streaked plumage on the underside municative signals.Although the classicalcases of the body, than mainland individuals (Saun- of adaptive radiation in birds are satisfactorily ders and deRebeira 1985). I used tape recording explainedby referenceto ecologicaldivergence and playback experiments to determine wheth- (Amadon 1950, Grant 1986), the initial phase of er there was any loss of communicative func- differentiation and, particularly, the effectsof tion in the vocal signalsof Singing Honeyeaters subsequent new colonizations or recoloniza- from the Rottnest Island and mainland popu- tions by the sameor other forms may be influ- lations. enced by reproductive signals involving terri- tory defense and mate choice. Becausemany MATERIALS AND METHODS speciesof songbirdslearn their songs,it might be expected that in such speciesthe coloniza- The Singing Honeyeater is one of 67 speciesbe- tion of an island would lead to establishment longing to the Meliphagidae,one of the mostspeciose families of Australian passerines(Thompson 1964). Many honeyeater speciesare abundant. Most feed ' Present address: Biology Department, Colorado principally on nectar. The Singing Honeyeater (20- State University, Fort Collins, Colorado 80523, USA. 30 g) is the most widely distributed honeyeater in 178 January1994] SingingHoneyeater Vocal Recognition 179 Australia (Blakers et al. 1984) and seems to be es- wheneverthere was a changeof distancethat resulted pecially successfulin the abundantly flowered gar- from movement initiated by the bird. In addition, a dens of suburban areas.There are no published rec- synthetic variable, sumbeh, was created by summing ords of this species'vocalizations. vocalizations,hops and flightsto providean index of The SingingHoneyeater has a discretedawn chorus the total activity. The distanceestimates were aided that commencesabout 20 to 30 rain beforelight, and by pacingbetween key featuresof the habitatin the lasts about 40 to 60 min. Typically, the male roosts vicinity of the speakerafter the test was completed. overnight in a tree and sings from that location in For analysis,the distanceestimates were averagedfor the pre-dawn period. This songis heard infrequently the trial to give a single value of distance for that at other times of the day. Along a residential street subject'sresponse to the stimulus.Hops occurredusu- it is commonto find a singing male nearly every 50 ally where a subjectremained within a single bush m in the treesalong the vergeand in the front gardens or tree for a period of time and changedlocations of the homes.A territory is defended physicallyand without flying. Flights were usuallybetween treesor vocally, defenseoften being centeredaround nectar bushes,and were frequently to and from the bush sources. Access to these nectar sources is contested near the playbackspeaker. All types of vocalizations frequently by the Red Wattlebird (Anthochaeracarun were summed in a single category.There was con- culata)and the Brown Honeyeater (Lichmerai•distinc- siderablevariety in the vocal performanceof subjects ta). Vocal defense during daytime is usually not by during tests,but the complexitywas too great to make songbut by a number of different calls.These appear reliable categoriesof vocalizations.This would re- to indicate a range of motivational conditions from quire a separate study. The kinds of vocalizations I an initial "distant alarm," given when an intruder heardduring playbacktests were the sameas I heard first enters the spaceof another male, to intense chat- during my observationsof the aforementionedtwo tering and continuousmultiple chirping when two territorial males during their normal day's interac- birds are closetogether. Observations I made on two tions with intruder neighborsand other species. vocally distinct malesover a period of severalweeks In the playback tests on Rottnest, subjectsheard suggestthat the birds are monogamous.Most breed- songsof Singing Honeyeatersfrom Rottnestor from ing commencesin the Australasianspring (August- Nedlandsor heard GoldenWhistler (Pachycephalapec- September) and ends in December-January.Males toralis;Gws) songsfrom Rottnest.The Golden Whis- were singing persistently when this study was ini- tler stimuli were used to determine if the response tiatedon 1 October,but by mid- to lateJanuary sing- to Nedlands stimuli was equivalent to the level of ing had decreasedconsiderably. Songs were heard responsegiven to heterospecificsongs. The Golden occasionallyon into March. Whistler occurstogether with Singing Honeyeaters Males in three populationswere tape recorded.One in many locations on Rottnest Island. Two different population was located in suburbanNedlands (Ned), Golden Whistler songswere used as stimuli. There- near Perth, Western Australia. A second population fore, the statisticaldesign consistedof three treat- was located 25 km west of Nedlands on the island of ments (Rot, Ned, Gws) with five different stimulus Rottnest (Rot) in the Indian Ocean. Rottnest was iso- songswithin eachof Rot and Ned treatmentsand two lated about 6,500 yearsago as the sea level rose near different stimulus songswithin the Gws treatment. the end of the Holocene (Playford 1983). A third pop- Forty subjectswere tested(15 with Rot, 15 with Ned, ulation was located 25 km east of Nedlands in the 10 with Gws). suburbanareas on the western edge of John Forrest When evaluatingthe mainland population of Ned- National Park (Jfp). Songswere recorded during the lands, subjectsheard the samefive Nedlands stimuli dawn chorususing a Marantz cassetterecorder (PMD and five Rottneststimuli as did the Rottnestsubjects. 201), Sennheiser microphone (MD 402-K) mounted In addition, I used five Jfp songsas stimuli. Forty- in a 40-cm parabolicreflector, and TDK Type I tape. eight subjectswere tested(16 eachwith Ned, Rot,and Songsof five different maleswere randomly se- Jfp). lected from each population and transferred to 20-s Data were analyzed by nested ANOVA (stimuli continuous-loop cassettes.These served as playback nested within treatments). Fisher's LSD (Carruer and stimuli (Fig. 1), which were delivered at the rate of Swanson1973) was used to make multiple compari- one song per 10 s for 5 rain during each playback sonsbetween pairs of treatments. trial. A trial was initiated by locating a vocalizing Singing Honeyeaterand placingthe playbackspeaker RESULTS (Perma Power S-610) within 5 m of the individual and under or beside a tree or bush. I activated the stimulus and retreated 5 to 10 m to make a record of Rottnestsubjects.--The experimental birds on events for 5 rain on another cassette recorder. RottnestIsland respondedto the playback tests I recorded the occurrence of vocalizations (vocals), with