Birds sing the history of the Rainforest

September 2001

Birds usually learn their song whenyoung, from other individuals in theirlocal population, just as humans learna particular dialect from theirs. Birdsuse songs to:• assist in identifying individuals of

the same species,• attract and assess likely mates, and• identify and deal with possible

competitors for food and mates.

Such important functions tend to meansongs sung within a bird populationare very similar - anyone who sings adifferent song may not be understood.When populations are isolated fromeach other, there is the opportunity forsong differences to evolve, even withinthe same species. Factors which mightpromote this variation include differ-ent local habitat structure, changes inpopulation genetic make-up, mistakesduring song learning within one orboth populations, or different songpreferences in the two populations.

Birdsong reveals rainforest historyRainforest CRC researcher Dr DavidWestcott realised that if patterns ofbirdsong were affected by changes inthe distribution of habitat, variationsin birdsong could be used to predictthe effect of the geographical historyof the rainforest on birds. The ideas DrWestcott set out to test were:

• That the geographic isolation ofbird populations promoted songvariation.

• That the geographic distributionof song variation would matchvegetation distribution.

The tropical rainforest birdsToday there are more than 200 speciesof birds living in the tropical rainfor-ests of north Queensland. Thirteen ofthese are found only in these rainfor-ests, and of these, eight are restrictedto the cooler moist upland rainforestsabove 600 metres. They all sing!

The Upland bird species studied were:Grey-headed robin(Poecilodryas albispecularis)Golden bowerbird(Prionodura newtonia)Mountain thornbill(Acanthiza katherina)Bridled honeyeater(Lichenostomus frenatus)Fernwren (Oreoscopus gutteralis)Chowchilla (Orthonyx spaldingii)

Bird species with more widespreaddistributions studied were:

Grey fantail (Rhipidura albiscapa)Brown gerygone (Gerygone mouki)Eastern whipbird(Psophodes olivaceus)Spotted catbird(Ailuroides crassirostris)Victoria’s riflebird (Ptiloris victoriae)White-throated treecreeper(Cormobates leucophaeus)Lewin’s honeyeater(Meliphaga lewinii)

Measuring and Analysing BirdsongDr Westcott recorded birdsong fromsites across the Wet Tropics and thenanalysed the “advertisement” songs ofeach bird species to look for differencesin the song patterns. Advertisementsongs are used to attract females anddeter intruders.

Among the birds who sang for the study:

Top: the spotted catbird (Ailuroides

crassirostris) photo: WTMA

Centre: Victoria’s riflebird (Ptiloris victoriae)

photo: WTMA

Bottom: Lewin’s Honeyeater (Meliphaga

lewinii) photo: Terry Reis

• That upland species isolated onmountain tops would showgreater and more frequent songvariation than species with anuninterrupted distribution.

Variables used to characterise eachsong included:• the song length,• the maximum and minimum

frequency,• the number of calls per song,• time between various calls, and• the number of types of call.

Results showed that:• Geographic variation is a common

feature of the songs of northQueensland rainforest bird speciesand occurs at both the local andregional scale.

• This variation is most marked andmost common in the uplandspecies with their more restrictedhabitat and irregular distribution.

• Song variation occurs betweenblocks of continuous uplandforest rather than within the sameblocks.

The existence of variation at both thelocal and regional scale implies theusefulness of birdsong in document-ing the effects of relatively recentvegetation changes on birdpopulations. The fact that variation ismore marked in the upland speciesfurther suggests that the isolation ofpopulations on mountaintops hascontributed to the evolution of acultural trait in birds - in this case, oftheir song.

An Example: the golden bowerbirdThe golden bowerbird is an uplandspecies that lives above 700 metreswith populations which are isolatedfrom each other on different mountaintops.

Recordings of the advertisement song,consisting of four to six calls, weremade of 30 males at 5 different sites.

Analyses of the recordings showedlarge differences between the songs ofgolden bowerbirds from differentmountaintops. These differences, eas-ily detected both in sound analysis andby the human ear, are very importantto the birds. Dr Westcott and his col-league Dr Frederieke Kroon, playedthe different recorded songs back tomale bowerbirds to see if they coulddiscriminate between the differentdialects from different locations.

Males responded aggressively to localdialects, but either ignored or re-sponded less strongly to those fromdifferent locations.

ImplicationsIf birds do not adequately recogniseand respond to songs of their speciesthat differ from their own population,then birds singing different songsmay be at a reproductive disadvan-tage. In this way, song variance maycontribute to increasing reproductiveisolation and genetic variation be-tween populations. Studies elsewherein the world indicate that such culturaldifferences may play an important rolein the evolution of populations andraces into species.

The research to date highlights the factthat isolated populations of the samespecies of birds can be different fromone another. If birds are considered atrisk or endangered, it may not beappropriate to manage them as onespecies. Birdsong can indicate whetheror not a bird population is isolated anddistinct from other populations of thesame species. If appropriate, manage-ment strategies can then be properlyplanned for each population ratherthan the species as a collective.

For further information contact:Dr David WestcottCSIRO Tropical ForestResearch CentreAtherton, QLD 4870Phone: (07) 4091 8800Fax: (07) 4091 8888Email: david.westcott@tfrc.csiro.au

The golden bowerbird (Prionodura newtoniana)

photo: WTMA