The Wilson Journal of Ornithology 124(3):454—466, 2012 
SILVEREYES (ZOSTEROPS LATERALIS) SONG DIFFERENTIATION IN 
AN ISLAND-MAINLAND COMPARISON: ANALYSES OF A COMPLEX 
CULTURAL TRAIT 
MYRON C. BAKER 1 - 2 
ABSTRACT.—Members of the genus Zosterops are known for their colonizing ability and extensive phenotypic 
differentiation on numerous islands. There have been morphological and biochemical analyses of some Zosterops 
populations, but little study has been devoted to patterns of vocal communication signals, known to be important pre-mating 
barriers in many bird species and in possible diversification of taxa. I report on the song system ol one subspecies of 
Zosterops in a mainland population and an island population 15 km distant. I used both a traditional subjective classification 
of song elements and the multivariate procedure of linear discriminant analyses (I DA) of measured sound features. The 
syllables constituting songs exhibited a low level of stereotypy, disallowing a lexicon of syllable "types' to be constructed 
for individuals or a population, New syllables were continuously produced as a bird uttered more and more songs, possibly 
indicating an extremely large repertoire or an open-ended generation of vocal innovations. I.DA indicated songs ot the 
island population were moderately differentiated from and less variable than those of the mainland. This type of song 
system creates a problem for research on vocal signals, whether directed at comparisons between birds in a local area or 
between populations. I made a preliminary effort to address this problem and discuss mv results in the framework of 
Zosterops and its propensity for evolutionary diversification. Received II October 2<>l I. Accepted 6 March 2012. 
Phenotypic divergence on islands has long held 
interest for students of evolutionary biology 
(Darwin 1859, Mayr 1963, DeSalle and Temple¬ 
ton 1988, Grant 1999), stimulating empirical 
and theoretical work on adaptation and speciation 
(Grant 1968, Barton 1989. Mayr and Diamond 
2001). Comparing phenotypic differentiation 
of isolated populations on different islands or 
between mainland and island populations has 
been more clearly suggestive of causal processes, 
such as founder effect, ecological selection, or 
genetic drill (Kaneshiro 1980. Mayr 1982, Baker 
et al. 2006. Parker et al. 2010). Historically, 
studies ot island biotas emphasized patterns of 
morphological evolution, but more recently in¬ 
clude molecular evolution (c.g., Slikas et al. 2000; 
Clegg et al. 2002a, b: Moyle et al. 2009; reviewed 
by Grant 2001, 2002). However, behavioral 
components of the phenotype, especially those 
involved in mating, have received much less 
attention in island studies. 
Research on mating signals of birds has con¬ 
tributed a number of findings that raise questions 
on the possible contribution of learned behavior 
patterns and social selection to genetic differen¬ 
tiation of populations (West-Eberhard 1983. 
Biology Department, Colorado State University. Fort 
Collins, CO 80523, USA; and School of Animal Biology. 
University of Western Australia, Crawley, WA, Australia 
Current address: I27N Ambrosini Uane. Femdale, CA 
95536, USA; e-mail: mcbaker@colostate.edu 
MacDougall-Shackleton and MacDougall-Shack- 
lelon 2001, Slabbekoorn and Smith 2002, Price 
2008, Servedio et al. 2009). Acoustic features of 
learned vocal signals, for example, are frequently 
correlated with morphological patterns of popu¬ 
lation differentiation, giving rise to the question of 
their role, if any, in speciation (Grant and Grant 
1989, Lachlan and Servedio 2004. Price 2008). 
Regardless of how a vocal difference arose in an 
island population upon or following colonization 
(founder effect, drift, selection), if it is a signal 
involved in mating, the production of the 
differentiated form of the signal could have a 
role in evolutionary sequelae (c.g., reproductive 
isolation, genomic divergence) that would be 
tested, and possibly reinforced (Butlin 1989) in 
the event of subsequent contact with new 
immigrants. 
Much of what we know about adaptive ra¬ 
diation of species on islands, particularly the 
correlated vocal communication traits, we owe to 
the extensive studies of Darwin's finches on the 
Galapagos Islands (Grant 1999, Podos 2001 1 . An 
equally famous example of bird speciation is the 
genus Zosterops, a highly speciose group of some 
85 species (Gill and Donsker 2012) distributed 
widely in the world, but especially known for 
having colonized numerous islands throughout the 
Indian Ocean and tropical Pacific. Many of these 
species and subspecies are island endemics (Mees 
1961, Pratt et al. 1987). The possibility that social 
selection may be important in Zosterops is 
relevant to the ‘great speciator’ status of this 
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