Baker • SILVEREYES SONG EVOLUTION 
463 
sampling to examine if any differences discovered 
at one time are maintained across future samples. 
This appears not to have been accomplished in a 
song system of the rapidly-evolving kind exhib¬ 
ited by the Silvereyes 1 recorded. 
More slowly evolving song systems have been 
documented for a number of bird species, usually 
by examination of whole songs as the units of 
analysis. Song repertoires of individuals in most 
of these cases are finite and normally quite small 
and. in some, virtually all the local population 
shares the same song types. The half-life of the 
populations’ song types is on the order of one to a 
few years despite variations among these case 
studies (e.g.. Slater et al. 1980. Payne cl al, 1981. 
Trainer 1989). Changes in songs over time in 
humpback whale (Megaptera novaeangliae) popu¬ 
lations have also been noted with the rate of 
complete replacement of the characteristic song 
forms ranging from ~15 years to as few as 2 years 
(Payne and Payne 1985. Noad et al. 2000). These 
cases of cultural evolution are at least quantita¬ 
tively different from that of the Silvereyes in the 
present study and from the few other documented 
bird examples (e.g.. Kroodsma and Parker 1977. 
Huntsman and Ritchison 2002) in which the 
singers seem to be constantly producing new' 
variant songs or song elements as they continue 
to sing. However, there are numerous other 
songbird species in which the song types and 
components of songs are small in number, 
stereotyped, and in a given population both widely 
shared (song dialects) and stable over many years 
(e.g.. Harbison et al. 1999). Thus, the Silvereyes 
system I investigated lies at one end of a spectrum 
ranging from stable to constantly changing. 
It will be important to learn if other Zosterops 
exhibit an open-ended syllable production style of 
song creation for the wider issue of comparing 
songs across Zoster ops species and subspecies 
occupying various islands. The type of song 
production I found in Z I. gouldi in Western 
Australia may not be the norm in this genus, based 
upon verbal descriptions of song traits by workers 
visiting multiple islands in the tropical Pacific 
(Pratt et al. 1987, Diamond 1998). These 
experienced observers suggest in their descrip¬ 
tions of songs a great deal of heterogeneity across 
Zosterops species and subspecies on the various 
islands w here they conducted their research. Most 
of the descriptions have little similarity to the kind 
of songs 1 found in the Western Australia samples. 
or to the song style described by Slater (1993) for 
Silvereyes on Heron Island, Great Barrier Reef. 
Slater (1991. 1993) examined several aspects of 
vocal communication in Silvereyes (Z I. chlor- 
ocephalus) in the well-studied population on Heron 
Island, off Queensland. Australia, describing songs 
and behavioral response to playback of songs. The 
two main findings from that study were: (1) the 
songs of Silvereyes are extraordinarily complex 
and composed of highly variable constituent 
acoustic elements (syllables), and (2) playback 
tests revealed no differences in behavioral response 
to songs of conspecifie neighbors versus those of 
strangers (defined as occupying a territory at least 
50 in distant ). These hirds in other observations did 
not avoid overlapping their songs with others, 
showed no evidence they were in song interchang¬ 
es with others, nor w ere they stimulated by songs of 
others in any w'ay. A conclusion drawn from this 
previous research was that "...the continuous 
nature of the individual variation in the Silvereyes’ 
songs did not allow their perception of specific 
individuals or groups on the basis of song 
structure” (Slater 1991:36) and is concordant with 
my analyses of acoustic variation in Z I. gouldi. 
My examination of Silvereyes’ songs showed 
individuals have moderate degrees of distinctive¬ 
ness. but w'ith their ongoing production of new 
vocal innovations there may be little or no stability 
in individual vocal signatures, in which case it is 
difficult to imagine individual vocal discrimina¬ 
tion. Sampling across days and weeks would be 
useful for this issue. 
The present analysis of one member of the 
Zosterops lateralis complex indicates that among 
individuals within separated populations the 
acoustic traits of most song syllables may undergo 
continuous innovation. Consequently the songs 
would be in constant flux, making it likely that 
differences between birds also fluctuate. My 
comparison of an island and a mainland popula¬ 
tion of one subspecies showed they differ 
quantitatively but. without long-term sampling, 
it is not possible to say if the differences observed 
are stable. It is uncertain the differentiation of 
song between Esperance and Woody Island, from 
the data I have presented, is sufficient to serve as 
an impediment to immigrants, and gives only 
modest support to behavioral isolation as an 
hypothesis addressing the ‘great speciator’ para¬ 
dox. On the broader question of vocal differenti¬ 
ation among other Zosterops . especially island 
endemics of the Indian Ocean and the tropical 
