from African shrikes (Thorpe 1972) to Australian whipbirds (Watson
1969).
Duetting is most common in the tropics,and this probably relates to
the fact that birds there frequently hold year-round territories
(Farabaugh 1982).This in turn is associated with birds that form long-
term monogamous pair bonds.One other association often claimed is
that between duetting and sexual monomorphism, and although
Farabaugh (1982) failed to find this,she said that that could be because
her definition of duetting was a rather undemanding one.It is certainly
striking that many species with tight antiphonal duets that have been
studied are monomorphic.
Duetting may have a role in maintaining the long-term pair bond and
in keeping contact between members of a pair,especially in the dense
and noisy environment of a species-rich tropical forest (Hooker and
Hooker 1969).However,evidence on these matters is equivocal (Todt
and Hultsch 1982;Wickler 1976).Wickler (1976) maintains that,in addi-
tion to possible roles within the pair,duetting is primarily a signal used
in cooperative territory guarding.
The idea that duetting pairs are jointly defending their territories
raises the question of why this evolved in certain species but not in others
in which only the male sings.The answer must lie in detailed field studies
of the species concerned,and few of these have been conducted to date.
One study on bay wrens (Thyothorus nigricapillus) in Panama suggests
an intriguing answer (Levin 1996a,b).In many duets,one bird sings an
initial section that is followed by a reply from the other.It has often been
assumed that the duet is initiated by the male,with the reply being the
contribution of the female.However Levin showed that this is not so in
bay wrens.Although these birds are monomorphic,she examined them
using a technique called laparotomy and found that the individuals
leading the duets were female.She suggests that duetting in these birds
may have originated because,for some reason,females are the more ter-
ritorial sex.They therefore sing just like female European robins
in winter to defend their territories and attract prospective mates.
However,bay wrens are monogamous,and once a female has attracted
a male,he deters others by adding a coda to her song.She thus keeps
females out of their territory while he puts off other males.
This idea for different roles of the sexes in duetting species is an inge-
nious one and may also apply to other species.Despite the fact that the
phenomenon has been extensively documented,few studies in the field
went beyond the stage of observation and description,and the subject
of duetting calls for more experimental work.As yet,any possible link
between this aspect of birdsong and coordinated singing in humans
would be decidedly tenuous!58 Peter J.B.SlaterMUS4 9/14/99 11:59 AM Page 58