rival repulsion.At the start of the breeding season many male songbirds
fight each other for territories.Neighbors also often have duels with song
across territorial boundaries and,if the birds involved have repertoires of
different song phrases,each tends to match the song of the other as they
sing.A male red-winged blackbird (Agelaius pheoniceus) that cannot sing
suffers more intrusions onto his territory by others than one that can
(Smith 1979).If a male great tit (Parus major) is removed from his terri-
tory,it will be less rapidly invaded if recordings of the song of his species
are played from loudspeakers (Krebs 1977).Experiments such as these
provide the best evidence for song having a role as a “keep out”signal.
What of song’s part in attracting females? Many male birds stop
singing once they are mated (e.g., sedge warbler, Acrocephalus
schoenobaenus;Catchpole 1973),and song increases enormously if a
male loses his partner for any reason (e.g.,great tit;Krebs,Avery,and
Cowrie 1981).In several species,song attracts females (e.g.,in European
flycatchers,Ficedulasp;Eriksson and Wallin 1986),and in canaries
(Serinus canaria) it increases nest-building behavior and boosts the
growth of eggs in the ovaries (Kroodsma 1976).Female birds that are
ready to mate show a particular display to the male,referred to as solic-
iting,during which they adopt a horizontal posture,spread their wings,
and flutter their tail up and down.Perhaps no surprise,females do not
normally show this display when song is played from a loudspeaker.
However,those of a number of species do so if made highly receptive
by treatment with the female sex hormone estrogen (Baker et al.1987).
In some cases males have only a simple song that labels them as belong-
ing to their species,and this seems to be adequate both to keep rivals out
and to attract a mate.However,in other cases females are known to be
most attracted by males with large repertoires,such as the sedge warbler
(Catchpole 1980) and starling (Sturnus vulgaris;Eens,Pinxten,and Ver-
heyen 1991a).If,for whatever reason,females prefer males with larger
song repertoires,males with the most elaborate songs will be most suc-
cessful or rapid in attracting a mate and therefore likely to have greater
breeding success.This process of sexual selection is thought to be a prime
reason why animals have large repertoires of different sounds.Here we
have a distinct difference from language,because the message of each
sound is the same:“I am a male sedge warbler in breeding condition.”But
the male that can say it in the most varied way is more attractive to
females,and thus most likely to be successful in leaving his genes to the
next generation.In such birds,unlike those with small repertoires,it is
much less common for the same song to be repeated several times in a
row,as the main message is variety itself (Slater 1981).
A crucial feature that songbirds have in common with humans is that
learning plays an important role in their vocal development.In this they
differ from their closest relatives,suboscines in the case of songbirds and51 Birdsong RepertoiresMUS4 9/14/99 11:59 AM Page 51