96
Hans Winkler
slowly, yet, songs of most birds are clearly concentrated or confined to very brief
periods at dawn and dusk. In other words: no ecological causes can be invoked as
explanation.
Signals should be honest and uncheatable, particularly if the interest of sender
and receiver are different. As stated in the much-discussed handicap principle,
signals are reliable indicators of the signaler's quality provided they are costly
(Zahavi 1977, Grafen 1990). A signal that indicates only the presence of the
signaler can be cost-free and reliable at the same time when there is no conflict of
interests (Maynard Smith 1994). As an explanation for the dawn chorus, the
handicap principle would require that singing is costly, and that these costs are
particularly high at dawn. It would predict not only that there is a strong
correlation between the quality of the singer and dawn song output, but also that
the variance among singers is highest at this time of day (Staicer et al. 1996). Note
that ecological hypotheses assumed that singing at dawn is more efficient and
presumably less costly. High predation risk in the morning is incompatible with
the predation hypothesis, but would represent just the type of handicap honest
signaling and sexual selection models would require.
Mate-guarding is one important means by which males can prevent their mates
from engaging in extrapair copulations (Birkhead and Moller 1992). Singing in the
dawn may ward off potential intruders and also may also persuade the female not
to leave for other males. It has been believed for some time that extrapair
copulations are most successful at dawn because a fertility window would be open
at that time. This early fertility window seems, however, not to be a general
phenomenon (Sheldon and Burke 1994). Singing and accompanying the female,
the most effective way of guarding her, may not be compatible activities. This,
and the fact that in most species males do not sing in the fertile period of their
females do not render paternity assurance as a plausible cause for the dawn chorus
(Staicer et al. 1996; Gil et al. 1999).
The territory defence hypothesis rests on the assumption that territorial intrusion
peaks at dawn (Staicer et al. 1996). It also implies that listeners use the
information about presence and status of the singer immediately. Thus, it would
predict that the timing of singing matches the timing of territory take-overs, and
suggests corresponding playback experiments (Staicer et al. 1996).
Staicer et al. ( 1996) propose a new hypothesis that stresses the role of singing
for adjusting social relationships among territorial neighbors. Among other things,
they suggest that males who assert their status at dawn benefit from that during
social interactions during the day. The main prediction of the social dynamics
hypothesis is that variation in dawn singing is correlated with the instability of
social relationships. The higher the instability, the longer or intense the dawn
chorus would be (Staicer et al. 1996). Factors that contribute to this instability
would include overnight mortality, frequent extrapair copulations, and dynamic
territorial boundaries. This hypothesis is attractive because it combines
ecologically relevant factors (mortality) with aspects of communication; but, it is
the latter aspect that remains somewhat elusive, for it is rather unspecific about
Hans Winkler
slowly, yet, songs of most birds are clearly concentrated or confined to very brief
periods at dawn and dusk. In other words: no ecological causes can be invoked as
explanation.
Signals should be honest and uncheatable, particularly if the interest of sender
and receiver are different. As stated in the much-discussed handicap principle,
signals are reliable indicators of the signaler's quality provided they are costly
(Zahavi 1977, Grafen 1990). A signal that indicates only the presence of the
signaler can be cost-free and reliable at the same time when there is no conflict of
interests (Maynard Smith 1994). As an explanation for the dawn chorus, the
handicap principle would require that singing is costly, and that these costs are
particularly high at dawn. It would predict not only that there is a strong
correlation between the quality of the singer and dawn song output, but also that
the variance among singers is highest at this time of day (Staicer et al. 1996). Note
that ecological hypotheses assumed that singing at dawn is more efficient and
presumably less costly. High predation risk in the morning is incompatible with
the predation hypothesis, but would represent just the type of handicap honest
signaling and sexual selection models would require.
Mate-guarding is one important means by which males can prevent their mates
from engaging in extrapair copulations (Birkhead and Moller 1992). Singing in the
dawn may ward off potential intruders and also may also persuade the female not
to leave for other males. It has been believed for some time that extrapair
copulations are most successful at dawn because a fertility window would be open
at that time. This early fertility window seems, however, not to be a general
phenomenon (Sheldon and Burke 1994). Singing and accompanying the female,
the most effective way of guarding her, may not be compatible activities. This,
and the fact that in most species males do not sing in the fertile period of their
females do not render paternity assurance as a plausible cause for the dawn chorus
(Staicer et al. 1996; Gil et al. 1999).
The territory defence hypothesis rests on the assumption that territorial intrusion
peaks at dawn (Staicer et al. 1996). It also implies that listeners use the
information about presence and status of the singer immediately. Thus, it would
predict that the timing of singing matches the timing of territory take-overs, and
suggests corresponding playback experiments (Staicer et al. 1996).
Staicer et al. ( 1996) propose a new hypothesis that stresses the role of singing
for adjusting social relationships among territorial neighbors. Among other things,
they suggest that males who assert their status at dawn benefit from that during
social interactions during the day. The main prediction of the social dynamics
hypothesis is that variation in dawn singing is correlated with the instability of
social relationships. The higher the instability, the longer or intense the dawn
chorus would be (Staicer et al. 1996). Factors that contribute to this instability
would include overnight mortality, frequent extrapair copulations, and dynamic
territorial boundaries. This hypothesis is attractive because it combines
ecologically relevant factors (mortality) with aspects of communication; but, it is
the latter aspect that remains somewhat elusive, for it is rather unspecific about