The Ecology of Avian Acoustical Signals
95
6 Dawn Chorus: Ecology, Sex, or Updating?
Die! patterns in acoustic signaling are universal among animals in general, and in
birds in particular. Many species of birds have a main peak of song at dawn and a
minor one at dusk. Many explanations have been proposed to explain this pattern.
In their review, Staicer et al. (1996) list 12 hypotheses and group them into those
that rest on physiology or the internal state of the singer, those that emphasize
environmental and other ecological constraints, and those that invoke social
necessities. Since song may have several functions, these hypotheses are not
mutually exclusive.
It has been known for some time that song influences reproductive behavior and
hormonal levels of females (Wingfield and Farner 1993; Cheng et al. 1998), and
may even control gene expression in certain parts of the brain (Ball and Gentner
1998; Jarvis et al. 1998). Singing may also stimulate the signaler's own hormones.
The self-stimulating hypothesis posits that singing changes the hormone levels of
the singer, and that this is most effective at dawn. The daily cycle hypothesis
proposes a circadian fluctuation of hormone levels with the dawn chorus as being
a nonfunctional consequence (Staicer et al. 1996). Thorough testing of these
possibilities must wait until methods exist which allow one to monitor and
manipulate hormones in the course of a day while the birds are singing.
At low light levels neither day- nor night-active hunters may find ideal
conditions. Hence, dawn and dusk would be ideal periods for singing. The
predation hypothesis implies that males could sing from more exposed locations at
dawn and dusk than at other times (Staicer et al. 1996). Neither observational nor
experimental evidence supporting the predation hypothesis seems to exist.
Foraging and singing may interleave, as in vireos (Vireo, Vireonidae), may
constrain each other to some extent, or represent activities clearly separated with
respect to microhabitat (e.g. Hunter 1980) and timing. Experiments on captive
birds show that the birds are able to forage only at luminances that are reached
after the time of the dawn chorus (Kacelnik 1979). Besides this, the hypothesis of
singing at periods when foraging is inefficient has received little support. A more
complex hypothesis derived from dynamic programming models (Hutchinson et
al. 1993, Houston and McNamara 1999) shares with the former the assumption
that foraging and singing are mutually exclusive activities, and the complete
neglect of factors related to the receiver. These models predict that, if there is
uncertainty about daily energy requirements, time for singing is optimally
allocated at dawn and dusk, Hutchinson et al. (1993) assume that females deem
continuous songs most attractive and that singing is costly (mainly in terms of lost
opportunity for other activities). Although there is little empirical support for this
hypothesis, it is attractive because it also explains the dusk chorus. Like the
previous hypothesis it suggests that the less singing is separated from foraging, the
less it should be restricted to dawn and dusk. Environments differ in the way in
which atmospheric conditions change in the course of the day. For instance, in the
rainforest studied by us, temperature gradients and turbulence build up only
95
6 Dawn Chorus: Ecology, Sex, or Updating?
Die! patterns in acoustic signaling are universal among animals in general, and in
birds in particular. Many species of birds have a main peak of song at dawn and a
minor one at dusk. Many explanations have been proposed to explain this pattern.
In their review, Staicer et al. (1996) list 12 hypotheses and group them into those
that rest on physiology or the internal state of the singer, those that emphasize
environmental and other ecological constraints, and those that invoke social
necessities. Since song may have several functions, these hypotheses are not
mutually exclusive.
It has been known for some time that song influences reproductive behavior and
hormonal levels of females (Wingfield and Farner 1993; Cheng et al. 1998), and
may even control gene expression in certain parts of the brain (Ball and Gentner
1998; Jarvis et al. 1998). Singing may also stimulate the signaler's own hormones.
The self-stimulating hypothesis posits that singing changes the hormone levels of
the singer, and that this is most effective at dawn. The daily cycle hypothesis
proposes a circadian fluctuation of hormone levels with the dawn chorus as being
a nonfunctional consequence (Staicer et al. 1996). Thorough testing of these
possibilities must wait until methods exist which allow one to monitor and
manipulate hormones in the course of a day while the birds are singing.
At low light levels neither day- nor night-active hunters may find ideal
conditions. Hence, dawn and dusk would be ideal periods for singing. The
predation hypothesis implies that males could sing from more exposed locations at
dawn and dusk than at other times (Staicer et al. 1996). Neither observational nor
experimental evidence supporting the predation hypothesis seems to exist.
Foraging and singing may interleave, as in vireos (Vireo, Vireonidae), may
constrain each other to some extent, or represent activities clearly separated with
respect to microhabitat (e.g. Hunter 1980) and timing. Experiments on captive
birds show that the birds are able to forage only at luminances that are reached
after the time of the dawn chorus (Kacelnik 1979). Besides this, the hypothesis of
singing at periods when foraging is inefficient has received little support. A more
complex hypothesis derived from dynamic programming models (Hutchinson et
al. 1993, Houston and McNamara 1999) shares with the former the assumption
that foraging and singing are mutually exclusive activities, and the complete
neglect of factors related to the receiver. These models predict that, if there is
uncertainty about daily energy requirements, time for singing is optimally
allocated at dawn and dusk, Hutchinson et al. (1993) assume that females deem
continuous songs most attractive and that singing is costly (mainly in terms of lost
opportunity for other activities). Although there is little empirical support for this
hypothesis, it is attractive because it also explains the dusk chorus. Like the
previous hypothesis it suggests that the less singing is separated from foraging, the
less it should be restricted to dawn and dusk. Environments differ in the way in
which atmospheric conditions change in the course of the day. For instance, in the
rainforest studied by us, temperature gradients and turbulence build up only