Feb 26: Song Dialects: Birds
Why Do Birds Sing?
Why this is an ill-posed question
• need to specify the correct level of analysis (for now, we are referring to the
functional question of survival value)
• even considering it as a functional question, it is TOO GENERAL
Three more specific questions about bird song
• Why do different species sing different songs?
• Why do only males sing (usually)?
• Are dialects (interpopulation variation within a species) adaptive?
Feb 26: Bird Song Dialects: Digression
A Functional Perspective on Communication
What communication is--a broad definition:
Production of signals by one individual (sender) that change the behavior of other
animals (receivers)
This broad definition is TOO broad
• Includes signals that change the behavior of receivers to the benefit of the receiver
but to the detriment of the sender (e.g., the rustling of a mouse that attracts a
predator)
• Natural selection would not favor traits in sender that work against the interests of
the sender (there is no "function" of the mouse's rustling)
A narrower definition: Production of signals by one individual that change the behavior
of a receiver in a way that benefits the sender
• This type of communication need not only occur among members of the same species
• Also, the receiver might also benefit by having changed its behavior in response to
the signal
• Thus, a rattlesnake's rattle signals to potential predators that the snake is dangerous
(this is good for the snake as well as for the would-be predator)
Feb 26: Bird song dialects: Why do different species sing different songs?
Why Do Different Species Sing Different Songs?
H1: Songs diverge by chance (genetic drift)
after speciation (perhaps associated with
selection for consistency within species)
Most likely hypothesis for most species
H2: perhaps differences reflect adaptation to
different local conditions (e.g., different
acoustical environments
Some evidence of this in Great Tits (in
Europe)--woodland populations have lower
frequency songs than grassland populations
H3: Selection for divergence of traits involved
in species recognition--to prevent hybridization
No evidence for more pronounced song
differences in indigo and lazuli buntings
where their ranges overlap (in Nebraska)
Feb 26: Bird song dialects--Why only males?
Why Do Only Males Sing?
H1: Tendency of males to sing (and females to not sing) is side-effect of
sexual differentiation of reproductive behavior
H2: Song involves costs (time, energy, exposure to predators);
Perhaps only the males reap benefits of singing in most species
Feb 26: Bird song dialects--Why only males?
Why Do Only Males Sing? (cont’d)
Hypothesis 2a: Males benefit by attracting females
• Choosy females use song as indicator of male quality (we postpone for now
the question of why females tend to be the choosy sex--see Chap. 12)
Evidence:
• In many species, males sing only when trying to attract mates
• In many species, females clearly choose among
males based on song--e.g., song complexity
• In starlings, males with larger song
repertoires are chosen earlier, and
have young earlier
Alcock Fig. 8.3
Feb 26: Bird song dialects--Why only males?
Why Do Only Males Sing? (cont’d)
Hypothesis 2b: Males benefit by more effectively repulsing rival males
• Female is a resource for which males compete
• Any trait that enhances competitiveness would be favored
• Perhaps songs serve to warn other males of presence and quality of singer
Evidence:
• In some species, males sing most when mates are fertile, hence when male
has most to lose by intrusion of rival male
• In white-throated sparrows, removal of singing
male results in intrusion by rival, but tape
recording delays intrusion
• In starlings, recordings of complex songs are more
effective than recordings of simple songs; thus, in
starlings, song complexity has survival value for both
mate attraction and rival repulsion
Alcock Fig. 8.4
Feb 26: Bird song dialects--Are dialects adaptive?
Are Dialects Adaptive?
H1: NO--they result from non-adaptive
genetic or cultural drift?
In indigo buntings, dialects do drift
around over time--doesn’t mean they
are non-adaptive
H2: YES--perhaps they represent
adaptation to local physical environment
Scant evidence of this
H3: YES--perhaps they represent
adaptation to local social environment
Feb 26: Bird song dialects--Are dialects adaptive?
Are Dialects Adaptive? - cont’d
H3a: Dialects are adaptive through process of
female choice: perhaps females prefer to mate
with male who sings like Dad (hence are adapted
to same environment in which Dad was
successful)
Evidence:
• In brown-headed cowbirds, females do prefer
to mate with males singing their own dialect
• However, ability of males to change dialect
over lifetime (e.g. white-crowned sparrow)
means that dialect doesn’t predict place of birth
H3b: Dialects are adaptive through process
of rival repulsion: perhaps males are more
effective in repelling rivals if they can “talk
the talk”--indicates familiarity with habitat,
staying power
• Song-matching in song sparrows
Alcock Fig. 8.7
Feb 26: Song Dialects: Birds
Why Do Birds Sing?
Three more specific questions about bird song
• Why do different species sing different songs?
Apparently a combination of selection for species recognition and drift
• Why do only males sing?
Intersexual selection (mate attraction) &
Intrasexual selection (rival repulsion)
• Are dialects (interpopulation variation within a species) adaptive?
Differences may be useful in indicating familiarity with local neighborhood