Environmental Constraints on Song Structure

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Environmental Constraints on Song Structure

• Forests pattern in amplitude modulation and
  spacing of sounds
• More attenuation of short wavelengths disrupts
  frequency modulation over distance
• Examples Northern Cardinal, Carolina Wren
• Grasslands pattern in frequency modulation
• Examples Eastern Meadowlark, Song Sparrow
• Upward defraction of sound leads to early
  singing, singing from high perches, song flights

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Duets

• Occurs 240 species in 44 families
• 20 species each shrikes, wrens, honeyeaters
• Antiphonal singing duets in which sexes
  alternate notes in what sounds like one song
• Associated with jointly-held year-round
  territories, especially in the tropics
• Possible function territory defense female (she
  starts song), mate-guarding male (he adds)
• Local example Carolina Wren

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Elements of Song Learning

• Innate template neural image in brain determines
  what pay attention to
• Sensitive period modify innate template to
  produce permanent motor tape based auditory input
  during a particular period
• Practice (song crystallization) during later
  period sing subsong, modify based on hearing self
  sing to produce adult song

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Experimental Evidence

• Birds isolated during sensitive period sing
  unmodified template, even if surrounded by other
  singing birds as adults
• Birds deafened after sensitive period sing
  subsong (no auditory feedback for practice)
• Birds deafened as adults sing normal song, do not
  need to hear self sing to produce perfect song

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Variations in Song Learning

• Specificity of template
• Swamp Sparrow learns only syllables from own
  species, Song Sparrow sometimes learns syllables
  other species, mimics learn everything
• Complexity of template
• White-crowned Sparrow simple, differs adult song
  Song Sparrow more complex, more like adult song

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• Timing of sensitive period
• Early, before practice
• Later, overlaps with practice
• Both early and later
• Annual, modify song every year
• How well copy what heard
• Copy song units exactly
• Copy syllables exactly, make own song units
• Improvise and invent as well as copy

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Outcomes of Variation

• Broad templates, inexact copying, long and late
  sensitive periods associated with large song
  repertoires
• Narrow templates, exact copying, early sensitive
  periods associated with small song repertoires
  and stereotyped songs
• Timing of sensitive periods often matches ecology
  to enable song matching

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Many sedentary species have fixed songs, early
sensitive periods (after dispersal) Example
Marsh Wren
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Many nomadic species relearn songs
yearlyExample Sedge Wren
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Dialects in Birds

• Regional differences in song
• Some are sharp, distinct, persistent
• White-crowned Sparrow example
• Some are gradual, so that populations far apart
  sound the most different
• Some may function to promote song matching,
  others may be accidental byproducts of the song
  learning system

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Elements Song Control System

• HVC (High Vocal Center) in hyperstriatum and RA
  nucleus in corpus striatum are the song control
  centers
• Connections here innate template that is
  modified into motor program for song
• New neurons, synapses form here during song
  learning, even in adults
• TSM nucleus controls muscles of syrinx to produce
  song
• TSM controlled by HVC and RA

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More Elements

• L nucleus receives auditory input, provides
  feedback to song control centers
• Provides input to modify template during
  sensitive period
• Provides input during practice
• Not involved in singing by adult birds
• ICo nucleus in midbrain controls motivation to
  sing (when to sing, not what to sing)

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Hormonal Control

• Hormones involved in seasonal, other longer term
  changes
• Hormones bind to neurons in ICo to control
  seasonal changes in motivation to sing
• Hormones involved in sensitive periods, control
  growth and other changes (based on learning) in
  song control centers