Elements of the Avian Sound Production System

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Elements of the Avian Sound Production System

• Internal Tympanic Membranes tension determines
  frequency of sound
• Air Sac pushes membranes into lumen of bronchi,
  pressure determines loudness
• Muscles of the Syrinx control tension of the
  membranes
• Trachea modifies sounds produced syrnix

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Role of the Trachea

• Acts as resonance tube in some species
• Extra-long trachea in some of these
• Acts as a complicated filter in most species,
  including the best singers
• Controls which frequencies are allowed pass
• Enables pure tones (at frequency of the
  membranes) to be produced (whistled song)
• Harmonic songs are also produced

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Variation in Syringeal Muscles

• Number determines range of frequencies
• Ratites, storks, New World vultures have none,
  can only grunt and hiss
• Most non-passerines, non-oscine passerines have 2
  pairs extrinsic muscles
• Oscine passerines have 2 pairs extrinsic muscles
  6 pairs intrinsic muscles, have the best range

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More on Structure

• Variation in song complexity in oscines is due to
  differences in what species are programmed to
  sing, not structure of syrinx
• Birds can sing two notes at once
• Usually alternate notes produced by two bronchi,
  but sometimes sing both together

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What is Song?

• Long, complex vocalization with functions of mate
  attraction and territory defense, mostly
  restricted to males in the breeding season

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Other Vocalizations (Calls)

• Mostly 5-14 per species
• Contact calls
• Aggressive calls, courtship calls
• Parental calls, begging calls
• Alarm calls, mobbing calls, arousal calls

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Vocalizations Function in Communication

• Birds use only vocal, visual communication
• Communication exchange information, alteration
  response probabilities of recipient based on
  signal from actor
• 2 views signal stands for motivation of actor
  signal used to manipulate recipient
• Signal has function (advantage actor), target,
  reason for recipient to respond

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Aerial predator alarm call is hard to localize
long, slurred, narrow frequency range makes hard
to detect beginning and end high pitch means
wavelength too short to easily localize
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Aerial Predator Alarm Call

• Risk to caller reduced because hard localize
• Target recipients may be mate, relatives
• Alarm calls also given in absence of mates and
  relatives
• Motivation view target recipient is predator
• Manipulation view target recipients are flock
  mates, used for cover

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Chickadees (and others) give false alarms at bird
feeders to obtain food when access being
restricted by dominant species

• This phenomenon is problematic for the motivation
  view, supports the manipulation view

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Terrestrial Predator Alarm Call

• Structure makes easy to locate, no risk
• Supports idea aerial predator alarm call is
  designed to reduce risk
• Refutes idea using flock mates for cover
• Several possible target recipients
• Mate and relatives
• Flock mates (recruit for mobbing)
• Predator

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Song as Communication

• Target recipients
• Potential mates (conspecific females)
• Territory rivals (conspecific males)
• One type of information species identity
• Structural result each species has unique song,
  divergent from those of similar species
• Song is overly complex for this single function,
  implying additional information conveyed

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The Complexity of Song

• Syllables groups into song units
• 4 syllables per unit in Rufous-collared Sparrow,
  50 in Winter Wren
• Species differ in how many song units an
  individual sings
• 1 (Rufous-collared Sparrow)
• A few (8 in Yellow-throated Vireo)
• 100 (30-150 in Marsh Wren)
• 1000 (2000 in Mockingbird, Brown Thrasher)

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Information Conveyed by Complexity

• Individual identity
• Probably universal
• Dear neighbor phenomenon
• Complex communication
• Different song units have different functions
  (mate attraction vs territory defense warblers)
• May provide location information (center vs edge
  territory)

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Much of complexity in song likely contains no
information

• Often song units sung do not vary with context in
  any consistent way
• That recipients respond differently to different
  song units seldom demonstrated
• It is likely that a complex repertoire is
  advantageous, but individual song units are
  meaningless

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Advantages of a Large Repertoire

• Diversity impresses females (mate choice)
• Diversity promotes territory defense
• Prevents habituation
• Enables song matching
• Dominance based on repertoire size
• Promotes mate acquisition, territory defense
• Evidence males largest repertoire have highest
  reproductive success, survival
• Mimicry is extreme case, result of strong
  selection for large repertoire size

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