Module 7: Song & Acoustic Biophysics

1.1 Anatomy & Position

Unlike the mammalian larynx (at the top of the trachea), the avian syrinx is located at the tracheo-bronchial junction — where the trachea bifurcates into two bronchi. Most songbirds have a tracheobronchial syrinx with intrinsic musculature in both bronchial branches, enabling independent bilateral control of the two sound sources.

Key structural elements (using oscine/songbird terminology):

• MTMMedial tympaniform membranes — thin, compliant membranes on the medial wall of each bronchus. The primary vibrating element. Thickness ∼5–20 μm; area ∼1–5 mm\(^2\).
• MLMedial labia — paired fleshy lips adjacent to MTM in some species. Co-vibrate with MTM.
• LLLateral labia — lateral valve controlling airflow. Tension affects both amplitude and frequency.
• RingsTracheal + bronchial rings — cartilaginous support. The pessulus is a central cartilage at the tracheo-bronchial junction.
• MusclesSyringeal muscles (6–9 pairs in oscines) — nXIIts (hypoglossal nerve) innervation. Control membrane tension, glottal aperture, and labial position.

1.2 Sound Production: Bernoulli & Membrane Vibration

Air flow from the air sacs through the bronchi past the MTM/labia follows the Bernoulli principle: in the narrow channel between the membrane and the bronchial wall, flow velocity increases and pressure decreases (the Bernoulli effect), drawing the membranes inward. The restoring elastic force then pushes them back outward, establishing oscillation.

The fundamental frequency of the membrane oscillation is determined by its tension \(T\), area density \(\rho_s\) (mass per unit area), and effective length scale \(L\). For a thin membrane under tension, the resonant frequency:

Frequency modulation during song occurs on timescales of milliseconds. The rate of frequency change depends on muscle force: for a finch producing a frequency sweep from 3–8 kHz over 50 ms:

The required rate of tension change is \(dT/dt \approx 2T f_0^{-1} \times 10^5\) — achievable by fast-twitch syringeal muscles with contraction times ∼5–10 ms.

1.3 Dual Voice: Two Independent Sources

The most striking capability of the oscine syrinx is the production of two simultaneous, harmonically unrelated frequencies. Wood thrushes (Hylocichla mustelina) produce complex chords by using each bronchial half of the syrinx independently. The two sides can operate:

• ● At different fundamental frequencies (independent pitch control)
• ● With different amplitude envelopes (independent volume)
• ● With independent spectral profiles (different harmonic content per side)
• ● With one side producing while the other is silent (unilateral phonation)

Nerve cutting experiments (severing the right or left n.XII branch) silence the corresponding side, confirming independent neural control. The left side tends to dominate in many species (left-hemisphere lateralization for song motor control — analogous to language lateralization in humans).

2.1 Source: Syrinx Harmonic Series

The vibrating membrane generates a harmonic series: the fundamental frequency \(f_0\) plus overtones at \(2f_0, 3f_0, \ldots, nf_0\). For a purely sinusoidal (single-frequency) vibration, only \(f_0\) is produced. The nonlinearity of membrane vibration under Bernoulli forcing generates harmonic distortion, rich in higher partials. The spectrum of the source signal:

2.2 Filter: Tracheal Tube Resonances (Formants)

The trachea acts as a quarter-wave resonator: open at the beak and approximately closed at the syrinx. Resonance condition:

The beak acts as a radiation filter. A closed beak attenuates high frequencies (low-pass), while an open beak emphasizes higher harmonics. Dynamic beak opening during song modifies the effective tube length and hence formant positions:

Air sacs (connected to the syrinx) add parallel resonant cavities, further shaping the spectral output. The complete filter transfer function \(H(f)\) is the product of tracheal, beak, and air sac transfer functions. The output song spectrum:\(P_{\text{out}}(f) = |H(f)|^2 \cdot P_{\text{source}}(f)\).

3.1 Song System Neural Circuits

During sensory phase (∼10–65 days post-hatch in zebra finch), the juvenile memorizes the tutor song into an auditory template in HVC and LMAN. During sensorimotor phase (∼25–90 dpn), the bird practices singing, comparing its output to the template via error-correcting plasticity in the AFP circuit (reinforcement learning). Dopaminergic projections from VTA to Area X signal reward and guide synaptic modification (BDNF/NTRK2-dependent LTP in RA).

3.2 FOXP2: The Language Gene

FOXP2 (Forkhead Box P2) is a transcription factor containing a forkhead DNA-binding domain. It is expressed in HVC and Area X in songbirds, and its expression level changes seasonally (higher during song learning periods). Key findings:

• ●Knockdown of FOXP2 in Area X of zebra finch during sensorimotor learning produces abnormal, inaccurate song that fails to match the tutor template.
• ●The human FOXP2 sequence differs from mouse at only 3 amino acids, but two of these are specific to the human lineage and arose after the human-chimpanzee split (∼6 Mya).
• ●Mutations in human FOXP2 cause a severe speech and language disorder (verbal dyspraxia), establishing it as critical for the fine oral-motor control underlying speech.
• ●FOXP2 regulates downstream targets including CNTNAP2 (axon guidance), SLIT1 (neuronal migration), and MAP1B (synaptic plasticity) in both birds and humans.
• ●Humanized mice (carrying human FOXP2 amino acid substitutions) show altered ultrasonic vocalizations and enhanced basal ganglia synaptic plasticity — suggesting FOXP2 drove changes in motor learning circuitry.