D4 Accelerometry

Topic

A miniaturized triaxial accelerometer, mounted on the bird's back or tail, allows for the reconstruction of wing-beat kinematics without the need for direct observation or bringing the bird into a laboratory. This device is the standard method for studying wing-beat kinematics under natural conditions, while the bird flies freely in its own environment.

The dorsoventral axis of this miniaturized triaxial accelerometer records the bird's body acceleration during each wing-beat cycle, generating a signal that rises and falls in rhythm with each stroke. Applying a Fourier transform to this signal yields the wing-beat frequency with high precision, while measuring the signal's amplitude allows for the estimation of the wing-beat angle—the angular amplitude at which the bird moves its wings during each cycle.

Based on the wing-beat frequency and angle derived from this signal, the Strouhal number is calculated; this parameter compares the frequency and amplitude of each stroke cycle against the bird's forward speed, indicating whether the wing-beat remains propulsively efficient. In practice, the Strouhal number calculated from the accelerometer signal serves as a direct functional diagnostic of the bird's propulsive efficiency, obtained without the need to observe it in flight.

This method is sensitive to two distinct anomalies, each leaving a unique signature on the accelerometer signal. The first is muscle damage: when the bird's pectoral muscle is damaged, both the wing-beat frequency and the wing-beat angle decrease relative to the values ​​the same individual would exhibit when healthy; consequently, the Strouhal number calculated using these reduced values ​​falls outside the optimal range associated with propulsively efficient wing-beats. The second anomaly is a neuromuscular disturbance: in this case, while the average wingbeat frequency and angle may remain within normal ranges, an asymmetry emerges in the duration of each wingbeat half-cycle, meaning the bird takes a different amount of time to complete the upstroke and downstroke phases of the cycle. This half-cycle timing asymmetry is the signature left on the signal from the miniaturized triaxial accelerometer by a nervous system that no longer coordinates the wingbeat with the same temporal precision as a healthy one.

These two distinct signal signatures—the simultaneous drop in wingbeat frequency, wingbeat angle, and Strouhal number on the one hand, and the half-cycle timing asymmetry on the other—make it possible to distinguish, using a single miniaturized triaxial accelerometer, whether a bird exhibiting anomalous wingbeats is suffering from muscle damage or a neuromuscular disturbance. Furthermore, by comparing the signal from the miniaturized triaxial accelerometer against predefined thresholds, the bird's current flight mode can be automatically classified—a direct application of this accelerometry method.

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