# Stroke Asymmetry in Bird Wing Dynamics During Flight from Video Data

## TL;DR

This study uses video and AI to show that birds' wing movements during flight are asymmetric, with differences in speed and wing area that affect flight efficiency.

## Contribution

The study introduces a novel method using DeepLabCut to quantify stroke asymmetry and effective wing area variation in free-flying birds.

## Key findings

- Wing oscillations show statistically significant velocity differences between upstroke and downstroke phases.
- Frigatebird wing area varies by ~19% during the wingbeat cycle, impacting lift generation efficiency.

## Abstract

The aerodynamics of avian flight provides critical inspiration for the design of bioinspired aerial vehicles, yet the quantitative characterization of free-flight wing kinematics remains challenging. This study employs a neural-network-based motion tracking approach (DeepLabCut) to analyze wingbeat kinematics in free-flying birds from video data. We automatically digitize key wing points and reconstruct three-dimensional trajectories to quantify asymmetric flapping patterns. Our analysis reveals that while wing oscillations approximate sinusoidal motion, they exhibit statistically significant velocity differences between upstroke and downstroke phases, confirming the stroke asymmetry of avian flapping. Furthermore, using video of a flying frigatebird (Fregata ariel), we quantify the changes in the effective wing area throughout the wingbeat cycle, showing a ~19% variation that significantly impacts lift generation efficiency. These findings provide quantitative benchmarks for avian-inspired wing design and offer insights for optimizing flapping kinematics in bioinspired aerial systems, particularly for enhancing takeoff and landing capabilities in micro air vehicles.