A velocity-loop view of avian gait: hodograph rotation sense characterizes the walking-to-running continuum in guinea fowl.

Terrestrial gaits are usually treated as discrete categories, yet the criteria that define them disagree in ground birds, which use grounded-running, bouncing dynamics without an aerial phase, over much of their speed range. We asked whether the rotation sense of the center-of-mass velocity loop, the hodograph, resolves gait-transition speeds in guinea fowl (Numida meleagris). We reconstructed center-of-mass motion from ground reaction forces over 2580 level overground steps (dimensionless speed range: 0.14 to 3.27). Hodograph rotation sense reverses from pendular to bouncing at a crossover speed of 0.58, close to 0.56 from kinetic-potential energy congruity and well below 1.06 at which aerial phases appear. Six established gait descriptors show the same structure: one principal component explains 50% of their variance and describes a pendular-to-bouncing continuum, and clustering agrees with hodograph rotation sense on 87.7% of steps. Gait in guinea fowl is a continuum whose most meaningful division is walking to grounded running, which can be determined based on one signed scalar per cycle from the velocity trajectory. The approach requires no measurement of height, making it less sensitive to integration drift, which may open opportunities for comparative gait studies in freely ranging animals equipped with inertial sensors.