Fig. 4: Evolution selects for both pitch stability and instability, but modern birds exhibit highly variable pitch agility and stability characteristics.
From: Birds can transition between stable and unstable states via wing morphing
a, b, We derived a pitch agility metric, which highlights that heavier birds are less agile (a) and have a reduced static margin (sm) range (b). Maximum and minimum values for each individual due to elbow and wrist ROM are plotted and the static margin is normalized by each specimen’s maximum root chord (crmax). An Ornstein Uhlenbeck model provided evidence of selection pressures acting on an unstable minimum (dashed line, Min θsm) and a stable maximum (dashed line: Max θsm) static margin, and \({x}_{\bar{{\rm{C}}}{\rm{G}}}\) (θCG). c, d, This \({x}_{\bar{{\rm{C}}}{\rm{G}}}\) position is stable if the neutral point is behind this position (c) and unstable if the neutral point is in front of this position (d). e, The investigated species exhibited a wide variety of static margins and absolute pitch agility. Dot colour and size represent the mean maximum and minimum value, respectively, for each species.
