Climbing parrots achieve pitch stability using forces and free moments produced by axial-appendicular couples.

During vertical climbing, the gravitational moment tends to pitch the animal's head away from the climbing surface and this may be countered by (1) applying a correcting torque at a discrete contact point, or (2) applying opposing horizontal forces at separate contact points to produce a free moment. We tested these potential strategies in small parrots with an experimental climbing apparatus imitating the fine branches and vines of their natural habitat. The birds climbed on a vertical ladder with four instrumented rungs that measured three-dimensional force and torque, representing the first measurements of multiple contacts from a climbing bird. The parrots ascend primarily by pulling themselves upward using the beak and feet. They resist the gravitational pitching moment with a free moment produced by horizontal force couples between the beak and feet during the first third of the stride and the tail and feet during the last third of the stride. The reaction torque from individual rungs did not counter, but exacerbated the gravitational pitching moment, which was countered entirely by the free moment. Possible climbing limitations were explored using two different rung radii, each with low and high friction surfaces. Rung torque was limited in the large-radius, low-friction condition; however, rung condition did not significantly influence the free moments produced. These findings have implications for our understanding of avian locomotor modules (i.e. coordinated actions of the head-neck, hindlimbs and tail), the use of force couples in vertical locomotion, and the evolution of associated structures.