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

Abstract

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 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.

Overcoming a ‘forbidden phenotype’: the parrot's head supports, propels and powers tripedal locomotion

“…These data may be compared to recent climbing experiments in parrots reported by [4], with two important notes regarding experimental differences between studies. First, unlike our continuous, 90° vertical substrate, parrots in [4] were observed climbing an instrumented ladder rail at an 81.5° angle.…”

“…One well-known, but anecdotal [6,11,12], example of tripedal locomotion is the climbing behaviours of parrots (Order: Psittaciformes) [1,6]. Unable to use their wings as grasping forelimbs, parrots have evolved to be resourceful climbers by co-opting their feeding apparatus as an additional ‘limb’ [4]. This tripedal gait has recently been shown to account for 5.23% of substrate-based locomotion in wild Psittaciformes [13].…”

Positional Behavior of Introduced Monk Parakeets (Myiopsitta monachus) in an Urban Landscape

“…Among birds, the parrots (Order: Psittaciformes) are a primarily arboreal lineage [ 26 , 27 , 28 ] that have evolved numerous anatomical features well-known to be associated with arboreal locomotion [ 29 , 30 , 31 ]. Briefly, these include long zygodactylous digits for grasping around supports [ 30 , 32 ], a distal elongation of the penultimate phalanx associated with a shortening of the proximal phalanges [ 32 , 33 , 34 , 35 ] that has been posited to increase grasping force [ 32 , 36 ], high mobility at the hip joint, and digital and tarsometatarsal pads that are textured and highly sensitive [ 37 ]. Further, Psittaciformes tend to have relatively short tarsometatarsi compared to other avian species [ 31 ].…”

“…This morphology serves to reduce limb length, allowing the animal to maintain closer contact to the substrate, thereby decreasing the gravitational pitching moment during climbing and rolling torques on thin arboreal substrates [ 38 ]. Parrots have also co-opted the feeding system and neck musculature to function as a third limb during climbing, an exaptation completely unique to the order [ 37 , 39 ]. Despite the well-known arboreal tendencies of parrots, there remains limited information about the way these animals exploit their environment in terms of locomotion and postures.…”

In vivo bite force in lovebirds (Agapornis roseicollis, Psittaciformes) and their relative biting performance among birds

“…Indeed, parrot bites are well known to cause severe injuries often requiring medical intervention (Dotson & Mullen, 2019; King et al., 2015). Moreover, parrots are an ancient arboreal lineage and exhibit numerous adaptations toward tree climbing including co‐opting their beak as a key component of their locomotor system when scaling vertical surfaces (Reader et al., 2022; Young et al., 2022). Thus, an improved understanding of relative biting performance within parrots may inform interpretations of both feeding and locomotor adaptations.…”