Birds land reliably on complex surfaces by adapting their foot-surface interactions upon contact

Abstract

Birds land on a wide range of complex surfaces, yet it is unclear how they grasp a perch reliably. Here, we show how Pacific parrotlets exhibit stereotyped leg and wing dynamics regardless of perch diameter and texture, but foot, toe, and claw kinematics become surface-specific upon touchdown. A new dynamic grasping model, which integrates our detailed measurements, reveals how birds stabilize their grasp. They combine predictable toe pad friction with probabilistic friction from their claws, which they drag to find surface asperities—dragging further when they can squeeze less. Remarkably, parrotlet claws can undergo superfast movements, within 1–2 ms, on moderately slippery surfaces to find more secure asperities when necessary. With this strategy, they first ramp up safety margins by squeezing before relaxing their grasp. The model further shows it is advantageous to be small for stable perching when high friction relative to normal force is required because claws can find more usable surface, but this trend reverses when required friction shrinks. This explains how many animals and robots may grasp complex surfaces reliably.

A Bio-Inspired Manipulator with Claw Prototype for Winged Aerial Robots: Benchmark for Design and Control

“…Some previous works have studied the morphology of bird feet and claw shape in relation to body size and lifestyle [20,21]. Other works have studied the foot-surface interaction when the birds perch [22], this explains how some animals may grasp complex surfaces reliably. The grasping force planar model proposed in [22] does not take into account lateral forces out of the plane, and hence only being valid once the bird's claws are fully wrapped.…”

“…Other works have studied the foot-surface interaction when the birds perch [22], this explains how some animals may grasp complex surfaces reliably. The grasping force planar model proposed in [22] does not take into account lateral forces out of the plane, and hence only being valid once the bird's claws are fully wrapped. In summary, the grasping force: (1) it is composed of the friction generated by toe pad and the strength of the force exerted by the claw; (2) it can be modeled as a static friction; (3) it depends on the contact surfaces at perch (surface and claw) as well as their orientation, texture, geometry, etc.…”

Getting to grips with how birds land stably on complex surfaces

“…With the rise in the use of aerial drones for a range of applications [1][2][3][4] , and the challenge of improving the aero dynamics and energy efficiency of drones, given their small size 5 , there is interest in developing drone design to boost their success in landing on a range of complex surfaces. Writing in eLife, Roderick et al 6 report their analysis of how Pacific parrotlets (Forpus coelestis) land on different types of perch, providing insights into the landing approach taken by these birds.…”

“…Force measurements and video-footage analysis now reveal that birds rely on rapid and robust adjustments of their toe pads and claws to land stably. 6 analysed perching using methods to assess the forces that a bird encounters during landing, and by studying high-speed video recordings. a, When a bird is about to land, its wings, body and legs are positioned in the same, predictable way, consistent with earlier work 8,9 suggesting that birds use visual cues to position themselves for landing.…”

“…For example, these technologies have enabled digital assays 4 that can measure the concentration of specific genes in a sample without calibration. They are also key to the single-cell genetic-sequencing techniques 5 currently used in the Human Cell Atlas, a project that aims to characterize every cell type in the human body 6 . Furthermore, microfluidics technologies are powering a wave of new point-of-care systems that bring diagnostic assays closer to the patient's bedside 7 .…”