Even casual observations of flying birds, bats and insects reveal the adept and seemingly effortless ability of these creatures to land and take off safely from a wide variety of surfaces, whether these are tree branches, telephone wires, flowers or rocks. By contrast, passenger aircraft usually require long, flat runways to accomplish the same feats, and, even so, accidents can occur during take-off or landing. With the rise in the use of aerial drones for a range of applications1–4, and the challenge of improving the aerodynamics and energy efficiency of drones, given their small size5, 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.
Previous work7 has examined how vertebrates such as birds, bats and terrestrial mammals grip surfaces, by studying their feet and claws. This work has relied mainly on approaches such as comparative morphological analyses to assess foot, toe and claw geometry, studies of animal motion (termed descriptive kinematics) or static tests of grip strength. Such methods have shown, for example, how claw shape varies depending on an animal’s size and claw use during its usual patterns of movement in its natural surroundings. For example, claws that are commonly used for running on the ground and manoeuvring usually have greater depth and are less curved than claws typically used for climbing. However, what has been lacking are studies of the dynamics and the forces that enable an animal to use its feet and claws to establish a stable support on landing, such as when birds perch.
Pacific parrotlets are tree-dwelling birds native to mountain forests of Ecuador and Peru. Roderick et al. studied how these birds landed (Fig. 1) on seven natural or artificial perches of differing diameters and textures, including rough, soft and slippery surfaces. Branches of three types of tree were tested, including one called a silk floss (Ceiba speciosa), found in the birds’ natural habitat.
To independently monitor the front and rear of the landing surface of a perch, the authors designed split perches so that each half was anchored separately to a force and torque sensor that recorded the timing and features of the landing force and the rotational force experienced by the birds; both forces are influenced by the landing approach. The authors also measured the squeezing forces produced by the birds’ feet and claws on landing. Combining these measurements with close-up, high-speed video recordings of the landing movements of the bird’s wings, body, legs, feet and claws provided detailed information about the landing events associated with achieving a stable perch (see videos from the paper at go.nature.com/2nbfhtq and go.nature.com/2perfs9).
The authors report that the birds approached their landing on any given perch in a consistent fashion in terms of the movements of their wings and legs, with the landing and rotational forces varying uniformly over the time frame of each landing process. Such a landing strategy is consistent with earlier work8,9 indicating that birds and insects approach a landing target using visual cues to accurately position their body appropriately for the estimated time when they will make contact with the landing surface.
This initial predictive phase of landing is followed by a rapid adjustment phase. It probably involves what is termed proprioceptive feedback from sensors in the bird’s skin, muscles and joints, and communication with the nervous system, as the bird squeezes the perch, dragging its toe pads and claws across the perch’s surface to achieve a stable grasp. Using laser scans and indentation tests to assess changes in the properties of the perch surface, Roderick and colleagues could relate the friction experienced by the birds’ toes and claws to the animals’ gripping movements, and showed how the movements of the bird’s claws are adjusted to anchor the claws to perches of differing diameters and surface features.
The birds curled their claws more on perch surfaces that were difficult to grasp, such as those of large diameter or that generated low friction on landing, than on easier perches. During this grasping phase, the friction forces experienced by the toes (which are fairly consistent for a given perch type) are subsequently reinforced and are accompanied by less predictable, but higher gripping forces exerted on the perch surface by the claw tip. This strategy provides a stable safety margin for gripping the perch that is comparable to analogous safety margins achieved by snakes10 and robots11, and is greater than the safety margins used by humans to grasp small objects12. Once stabilized on the perch, birds relax their grip, avoiding the unnecessary continued energy cost of muscle activation.
A limitation of Roderick and colleagues’ work is that it did not investigate the role of the nervous system in controlling how gripping establishes a stable landing. The authors report superfast (1–2 milliseconds) initial anchoring movements of the claws, which suggests that these might be rapid, intrinsic, elastic mechanisms that do not involve neural control. However, these superfast movements are followed by longer-lasting adjustments in toe and claw movements that probably help to establish the stable grasp allowing birds to then relax their grip. These slower adjustments probably require proprioceptive feedback through the nervous system. Such feedback control could be evaluated by recording muscle activation and force patterns over the course of landing and perching. Inhibiting the activity of the mechanosensory receptors in a bird’s toe pads with an anaesthetic would offer a way to determine whether the loss of sensory feedback from toe pads affects these foot movements and the bird’s landing ability.
The landing flights in this study were short and were made between perches on the same horizontal level. However, Pacific parrotlets probably fly to perches above or below the animal’s current location when foraging. It would therefore be interesting to examine whether body orientation and landing forces vary depending on the trajectory of landing flights. Perhaps such flights might show less consistent patterns in the early stages of the landing process than were found by the authors. Nevertheless, Roderick and colleagues’ detailed biomechanical analysis provides an important road map for future work on how feet, toes and claws enable animals to grip surfaces stably.
Nature 574, 180-181 (2019)