Aerodynamics of Slow Flight & Acceleration
In almost all birds, flight plays a critical role in survival, whether in foraging, attracting mates, defending territories, migrating, or avoiding predators. In order to accomplish all of these tasks in flight, a bird must first take-off and accelerate through a slow phase to reach a comfortable cruising speed. While crucial, slow flight is more energetically demanding than fast flight, and often a bird operates at its aerodynamic limits. With modern flow-visualization techniques, I am exploring the time-varying wake of birds in slow flight, and have established that a wingtip-reversal upstroke produces a unique aerodynamic signature that likely promotes additional force production during slow flight.
Image: Air velocities around a Diamond dove, taken using Particle Image Velocimetry.
Aeroecology: Flight in free-living birds
My research program aims to directly measure performance of birds in an ecological context, within the new sub-field of aeroecology - wherein the air itself is recognized as a habitat with it's own suite of selective pressures. Modern technologies allow us to quantify air in detail (pictured, sonic anemometer used in Warrick et al. 2016), coupled with high-speed videography. As part of a field-based team, we have discovered that wild barn swallows utilize wind gradients during foraging flights.
Image: Sonic anemometer used to detect minute changes in wind speed and direction, as used in Warrick et al., 2016
Performance consequences of natal environment
Development of locomotor morphologies and abilities varies in tremendously across the avian world (from precocial chickens to highly altricial hummingbirds), and can have sustained impacts on adult performance. We have recently established that the natal environment (specifically, clutch size) has lasting effects on the adult flight ability in a captive bred population of Zebra finches. Further, adult flight performance is positively correlated with fecundity, and thus may be a reliable measure for fitness (see Crino et al., 2017).
Image: Zebra Finches in nest
 While slow flight is the most energetically demanding form of flight, birds are capable of mediating take-off flight with propulsive leg forces. In general, it is easier to propel from a solid surface than air or water - and so birds rely heavily on their legs during the take-off phase, modulating their wings to produce additional forces following take-off. My research focuses on trade offs between force production by the legs and wings, as well as the interplay of natural materials on the ability to accelerate during take-off.
Collaborators: Pauline Provini, Anick Abourachid
Image: Transitional wingbeats, from Provini et al. 2012
The avian wing has specialized morphologically via fusion and reduction of skeletal and muscular elements in order to reduce distal limb mass. Surprisingly, then, despite a hypothesized four-bar linkage system (wherein action of only the proximal muscles allows the distal muscles to flex and extend), the bird remains capable of bending and twisting the distal hand-wing to provide additional aerodynamic forces during upstroke. From detailed morphological studies, it has been proposed that a majority of wing supination and extension is due to passive bone-bone interactions at the wrist (Vasquez, 1992). To address hypotheses of joint function, I am combining high-speed biplanar flouroscopy with micro-CT reconstructions of skeletal morphologies to recreate a digital model of motion, using XROMM techniques.
Image: 3D reconstruction of a japanese quail skeleton from a mico-CT scan
Unsteady, time-dependent airflow around the wing is critical for take-off, maneuvering, and even landing. Thus, in order to fully understand wing shape we must immediately move beyond assumptions that it performs as a rigid surface and fully embrace its function as a dynamic, living wing as it performs in nature. However, a large gap in knowledge exists as to how time-varying aerodynamic mechanisms contribute to the morphology and behavior of birds. To improve our understanding of the evolution of flight, I have designed a portable high-speed camera setup to record take-off flights in the wild. By coupling field recordings and habitat analyses with detailed morphological analyses (with the spread wing collection at the Burke Museum, left), I will elucidate correlations between performance, habitat, and wing shape.
Image: Simplified phylogeny of waterfowl wings (average mass inset)
Attachment Ability in Anolis
One of the most diverse genera of vertebrates are found in the anoles, a model group for studies of adaptive radiation and evolutionary convergence. For this reason, they serve as an excellent model clade to explore correlations between morphology, performance, and ecology. A defining characteristic of the Anolis clade is the subdigital toe pad, which consists of expanded modified subdigital scales that facilitate adhesion. It has been suggested that the adhesive toe pads are a key innovation: an important change that allowed for the occupation of niches unavailable to non-clinging vertebrates, thus facilitating the rapid radiation of anoles. Macro-scale toe pad morphology contributes to climbing and attachment performance, and ultimately may contribute to ecological fitness. Established correlations between toe pad morphology and habitat use support this association.
My research explores the potential mechanical and ecological tradeoffs in adhesive pads and claws by exploring organismal-level performance and morphology in relation to habitat use.
Image: Anolis sagrei hanging by a single toe-pad to a glass microscope slide
Micro-morphology and performance of biological adhesive structures
The subdigital toe pad has evolved in three genera of squamates (geckos, anoles, and skinks), and consist of subdigital scales that contain millions of microscopic hair-like structures (setae) that facilitate adhesion via molecular-level van der Waals forces. Mechanical models of van der Waals interaction suggest that geometry can dictate the limits of setae function; We are exploring the association between variation in microstructure morphologies (from Anolis sagrei, right) and performance. Exploring morphologies across species allows us to infer the evolutionary dynamics as well as establish general design principles.
Image: Microstructure of the anoline toe-pad: sub-digital adhesive scales (lamellae) contain microscopic hair-like structures (setae), with flattened tips (spatulae)
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