Repeated evolution of drag reduction at the air–water interface in diving kingfishers

Abstract

Piscivorous birds have a unique suite of adaptations to forage under the water. One method aerial birds use to catch fish is the plunge dive, wherein birds dive from a height to overcome drag and buoyancy in the water. The kingfishers are a well-known clade that contains both terrestrially foraging and plunge-diving species, allowing us to test for morphological and performance differences between foraging guilds in an evolutionary context. Diving species have narrower bills in the dorsoventral and sagittal plane and longer bills (size-corrected data, n ¼ 71 species, p , 0.01 for all). Although these differences are confounded by phylogeny ( phylogenetically corrected ANOVA for dorsoventral p ¼ 0.26 and length p ¼ 0.14), beak width in the sagittal plane remains statistically different ( p , 0.001). We examined the effects of beak morphology on plunge performance by physically simulating dives with three-dimensional printed models of beaks coupled with an accelerometer, and through computational fluid dynamics (CFD). From physically simulated dives of bill models, diving species have lower peak decelerations, and thus enter the water more quickly, than terrestrial and mixed-foraging species (ANOVA p ¼ 0.002), and this result remains unaffected by phylogeny (phylogenetically corrected ANOVA p ¼ 0.05). CFD analyses confirm these trends in three representative species and indicate that the morphology between the beak and head is a key site for reducing drag in aquatic species. Halcyon coromanda Halcyon

Morphological innovation and biomechanical diversity in plunge‐diving birds

“…Similar results are reported by Crandell et al. (2019) after correcting for body size. However, they showed that beak width was the only trait with significant difference between foraging regimes after phylogeny was taken into account.…”

“…Predicted impact forces were significantly correlated with recent empirical values (Crandell et al. 2019) of deceleration from 3D printed beaks (PGLS; P = 0.0061, N = 25; see Fig. S13).…”

“…Recently, Crandell et al. (2019) used physical models and computational fluid dynamics (CFD) to test whether the beaks of piscivorous kingfishers have evolved reduced drag compared to other, primarily terrestrial species. However, this study did not aim to investigate relationships between the outer layer of keratin (rhamphotheca) and the underlying bone (skeletal beak) or how foraging behavior might affect rates of beak evolution.…”

Water impact of a surface-patterned disk

“…Birds such as northern gannets and kingfishers dive into water to catch prey (Chang et al. 2016; Crandell, Howe & Falkingham 2019). These interesting biological behaviours have inspired the development of water-running robots and shape-morphing aerial unmanned vehicles (Floyd et al.…”

3D Printing for Ecology and Evolution: A Hands-on Guide to Turn an Idea into Reality

“…In functional ecology, key questions often involve studying the impact of specific morphological properties of the organism (or part of it). For example, heads of bat species were scanned and 3D printed for their aerodynamical properties to be tested in a wind tunnel (Vanderelst, Peremans, Razak, Verstraelen, & Dimitriadis, 2015); similarly, 3D printed beaks of kingfishers were used to study the evolution of drag reduction at the air-water interface (Crandell, Howe, & Falkingham, 2019). 3D printing furthermore allows to go one step further than replicating biological material: creating highly controlled variants in order to manipulate morphological features independently.…”