How seabirds plunge-dive without injuries

Abstract

In nature, several seabirds (e.g., gannets and boobies) dive into water at up to 24 m/s as a hunting mechanism; furthermore, gannets and boobies have a slender neck, which is potentially the weakest part of the body under compression during high-speed impact. In this work, we investigate the stability of the bird's neck during plunge-diving by understanding the interaction between the fluid forces acting on the head and the flexibility of the neck. First, we use a salvaged bird to identify plunge-diving phases. Anatomical features of the skull and neck were acquired to quantify the effect of beak geometry and neck musculature on the stability during a plunge-dive. Second, physical experiments using an elastic beam as a model for the neck attached to a skull-like cone revealed the limits for the stability of the neck during the bird's dive as a function of impact velocity and geometric factors. We find that the neck length, neck muscles, and diving speed of the bird predominantly reduce the likelihood of injury during the plunge-dive. Finally, we use our results to discuss maximum diving speeds for humans to avoid injury.diving | seabirds | buckling | injury | water entry N ature contains several species of creatures that interact with the air-water interface (1). A number of bird species are able to dive into water from the air as a hunting mechanism (e.g., kingfishers, terns, and gannets), a behavior known as plunge-diving (2, 3). Some seabirds, like the northern gannet, are highly specialized plunge-divers, making 20-100 dives per foraging trip, diving from heights of 5-45 m, and attaining speeds of more than 20 m/s (4-7). Thus, the bird's structure and behavior have presumably evolved to withstand a variety of high dynamic stresses, because no injuries have been reported in plunge-diving seabirds. Biologists have previously focused on the diving behavior in terms of ecological factors, such as diving depths, prey species, and hunting success rate (8-10), and physiological features, such as the role of vision while crossing the air-water interface (11,12). Unique kinematic and morphological features during the dive have also been observed, such as having a sharp, arrow-like body posture and a straight, long, and slender neck (13,14). However, a mechanical understanding of plunge-diving birds is not well-established.To study such a phenomenon, Morus bassanus (hereafter gannets) and Sula leucogaster (hereafter boobies), from the Sulidae family, are used as a model species due to their highly specialized diving characteristics (5, 13). First, they plunge-dive at very high speeds, using that momentum to carry them to some depth. Then, they use their webbed feet and/or wings to propel themselves further underwater, like penguins and cormorants (15, 16). Although plunge-diving at high speeds allows the bird to dive deeper, it induces much larger stresses on the seabird's body than pursuit diving alone (13). The two main forms of plunge-diving observed are known as the V-shaped dive and the U-shaped dive (5). During ...

Search and foraging behaviors from movement data: A comparison of methods

“…Northern gannets ( Morus bassanus) , hereafter gannets, are a well‐studied species that occur principally in the temperate shelf seas of the North Atlantic during the breeding season. Gannets are visual predators (Cronin, ) and undertake plunge‐diving from height, entering the water at speeds of up to 24 m/s (Chang et al., ). Prior to diving, gannets typically slow their flight and increase their path sinuosity (Wakefield et al., ; Bodey et al., ; Patrick et al., ; Warwick‐Evans et al., ).…”

“…Northern gannets (Morus bassanus), hereafter gannets, are a wellstudied species that occur principally in the temperate shelf seas of the North Atlantic during the breeding season. Gannets are visual predators (Cronin, 2012) and undertake plunge-diving from height, entering the water at speeds of up to 24 m/s (Chang et al, 2016).…”

How localized force spreads on elastic contour feathers

“…There are three notable phases during a plunge-dive: (1) the impact phase, (2) the air cavity phase and (3) the submerged phase [16]. It was shown during the impact and air cavity phase that the bird's neck is at greater risk of injury [17]. While northern gannets typically dive at around 24 m s −1 in nature [14], their hypothetical maximum diving speed would be up to 80 m s −1 before neck injuries due to the morphological properties of the skull and strength of neck muscles [17].…”

“…It was shown during the impact and air cavity phase that the bird's neck is at greater risk of injury [17]. While northern gannets typically dive at around 24 m s −1 in nature [14], their hypothetical maximum diving speed would be up to 80 m s −1 before neck injuries due to the morphological properties of the skull and strength of neck muscles [17]. Near the end of the second phase, the air cavity closes, or pinches off, and the bird's upper chest at the shoulder girdle hits water at high speeds (figure 1).…”

Water entry of deformable spheres

“…Both of these methods generate asymmetrical cavities and cause the impacting body to veer from the primary axis of travel. Some groups have extended the work to biological organisms, for instance, Chang et al (2016) experimentally investigated plunge-diving birds using a simplified model. Their experiment involved an elastic beam attached to a rigid cone (representing the bird neck and head, respectively), and focused specifically on when buckling occurs as it relates to possible physical damage, as opposed to how significant deformations affect cavity shape and entry dynamics as discussed herein.…”