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 ...