Northern gannets, like many seabirds, like to eat fish, but highly sought prey such as mackerel live well below the water's surface. It's exhausting for the birds to swim so far. Instead, gannets let gravity do the work: they dive from up to 40 meters above, reaching depths of around 11 meters without so much as a waggle of their feet.
Northern gannets are masters of this art. "I am regularly impressed by the way in which these animals forage," enthuses Samantha Cox, a PhD candidate specializing in seabird foraging behavior at Plymouth University in the UK. "They hit the water at extremely fast speeds with great precision and in a highly streamlined position."
In fact, these birds are hitting the water at a staggering 85 kilometers per hour.
Handled incorrectly, a high-speed crash into water can be devastating. But as evolution would have it, northern gannets are well-adapted for their high-impact lifestyle, possessing airbag-like structures in their lower body, and covered nostrils to prevent water shooting into their nasal passages. Jung, however, was curious about what he saw as an obvious weak spot: their long, slender necks. Why, he wondered, don't gannets' necks buckle and fracture as they enter the water?
To find out, his team froze a bird into its diving position and then repeatedly released it into a tank of water. From high-speed videos, they were able to rule out direct hydrodynamic forces acting on the neck, as they could clearly see protective air cavities forming around it upon entry. Instead, in the next phase of the experiment they focused on how much force acts on the bird's head, and, thus, on its neck.
The team produced model gannets using elastic beam "necks" and cone "heads" of varying lengths and proportions. These models were sent plunging into the water at different speeds.
Together with an anatomical analysis of the frozen gannet, the team's simplified models demonstrated that injuries are prevented through a combination of the bird's optimized neck length, streamlined beak, and neck muscles that contract just before impact. Incredibly, it appears that the gannet's neck could withstand speeds of up to 288 kilometers per hour before it buckled. This suggests these birds are quite conservative, diving three times slower than they feasibly could.
"Most biological systems try to have some sort of safety factor," explains Jung.
Despite the evolutionary adaptations, diving is not entirely risk-free, even for such supremely adapted birds. Back in 2011, nutritional ecologist Gabriel Machovsky-Capuska observed the carnage that can result from mass dives, with video footage and autopsies of New Zealand gannets revealing dozens of birds colliding underwater and sometimes impaling one another with their sharp beaks.
Unless gannets want to end up like their frozen friend, playing it safe seems like an advisable option when taking a plunge.