While beside a creek, pond, or at the ocean, you may have been fortunate to spy a kingfisher. Found the world over, kingfishers are a family of birds containing over 100 species, often visiting bodies of water, where, as their name suggests, they are masters at catching fish.
The method kingfishers use to catch fish seems simple enough. Once a kingfisher spies a fish (using special glare-reducing cells in its eyes), it leaves a perch and plunges into the water to grab it in its beak. Fish, however, have a defensive strategy that is hard to overcome. Specialized receptors along a fish’s body, known as a lateral line, sense disturbances in the flow of surrounding water. Any sudden movement of water, such as a compression wave from a diving bird, and fish are gone with a flick of the tail. If you’ve ever tried to catch a fish with your hands, you know how difficult it is to escape their detection as soon as your hand touches the water.
So how does a kingfisher, hitting the water’s surface at over eleven meters per second, manage to grab a fish in its beak before the fish detects it and flees? The secret is in the shape of the kingfisher’s beak. A long and narrow cone, the kingfisher’s beak parts and enters the water without making any splash or creating a compression wave. This buys the bird crucial milliseconds to reach the fish before the fish knows to flee. The longer the beak, the smaller the angle of the wedge hitting the water, reducing impact. A shorter, fatter, or rounder beak would increase the wedge angle striking the water, resulting in a splash and a compression wave.
Eiji Nakatsu, chief engineer of the company operating Japan’s fastest trains, wondered if the kingfisher’s beak might serve as a model for how to redesign trains to run more quietly and efficiently. Sure enough, as his team tested different shapes for the front of the new train, the train became quieter and more efficient as the geometry of its nose became more like the shape of a kingfisher’s beak, requiring 15% less energy while traveling even faster than before.
“[W]e had another challenge that we pursued to the test run phase. Half of the entire Sanyo Shinkansen Line (from Osaka to Hakata) is made up of tunnel sections. When a train rushes into a narrow tunnel at high speed, this generates atmospheric pressure waves that gradually grow into waves like tidal waves. These reach the tunnel exit at the speed of sound, generating low-frequency waves that produce a large boom and aerodynamic vibration so intense that residents 400 meters away have registered complaints. For this reason, we gave up doing test runs at over 350 km/h.
“Then, one of our young engineers told me that when the train rushes into a tunnel, he felt as if the train had shrunk. This must be due to a sudden change in air resistance, I thought. The question the occurred to me – is there some living thing that manages sudden changes in air resistance as a part of daily life?
“Yes, there is, the kingfisher. To catch its prey, a kingfisher dives from the air, which has low resistance, into high-resistance water, and moreover does this without splashing. I wondered if this is possible because of the keen edge and streamlined shape of its beak.
“So we conducted tests to measure pressure waves arising from shooting bullets of various shapes into a pipe and a thorough series of simulation tests of running the trains in tunnels, using a space research super-computer system. Data analysis showed that the ideal shape for this Shinkansen is almost identical to a kingfisher’s beak.
“I was once again experiencing what it is to learn from Nature, seeing first hand that a solution obtained through large-scale tests and analysis by a state-of-the-art super-computer turned out to be very similar to a shape developed by a living creature in the natural world. The nose of our new 500-Series Shinkansens has a streamline shape that is 15m in length and almost round in cross section.
“This shape has enabled the new 500-series to reduce air pressure by 30% and electricity use by 15%, even though speeds have increased by 10% over the former series. Another benefit has been confirmed through a favorable reputation among customers that these trains give a comfortable ride. This is due to the fact that changes in pressure when the trains enter tunnels are smaller.” (Kobayashi: 2005)
Shinkansen Technology Learned from an Owl? The story of Eiji Nakatsu
“Our data show that diving kingfishers have morphological adaptations associated with aquatic foraging. Further, aquatic foraging species’ beak shapes produce less hydrodynamic drag than terrestrial species, measured as lower peak deceleration during the impact with the water, and as drag force in CFD simulations. Collectively, we find evidence that supports adaptations for improved diving performance in aquatically foraging kingfishers relative to terrestrial and mixed-foraging species.” (Crandell et al. 2019: 7)
“Notably, no apparent bow wave, where water is pushed forward in front of the animal, appears at the tip of the Ceyx or Ceryle kingfisher bills in the CFD [computational fluid dynamics] simulations.” (Crandell et al. 2019: 8)
Repeated evolution of drag reduction at the air–water interface in diving kingfishersInterface
“Here, we investigate the impact forces and splash evolution of wedges entering water as a function of the wedge opening angle. A gradual transition from impactful to smooth entry is observed as the wedge angle decreases.” (Vincent et al. 2018: 508)