There are a few birds that have the remarkable ability to walk or run across the surface of water without sinking. This feat requires highly specialized adaptations that allow them to distribute their weight and avoid breaking the water’s surface tension.
The Basics of Walking on Water
Normally, the weight of an object is too much for the surface tension of water to support, so the object sinks. For an animal to walk on water without sinking, its legs and feet need special properties to allow it to distribute its weight over a large surface area and avoid puncturing the water’s surface. The animal’s legs also need to move rapidly to keep its feet above the water between steps.
Only a handful of birds have evolved the right combination of traits for water-walking. Wading birds in the family Rallidae, which includes coots and moorhens, are the most common water-walking birds. However, other birds like the Western grebe can also run across the water for short distances.
Adaptations for Water-Walking
Coots and moorhens have several key adaptations that enable them to walk on water:
- Wide, flat feet distribute their weight over a large surface area to avoid sinking.
- Long toes and flexible foot joints allow them to splay their toes widely.
- A light skeleton makes them very buoyant.
- Densely packed feathers help repel water.
- High stepping gaits and fast foot speed keep their feet on the surface.
Their feet have lobes on the toes that flatten out when splayed, creating a larger surface that sits right on the water’s surface tension. When they retract their feet, the lobes close up to cut through the water. Their lightweight skeletons and dense plumage also help make them buoyant.
The Jesus Christ Bird
One water-walking species, the Purple Swamphen, has gained notoriety for appearing to walk on water like Jesus Christ. Its scientific name is Porphyrio porphyrio, but it’s known by many other names including:
- Purple gallinule
- Purple moorhen
- Allen’s gallinule
- “Jesus Christ bird”
This bird inhabits wetlands worldwide in Africa, southern Europe, and southern Asia down to Australasia. It has brilliant blue plumage on its head, neck, back, and underparts, with a purple-red breast and red bill and legs. The striking coloration and its ability to seemingly walk on water like the biblical Jesus Christ inspired its nickname.
The Western Grebe’s Water Ballet
While not technically walking, the Western grebe performs an elaborate synchronized “water dance” with its mate that allows it to run across the water’s surface:
- To lift their bodies out of the water, the grebes flap their feet and wings furiously under the water.
- Once enough speed is generated, they rise up and skim across the water on both feet.
- Mates mirror each other’s movements, nearly running over the water in a dazzling courtship display.
Interestingly, young grebes cannot perform the water dance. They must practice underwater wing-flapping extensively before successfully running across the water’s surface like their elders.
Other Water-Walking Birds
While coots, moorhens, and grebes are the most accomplished water-walkers, a few other birds demonstrate limited water-walking abilities:
Gulls
Gulls sometimes can run or take a few walking steps across calm water while flapping their wings hard. However, they cannot smoothly walk for more than a step or two.
Raptors
Birds of prey like ospreys may splash across the water’s surface briefly when taking off from the water with prey. Their broad wings provide lift, while their feet skim the water momentarily.
Herons
Wading birds like herons sometimes appear to run across the water while spreading their wings to gain speed and lift their bodies higher to avoid getting wet. However, they do not truly walk on the water’s surface.
Physical Explanations
The physics behind water-walking relies on the property of surface tension. Water molecules are polar and attracted to each other, creating an invisible “skin” across the liquid’s surface. This allows lightweight creatures to essentially sit on top of this surface skin without sinking if they distribute their weight properly.
For a creature to walk on water, the force of its weight must be less than the upward force supplied by the water’s surface tension. The surface tension (F) depends on the coefficient of surface tension (γ) and the length of the contact perimeter (l):
F = γ * l
Birds increase l by splaying their toes widely so the perimeter of their feet presses down across a large area. Meanwhile, their lightweight skeletons and feathers decrease their body weight. By optimizing these traits, they can achieve F > body weight and remain above the surface.
Surface Tension Demonstration
You can observe surface tension at work by floating light objects on water. Carefully place a razor blade, needle, or paperclip flat on the water’s surface. Even heavy metal objects will float this way due to surface tension!
Conclusion
Only a select group of birds possess the remarkable ability to seemingly walk on water. Their specialized adaptations allow them to distribute their weight and stride quickly across the surface, taking advantage of the water’s surface tension. Next time you see a purple gallinule strutting across a wetland, you’ll know the physics behind this Jesus Christ “miracle” bird!