Blog #385 Diving A Perfect 10
A chronophotograph of an Osprey diving, courtesy of Chen Chengguang.
An Osprey’s famed dive is pure artistry. Watch a majestic osprey cut through the wind, averaging 30 mph as it soars across the sky. Suddenly it will stop mid-flight at a height of 30–130 feet to hoover, slowly beating its wings to stay in one place, like a giant kingfisher or hummingbird, while peering down with eyes that can see into the shallows from a hundred feet up. Just a glint or a shadow of a movement below means one thing—a fish. Once a potential prey has been spotted the Osprey hovers briefly and quickly decides whether to dive or move on for a better opportunity. Ospreys can dive without hovering when the opportunity for a potential catch arises.
Once the Osprey commits to the dive, it tucks its folded wings and plunges on a vertical or diagonal trajectory up to 80 mph while still keeping its eyes on the prey, calculating its size and movement, and the refraction of the fish's image in the water. To gain speed or cover the extra distance, the bird may take extra wing beats, bending their wings at the elbows. Their tail acts as a rudder, slowing the dive and helping to pull the bird out of the water. They thrust downward on their tails and upward with their wings to dive. At the last second before hitting the water, he throws his wings back and juts his legs and talons forward ready to strike feet first. This allows the bird to keep the fish in sight throughout the dive, and the feet hitting the water first cuts through the water’s surface and protect the head. Ospreys also have closable nostrils to keep out water during impact. He plunges with a splash, immersing his entire body in the water.
The long, curved talons and spicule-covered feet aid in fishing, courtesy of Chen Chengguang.
Grabbing and trapping the fish in his talons, he pierces it and clutches its slippery, cold-blooded body. Backward-facing scales on the talons and needle-sharp, horny scales known as spicules covering the underside of the foot and lower leg help the Osprey hold on to its catch. The outer talons can rotate forward or backward, much like an opposable thumb, allowing the osprey to align a captured fish parallel to its own body for better aerodynamics in flight. All told, an adult osprey can take off with a fish half the length of its own body provided the fish is less than 25 percent of its 2- to 4-pound body weight.
Fishing Ospreys have an extraordinary success rate, some catching fish on well over fifty percent of their dives. In part, a few particularly adroit birds catch close to ninety percent, in part due to the careful preparatory searches.
Osprey submerges to catch a fish, courtesy of YouTube.
While still submerged, the Osprey secures its hold on the fish using its spicules. t may take a moment or two to fully grasp the struggling fish before the bird takes off. If the attempt fails, the bird will take off quickly. It takes a great exertion for the bird to lift its soaked, heavy body while hoisting heavy prey. Its dense, oily plumage prevents its feathers from getting too heavy and waterlogged. Beating his extremely strong wings furiously in backward half-circles, it rises and shakes off the excess water like a wet dog. The male typically takes the meal to a tree where he can eat the protein-rich head beginning with the collagen-filled lips, after which he brings the remainder of the fish to his family.
Ospreys have an extraordinary success rate at fishing, some catching well over fifty percent of what they dive for, (although a few particularly adroit birds catch close to ninety percent), in part due to the careful preparatory searches. Just as the bird is about to launch, it makes careful final adjustments to the dive. After the bird has tucked its wings and dropped down thirty feet, it may pause and readjust again or abort the dive. Osprey dives require full commitment, abandonment, and immersion.
Their third eyelid—the nictitating membrane—covers their eyes like goggles, permitting excellent vision underwater. The Osprey’s exceptional vision is due to three main adaptations that they share with other birds of prey. Humans have only one fovea, which is a depression in the retina densely filled with color-detecting cone cells. The fovea is responsible for sharp vision and detailed focus. Birds of prey have two foveas, which creates a larger field of sharp focus. They also have a structure called “pecten,” which contains photoreceptor cells responsible for vision resolution. Humans have photoreceptors in the pecten at a density of 200,000/square mm but birds of prey have an approximate density of 1,000,000/square mm. Finally, birds of prey have Crampton’s muscles, which aid ocular lenses to see prey and objects at great distances.
The vertical edge of the third eyelid is visible in this juvenile Osprey, courtesy of Osprey Tales.
A chronophotograph of an Osprey diving on a diagonal, courtesy of Chen Chengguang.
Ospreys also use behavioral adaptations that help them catch fish underwater. Ospreys position themselves at steeper diagonal angles relative to the potential prey, which is crucial to reducing the effect of the refraction caused by the water. They also use the cushioning effect of water to attack their prey at steep angles.
When is the best time to watch Osprey perform these spectacular dives? In fair weather, they typically hunt at mealtimes; in inclement weather, it’s catch as catch can. Hunting times vary and are influenced by weather conditions, particularly wind speed, and the family’s demand for fish. A study of foraging Ospreys found they tend to hunt more often at dawn and dusk. An Osprey’s hunting schedule and success are affected by wind speed with higher success during calm wind hours. Increasing wind speed makes the water choppier, limiting their fishing success. Therefore, Ospreys tend to avoid hunting during high winds, but their hunting success is not affected by overcast skies, bright days, or even rain.
Diving Osprey looks like it's falling out of the sky, courtesy of Chen Chengguang.
Eyes to the sky!
Candace
Candace E. Cornell
Friends of Salt Point, Lansing, NY
Cayuga Lake Osprey Network
Guest photographer:
Chen Chengguang (aka Joinus12345), is a Taiwanese photographer famous for his chronophotograph pictures of ospreys diving toward their prey taken in several frames and then composited into one image. Made popular in the mid-19th century, chronophotography is a technique in which movement is captured in several frames and then either laid side by side or placed in one frame to demonstrate motion. It's the predecessor to animation and cinematography and was originally used by scientists to study objects in motion.
By using this older technique, Chen illustrates every detail of the hunting osprey. Using three to five movements, one can see the osprey, its eyes fixed in concentration, talons outstretched, and wings pulled back, ready to snatch its next meal. Chen captures each movement of this skilled hunter in crisp detail—a testament to the photographer's technical abilities.
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