Wednesday, May 22, 2013

Camouflage!

In addition to being silent flyers, owls are remarkably cryptic. This photo shows a barn owl, possibly mother Tess, perched just outside the nest cavity. She has flattened herself against the tree and is holding one wing down. Her barred wings break up her outline, making her difficult to see against the tree's rough bark.

Camouflage exists in two basic forms: crypsis and mimesis. Cryptic camouflage makes animals hard to see, while memesis, or mimetic camouflage, disguises them as something else.

Visual crypsis can be achieved in many different ways. When Tess is perched as she is above, her colors and patterns resemble a natural background. Owls are adapted for exceptional camouflage when roosting and hunting. Many species have colors and patterns that mimic the bark of preferred trees, which may explain the incredibly variety of color and pattern morphs seen in many widely distributed owl species.

Disruptive patterns use strongly contrasting, non-repeating markings such as spots or stripes to break up outlines. Tess's barred wings, plumage, and dark spots help break up her outline, making her difficult to see. Her spots also make her more attractive to male barn owls, who exhibit a clear preference for spotted females. The bigger and darker the spots, the better!

Cryptic patterns and coloration don't work as well when an animal is moving. While the fledgling owls aren't acting particularly cryptic, adult owls have very cryptic behavior. They perch quietly and hunt stealthily. Their soft feathers reduce turbulence, muffling the sound of air flowing over their wings. As the young owls learn how to fly and hunt, they will become increasingly cryptic as well.

The combination of stealthy coloration, disruptive patterns, and cryptic behavior make owls very cryptic, or hard to see. But some people believe that kestrels practice mimetic camouflage. The picture to the right shows the kestrels nesting at Xcel Energy's Pawnee Station earlier this year. The male is on the left and the female is on the right. They are facing forward, so we can see the dark 'eye spots' on the back of their heads. This mimetic false face might confuse predators into thinking that the back of a kestrel's head is actually its front.

Like many ground-nesting birds, Killdeer lay very cryptic eggs. I find it extremely difficult to see the Killdeer egg in this video, even though she lays it right in front of us (it can briefly be seen about 52 seconds into the video). Both of the adults are quite cryptic against the gravel road. However, they also practice a distraction display that disguises a healthy bird as an injured one and helps protect the nest. When a threat approaches the nest, the killdeer holds it wing in a position that simulates an injury and emits a distress call. The 'injured' killdeer lures the predator away from the nest by appearing to be easy prey. Once the predator is far enough away from its nest, the killdeer flies away. This behavior also strikes me as mimetic, since a healthy bird is mimicking an injured one.

Can you see the Killdeer egg in this picture?


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