How things don't go bump in the night

Posted: Friday, October 29, 2010

As Halloween creeps closer, a wildlife spy's thoughts can turn to mysterious flying creatures. Not mythological ones like witches and ghouls, but instead critters, like owls, bats and flying squirrels, that come out when the sun sets and darkness creeps into the woods.

But as nighttime activity begins to ramp up, how do these creatures - literally - keep from going bump in the night?

Although seeing one is rare, owls are surprisingly common around Juneau. Along Douglas Highway alone I've heard four different species of nocturnal owls: the barred, western screech, great horned and the northern saw-whet. Although they're most vocal in late winter, these nighttime predators remain active year-round. Even on the darkest, most overcast nights, they still need to eat.

Owls have very large eyes for their size. A great horned owl, for example, has eyes almost as large as a human's which, makes it easy to see prey scurrying about in low-light conditions. Such enormous eyes, however, leave no room for surrounding musculature. They are immobile within the eye socket. Instead, owls have extremely flexible necks, allowing them to swivel their heads 270 degrees, or three-quarters of a circle. They are tricky to sneak up behind!

A nocturnal owl's real secret weapon, however, is its ears. Unlike mammals, birds don't have much of an external ear, and it's usually hidden by feathers. In fact, the "horns" on a great horned owl are decorative feather tufts and aren't related to hearing.

An owl's ears are slightly offset from each other vertically; usually the right one is higher and often at a slightly different angle than the left. The feathered disc pattern surrounding each half of an owl's face helps funnel sound into the ear. The differences in the intensity of the sound and the time it arrives in each ear allow the owl to zero in on the sound's origin. When it turns its head so the sound is the same in both ears, its face is aimed directly at the source and the owl is ready to launch its attack.

Another seldom-seen creature common to Southeast is the northern flying squirrel - and a tasty treat for owls. Technically, flying squirrels don't fly; they glide from tree to tree using furry membranes that connect the front and back leg on each side. When they leap off a tree, they spread-eagle their legs so the membrane catches the air like a kite. These prolonged leaps can be short, but can sometimes stretch hundreds of feet.

How do flying squirrels avoid crashing into trees in the dead of night? Like owls and many other night animals, they have enormous eyes. But beyond their size, there are several other physical characteristics of these eyes that allow flying squirrels to turn even the dimmest light into "daytime." The retina is the light-sensitive inner surface of the eye. It has two kinds of cells that react to light: Cones, which work in bright light and detect color, and rods, which work in low light and detect light-dark changes, motion and shape. Rods only have one type of pigment, so they don't detect color. When you look at something in low light and colors are hard to distinguish, more rods and fewer cones are operating.

The first night vision instrument for flying squirrels are the rods. Flying squirrels have almost all rods in their retinas. Having large eyes for their size allows them to collect more light than human eyes, triggering the rods with a fraction of the light humans need.

The second night vision instrument is called the tapetum lucidum (Latin for "bright tapestry"). It's a mirror-like membrane behind the retina present in nocturnal animals. When you see an animal's eyes reflected in headlights, you're seeing their tapeta lucida in action. Light that passes through the retina reflects off the tapetum lucidum back through the retina a second time for another shot at activating rod cells, effectively doubling the available light.

Researchers have discovered a third instrument in the last few years. The structure of the nucleus, or core, of the rod cells of animals with good night vision is different from that of animals who rely on day vision. In daytime animals, who are active when light is plentiful, the structure scatters light. In nocturnal animals, a different setup allows the nucleus to efficiently channel the light toward the sensitive part of the rod cell, enhancing the squirrel's night vision.

Sitting on a porch on Douglas in late summer after sunset, something darted through my peripheral vision. Looking up, the silhouettes of small bats darted against the darkening sky. Little brown bats (a very imaginative name) are the most common bats in Southeast Alaska, although there are several bat species that roam the skies. Bats gobble up insects faster than trick-or-treaters consume candy corn. In a single night, a little brown bat can eat nearly 1,000 flying insects, including mosquitoes.

How do bats achieve their acrobatics in the dark? Instead of passive listening like owls, they generate sounds, then listen to the returning echoes to create a detailed picture of their surroundings and potential prey. These echolocation calls are mostly higher pitched than humans can hear. Researchers use a special device called a bat detector to hear bat calls. The machine's microphone captures the sounds and lowers them to a range humans can hear. In a split second, bats use several tricks to target tasty morsels. The time it takes for the echo to return tells the bat how far away a juicy mosquito is. Like owls, bats use the difference in timing between when a sound reaches the left and right ear to determine horizontal position. Folds in the bat's ears help it detect vertical position. Sounds from above or below hit the folds differently, creating a different sound for the bat. The intensity of an echo tells the bat the size of its prey: The larger the mosquito, the more sound waves it bounces back and the more intense the sound. The bat can even tell if the mosquito is moving toward or away from it. Like when you hear a truck pass by on the road, the pitch rises when the insect is moving toward the bat, and lowers when it's moving away.

If you decide to try some night-time spying, red light helps you keep your rods adjusted for night vision while illuminating the scene. There may be more night light than you think if you let your eyes adjust this way. Some head lamps or flashlights have a red setting, or you can put red film over a white light. That way you can scout for night-shift creatures and at the same time avoid going bump in the night.

• Beth Peluso is a freelance writer, illustrator, and avid birder. She enjoys spying on wildlife around Alaska.

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