High above a field, a common barn owl, Tyto alba (TIE-toe AL-buh), soars in a nighttime breeze. Suddenly, the bird's expert ears pick up a faint squeak below. Its eyes pan the ground and spy the source of the sound: a delectable morsel, a mouse. The owl pumps its wings silently through the air. In seconds, it nabs its surprised victim.
Watching from below, scientists like Eric Foreman of the U.S. Forest Service study these raptors (birds of prey). Their goal? To learn about owls' hunting techniques, habitat needs, and threats to their survival.
Many of these silent stalkers are listed as endangered species (in danger of dying out) -- from the barn owl found on every continent except Antarctica, to the northern spotted owl, Strix occidentalis cawrina (stricks awks-ih-denTAU-lis cohr-EE-na), of the Pacific Northwest. That's because suitable habitat is disappearing to make way for development. Animals preferring similar habitats are forced to share the land and compete for food and shelter. "In Washington State, the population of the northern spotted owl has declined dramatically in the past 10 to 15 years. We suspect it's because barred owls, Strix varia (stricks VEH-ree-ah), have invaded the region," says Foreman.
When animal populations shift, it can upset the balance of an ecosystem (system of interactions between living and nonliving things), says Marge Gibson, executive director of the Raptor Education Group in Antigo, Wisconsin. Example: Barn owls keep mice populations in check. "They're the princes of rodent control," Gibson says. Losing their habitat to animals that don't hunt rodents means mouse populations may rise.
Indeed, barn owls are champion hunters. They skillfully snap up one-and-a-half times their body weight in prey each night.
Barn owls have the uncanny ability to swoop down noiselessly on their supper. “If it wasn’t for silent flight, owls would starve. They wouldn’t be able to catch anything, because their prey would [hear them and] run away,” says Gibson.
That’s why the front edge of an owl’s wings sports a comblike fringe of stiff feathers. The comb’s “teeth” break up turbulence (irregular air currents), smoothing airflow over the wing. The less an owl disturbs the surrounding air molecules (particles of two or more atoms joined together), the quieter its flight will be. The raptors also have a covering of downy feathers on their wings that help muffle the sound of their movements.
Owls have some of the largest eyes per body size of any animal. “If humans’ eyes were similarly sized, we would have eyes the size of tennis balls or larger,” says Gibson. An owl’s extra-large pupils (opening at the eye’s center) and an abundance of rods (cells that detect light) make the raptor’s eyes about 100 times more sensitive in the dark than human eyes. That’s important for nighttime hunting. But having such huge eyes has one drawback: Owls can’t “roll” their eyes from side to side in their eye sockets like humans can. Instead, they have the handy ability to rotate their heads 280 degrees -- almost a full circle.
Barn owls are nocturnal (active at night), so hearing their next meal is more important than seeing it. An owl’s facial disk -- the flat feathers surrounding its face -- collects sound waves (vibrating energy waves that travel through a medium) just like a radar dish does. These feathers bounce sounds back to the owl’s ears, or the slits on either side of its head.
These opening are asymmetrical, with the left ear positioned slightly higher than the right, so sound waves enter one slit a fraction of a second sooner than they hit the other. The owl’s brain calculates this time difference to tell whether its prey is left, right, up, or down.