There are 19 species of owls in North America, but only one, the snowy owl, is white. The reason it is white is because it spends winter, and parts of summer, in snow. The white is protective coloration.
Yet, the snowy owl isn't snowy all of its life. As a hatchling in its dark-colored tundra nest, a baby snowy owl is gray. As it matures, it grows white feathers, but is flecked with black until it is fully mature at two years. Then, except for a tiny bit of flecking, and golden eyes, it is an all-white bird that is a spectacular sight to behold.
During most winters, snowy owls fly south in search of food, mostly rodents, such as lemmings, hares and ground squirrels. It is during lean years in the North that we get to see these white ghosts flying on silent wings. It is not unusual to spot a snowy owl sitting on a fence post, or in a tree overlooking a farm landscape in the lower 48 states. Some snowy owls will frequent airport runways, where rodents are common.
Snowy owls are not always silent. The song of the male is a deep, muffled hoot that is repeated; the female may bark in alarm, and the male may quack, or make a slurred whistle.
Healthy snowies are usually nocturnal, but those that are starving, or stressed by a lack of food in winter will hunt in daylight, and that's when people have the best opportunity to see them.
-- George H. Harrison