Everybody knows that blue birds nest in spring and summer. The rest of the year bird boxes are empty.

Really?

Ever thought of going out on a winter's day and checking your bluebird boxes? Carefully open the top and look inside.

You might be surprised to find a globular nest inside about the size of a grapefruit. It's made of strips of inner bark from yellow poplar twigs or grass stems or other such materials.

To look inside, get a couple of little sticks -- about the size of chopsticks. Gently tease a hole in the top of the nest. Don't do this with your fingers. The resident might bite!

Do you see something furry, soft and silky with large eyes? What is it? A flying squirrel.

Flying squirrels often use bluebird boxes for their nests. They prefer a box mounted on a tree or on a post in a grove of hardwood trees. For best results, place the house with the hole right next to the trunk.

Boxes out in the middle of clearings are less desirable because flying squirrels like to jump from the house to a tree rather than run along the ground.

Flying squirrels are tiny squirrels, smaller than a chipmunk. The body is about five inches long plus four inches of tail. There are two species. The southern flying squirrel has a clear white underside and the northern species has a sooty white underside.

Loose folds of furry skin between the front and back legs form the sails with which they glide, or "fly," from tree to tree.

To attract both flying squirrels and bluebirds to your wild garden, put a box intended for bluebirds in the middle of the garden or yard -- out away from the trees. The bluebirds seem to know this is a good place. The squirrels, however, will prefer a box in the woods.

In fall and midwinter, several flying squirrels may crowd into one bird box for a day of sleeping, but before bearing her young, a pregnant female is likely to take over the box for her family and drive off visitors.

The young are born in winter and spring. To encourage flying squirrels to use a bird box, fill it half-full with soft rags. They will use these for nesting material.

Flying squirrels also nest in natural tree cavities and in leaf nests made by gray squirrels. In the deep south, they may take refuge in Spanish moss.

Sometimes after dark, you can hear a faint, high-pitched squeaking coming from up in the trees. This is the flying squirrels' call.

When they're active at night, they're vulnerable to being caught by owls. By day, rat snakes may catch them in their nests.

Don't worry about this, though. Flying squirrels have about three young at a time, and their populations quickly replace such losses.

You can attract flying squirrels to a bird feeder with nutmeats, peanut butter and sunflower seeds. The feeder need not be fancy -- a board nailed to a tree trunk will do. Nursing females are especially likely to visit feeders in spring.

Flying squirrels will also eat bits of meat -- surprisingly to some, flying squirrels are predators. They often eat nestling birds and insects, and flying squirrels have also been known to attack, kill and eat mice.