When autumn's chill creeps into the air, the animals with which we share this urban area begin looking for snug winter retreats.

Most people are not aware of the large wild animal population that exists in a big city. But when an animal decides to move in with you, or to transform your garbage can into an all-night diner, awareness is thrust upon you . There is little choice.

Most complaints deal with possums and squirrels. Sometimes raccoons, and sometimes wild cats. Not wildcats but cats that run wild. If you cross one, you could be forgiven for thinking it was a wildcat.

Squirrels move into attics and chimneys when the weather gets cold. They are not easily persuaded to move out.

Possums and raccoons break into garbage cans. They take what they want and strew the remainder over a wide area. Householders who are visited by such anmals dislike cleaning up after them, but many wonder how to get rid of the pests humanely. Most people prefer not to harm animals.

"Get a new can with a top that fits snugly," a neighbor advises. The new can is bought, but it is opened by the possum as easily as he opened the old can. "Buy a tight spring that runs from handle to handle and across the top of the can," another neighbor suggests. Your friendly neighborhood hardware merchant knows just what you want. And the possum knows just how to get the spring off. He's strong, and he's smart. So you put some bricks on top of the snug lid that's held in place by a heavy spring on your new can. In the morning, you find the bricks on the ground, the spring sprung, the lid off, and garbage all over the place as before.

Is there no way to deal with nocturnal visitors? Readers tell me there is one thing that can be done. It is successful, at least temporarily.

Call your local Humane Society and borrow or rent a humane animal trap. Bait the trap with food. When you catch a possum, take him to a wooded area and open the trap. The possum will flee at high speed. So will cats and squirrels, I'm told. Your troubles will be over until other animals move into your neighborhood, or come up out of the sewers in which many take refuge. i

One reader reported that he caught a possum one night and the next morning released it in Rock Creek Park. That night, his garbage can was raided again.The next night, he caught another possum. "The second one was mad as hell," he told me. But when the second one was released, "it made a beeline for the same tree the first one had disappeared into."

If you look in the phone book for the number of your local Humane Society, keep in mind that there is a national organization, which is not what you want, and there are local groups, most of which will be what you do want.

The one in the District of Columbia is at 7319 Georgia Ave. NW (about two blocks short of the District line). Its phone number is 333-4010. Its phones are usually answered between 10:30 a.m. and 4:30 p.m., Monday through Saturday, but the staff consists of only six people and sometimes the person in the office must go out on an emergency call. If you call at such a time, you get an answering service. Later somebody calls you back.

The Washington Humane Society does not charge for its animal traps or its advice on how to use them. You can borrow a trap for a week by putting down a $20 deposit.

The society is a nonprofit organization and gets no government funds. It exists on voluntary contributions. If you borrow a trap that helps rid you of unwanted guests, a voluntary contribution is a nice way to say "Thank you."

Keep in mind that local societies are not branches of the national organization. Each has its own rules and regulations. "The Alexandria chapter, for example, does not place a dog for adoption in a home outside Alexandria. We do." The "we" refers to the Washington group, which will place an animal for adoption in any home that seems suitable after an investigation. SMILE A WHILE

The November issue of Changing Times has arrived with its usual assortment of interesting comments. Have a few free samples:

"There are three ways to stretch your food budget: Plan meals carefully. But in bulk. Have Thanksgiving dinner at mother's house."

"Speaking of corporate takeovers, we just heard about the Indiana housewife who accumulated enough cents-off coupons to gain control of the company that issued them."

"If you're introduced to a mortgage banker at any time in the next few months, the correct salutation is, 'Hi!'"

"American ingenuity lost" Nonsense! Why, right now we're brewing more foreign beers than all the rest of the world combined."