Birds often seek the refuge of a chimney to nest in or raise their babies. The trouble is, these feathery creatures can quickly become barbecued unless they are removed from the chimney. Actually taking a bird out of a chimney is practically impossible; therefore, the name of the game is prevention. A bird will fly out of a chimney eventually, usually in search of food or nesting material. When it does, it's time to seal your chimney and keep the birds from coming back.
Monitor the activity around your chimney. If you notice birds coming back with sticks or paper, they are building a nest, probably in preparation for a few little ones. If they are bringing back worms or insects, they already have baby birds in a nest inside the chimney.
Contact a wildlife service to remove the bird's nest from your chimney, if you notice the birds are bringing back nesting material. If you attempt to remove the nest yourself, you could injure the birds or prevent the mother from coming back and feeding the baby birds, causing them to die from starvation. A wildlife service uses special equipment when removing nests to cause as little disturbance as possible.
Watch the birds' activity and when you know they are out of the chimney, close the damper. Doing so will prevent them from being able to enter the chimney. However, if you close the damper but are unsure of whether the birds are still inside the chimney, you could trap them in your fireplace. If you aren't using your fireplace, push a large piece of foam rubber up the chimney through your fireplace to prevent any birds that may somehow enter your chimney from falling into your fireplace.
Lower the arrester cap on your fireplace to eliminate any gaps between the cap and your chimney to prevent the birds from going inside. You could also purchase a chimney cap that sits over the top of the chimney opening to prevent birds from entering inside. If you cannot lower the cap or cover the chimney opening, place bird spikes around the chimney to prevent the birds from going near the opening.
Call a chimney sweep company to lower the arrester cap or place a cover over the chimney if you do not feel comfortable doing it yourself.