It is most likely that they are chimney swifts, and they are roosting in your chimney. A few may have nested in your chimney, but the breeding season has passed, and they have gathered in large flocks, and may be migrating. Because the birds will probably move on soon, I would suggest that you delay having your chimney cleaned until you no longer see the birds in the evening. They may return in the spring, and possibly nest there next summer.
