In most cases its a Chimney Swift. This is the only bird that can fly up and down your chimney. And they do, looking for places to make their nest. You’ll know you have these uninvited guests when you hear the baby chimney swift birds "screeching". Once these swifts have made their home in your chimney, it is unlawful to remove these birds due to their endangered status. If you have swifts in your chimney, we have to wait until the fall when these birds migrate to remove the nest and install a chimney cap.