It keeps birds and rain out but I have two mounted side by side. When the fireplace is burning and the oil burner is off, the smoke travels down the burner chimney and into my basement. This didn't happen without the caps. So heat up your oil burner chimney before you light the fireplace. These caps also collect tarry creosote. Clean it every year to avoid a buildup. I found Mineral spirits dissolved the tar best after trying various solvents.