He's cute and enjoying your hospitality. One reason to get a flue cap.
How often should I have my chimney swept? The simple answer is: The National Fire Protection Association Standard 211 says, "Chimneys, fireplaces, and vents shall be inspected at least once a year for soundness, freedom from deposits, and correct clearances. Cleaning, maintenance, and repairs shall be done if necessary." This is the national safety standard and is the correct way to approach the problem.