The sound of bird calls is approximately 45 decibels. By comparison, the noise inside a library is slightly quieter than 45 decibels, and the sound of a quiet conversation is slightly louder.
The human ear can detect a wide range of sounds up to around 140 decibels. Anything louder than this level causes pain and can lead to damage and rupture of the eardrums. A jet taking off just overhead is around the upper limit of human tolerance at 150 decibels. In contrast, the sound of breathing is barely audible and is approximately 10 decibels.
The word "decibel" means "one-tenth of a bel." A bel is a unit of sound named after the inventor Alexander Graham Bell.