Restoration, not masking
Attention Restoration Theory (Kaplan, 1995) proposes that natural environments let directed attention recover because they hold attention without effort. Ratcliffe, Gatersleben and Sowden (2013) found that bird sounds in particular were associated with perceived recovery from stress and attention fatigue, though some birds are experienced as alarming and the effect depends on the sound.
How to use it
Forest ambience masks less than rain or brown noise; birdsong is variable and can catch the ear. It is at its best for lighter work, for reading, and for the five-minute break, where the change of sound marks the change of mode. Some people run forest during breaks and rain during blocks.
Volume
Low. This is a background you glance at, not a wall you hide behind.
Kaplan (1995), Journal of Environmental Psychology, 15(3). Ratcliffe, Gatersleben & Sowden (2013), Journal of Environmental Psychology, 36.