There have been many scientific studies, and many in progress, about birdsong and its affect on the human brain. I am not a research student nor a scientist myself, so I'm not going to cite a bunch of sources. You are on the internet reading this, you can google "birdsong and its affect on mental health" and see for yourself if you wish. But it really is a thing, and I'll bet you have felt the healing affects when you are walking outside. Just being in nature, walking through trees and lush living plants is amazingly healthy for you. But the added note of birdsong is a cherry on the cake! I think it sums it up with a quote by Julian Treasure, author of Sound Business, "People find birdsong relaxing and reassuring because over thousands of years they have learnt when the birds sing they are safe, it's when birds stop singing that people need to worry."
Who doesn't walk in a forest and feels uneasy if they can't hear bird song? In how many horror movies has someone said "The birds are quite....something is wrong" and you just KNOW its ganna' be bad. Birds are the heartbeat of a forest, that is how you hear the life pumping through it.
More and more I am realizing just how important it is to GO OUTSIDE. So much outside stimulation is needed for your mind. Being in nature is not some hippie dream. It statistically is healthier for you. Opening a window is not enough. I understand it can sometimes be the hardest thing in the world to walk out your door, and everyone has their own personal barriers to work through. But eventually, in your own time, you can make it happen if you keep moving forward. Allow yourself a breather. Go outside and immerse yourself.... It is so much better for you than you may realize. There is not a lot of free medicine in this world, so I would take all the bird song you can get!