Birds need to get tan~
Do you know? Birds see richer colors than us under the sun!
It produces strong bones, beaks, and aids in feather production.
It builds the immune system.
It kills germs and bacteria on the feathers and skin (and it has been recently discovered that direct sunlight kills the deadly PDD virus on surfaces.)
It minimizes the chances of developing certain cancers.
It reduces anxiety and depression.
It enhances a bird’s vision.
Natural sunlight can only be reached outside.
Case by case, setting your bird’s cage by the window may not be enough. Some glass blocks out 90% of the sun’s UV rays, even screens block out 30%, so there may not be any gain for vitamin D production.（Source: http://www.birdtricks.com/blog/the-importance-of-natural-sunlight-for-your-parrot/）
In addition, birds will be able to see clearly the color, appearance, etc. of another bird under the sun, which is very important for their partnering!