Biophilia is a term that means love for life but it's not only about loving all other life on earth, it's also our innate need for nature. This has been proven over and over by studies: greenery and interacting with nature improves our mental and physical health - this is not an opinion, it is a FACT.
Now, considering that we, humans, are the ultimate domesticated animal, if we benefit from contact with nature, imagine how important it would be for undomesticated species like any of our birds! Colorful toys, little huts and stuff like that looks great in a cage and our birds enjoy them but, in reality, nothing is better for them than plain old nature so, please, remember this when you are changing things in their cages, give them natural things instead of plastic or metal: tree branches with the bark on (and if you can get some with leaves on them is even better!), pinecones and acorns, sisal ropes with knots on them, plants they can chew on (like spider plants, for example), etc and, if they like to sleep in a hut, do try to use natural elements for them - a member here made his GCC a little hut out of a woven grass mat.
Learn more biophilia here:
https://www.mnn.com/earth-matters/wilde ... 0-40378997