All birds have two brains. The front half is for feeding, flying, raising and procreating, and the back half is for getting angry. A happy bird does not often sing. It spends its time doing other things.
When birds are caged, the front half of their brains gradually atrophy and they still sing, but instead of singing about mating or asking for help, they sing curses. Birds are not magical creatures, but they are messengers of the forest, and Woodspirits can see their curses. Seeing a person with a curse, the Woodspirit will not heal him.
Birds don't like freedom, they just have to be free, it's in their blood, and whoever takes their freedom, it drags them to hell.