Seed-scattering birds:

 I think that adding more birds to minecraft would add atmosphere to the game, but I would not like these birds to be only aesthetic and have no purpose, so I propose to add birds that can spread the seeds of the trees, so that the forests can regenerate from naturally after being felled. For this you would have to add seeds for the different trees in the game, with which the birds can interact (acorns for the oaks for example), these seeds would fall from the leaves of the trees instead of the buds, they could be planted in the ground and after a while they would become shoots that later became trees.
  For oak and birch forests, jays could be added, they would pick up the seeds from the ground and plant them elsewhere to eat later, but sometimes this would not happen and a tree would grow.
  For the taigas, I think there is no better option than adding crossbills, birds specialized in opening the pine nuts of conifers.
  For the jungles, you would not have to add a new mob, but you could modify the behavior of the parrots, although adding toucans could also be fun.
  Finally, I believe that the savannahs do not need to have a seed-scattering bird, although turacos or Red-billed hornbills could do the trick.
 
If there is any error in the writing, I am sorry, my native language is not English and I am not fluent in it yet. So I used a translator for the most of the text.