Minecraft world is beautiful but “dead” in terms of sounds. It is just empty. When you walking in a real forest you hear birds singing.
There are some mods that add those kind of sounds, but they are “fake” because they are not produced by actual mobs in the game.
i suggest to add different kind of birds for different biomes with unique sounds. Some of them might be common for multiple biomes, but some are unique. So each biomes will sound different.
Great example are cave bats.
I suggest: woodpecker, cuckoo, nightingale, owl, seagull
That would be a great addition! Maybe they could add bird cages or bird houses as well!