Once your egg has turned into a Fledgling and hatched into a bird you will have a new addition to your Journal! You will also get Journal points for hatching new birds!
To fully merge and level up a bird you will need to hatch the same bird 5 times - and merge them together, and feed them the required amount of Gnats until they glow blue!
Once they have been fully merged you will get a maximum melodia harvest amount, and be on your way to completing an entire set.