21.0.16-beta

• Added 3 new Frog variants.
• Added Dragonflies. Normally, they can only spawn in river and swamp biomes in sunny days. However, they can spawn rapidly in most biomes when it's going to rain. They also have multiple colors.
• Added Birds. They have many variants and different sound effects, and can spawn in many biomes. They act like parrots, can be tamed and sit on the player's shoulder. But they cannot imitate other mob's sound.
• Added Pigeons. They are special birds and can be used to deliver mails. Use an mail to your own tamed pigeon, and it will send the mail to the specified mailbox depending on the recipient of the mail. The sending distance usually doesn't play a big role in this process.
• Added Letters. Use paper, feather and ink sac to craft a Paper and Quill. Just like the Book and Quill, you can edit it, but it only supports 5 pages at most. Then sign the paper, and it becomes a Letter.
• Added Envelopes and Mails. Use paper to craft envelopes. Use the envelope to open the gui, where you can put in a letter (or paper and quill) and an attachment item, or you can also leave it blank. Then write your recipient's name and confirm, and the envelope becomes a mail. To maintain secrecy, a mail can only be opened by its sender or recipient. If a mail lacks both then anyone can open it. Dispensers can automatically open the mail, only if the sender and recipient are all empty.
• Added Mailboxes, with every type of wood. They have 9 slots and are the terminal of posting system. Pigeons try to put mail into it. If the mailbox is full, the mail will be dropped as an item entity. Usually the mailbox's name would be consistent with its owner (the player who placed it down), but renaming it can change that, which may make a difference to the pigeon delivering.