The Pigeon Post is a kit by the Letter Writers Alliance to send a plastic carrier pigeon through the mail.
Wait, I can actually mail it without putting it in a box and it gets there?
Yes, really. And Yes, it actually gets to its destination. It may gain some scuffs along the way, but it makes it to the recipient.
How does this actually work without putting the pigeon in a box?
It violates no postal regulations. The pigeon is not hazardous, fragile, or perishable. It weighs under 13oz., so it is legal to place into a post box. Just because it isn’t a rectangle doesn’t mean it can’t be mailed.