An avian magnetic field redirecting device, or as most people call it, a "bird brain" is a small piece of metal (usually silver) with an identical jewel on each end. In the middle is an indentation that is useful for breaking the metal in half. On one side of the indentation is usually the name of the person carrying them around whole, and when they snap in half, latent magic energy allows the owner to alter the unlabeled half to write what he/she wishes on it.
A Bird Brain, when paired with a messenger bird, is a form of long distance communication. If someone wishes to keep in touch with someone, they will snap their bird brain in half along the indentation and give the half with their own name on it to the person they wish to keep in contact with. They will then use the latent magic energy to inscribe either the person's name that they just handed the other half to, or a quick not of who they are and how they met them, etc.
To use a bird brain, you first need a messenger bird. It cannot be any bird, however. It must fulfill two requirements: It must have great eyesight (so usually a bird of prey like a falcon or eagle) and have been trained and enchanted by the mage's guild. If you have such a bird, attach your message to the bird's leg, and show it your half of the bird brain. The bird will take off and head towards wherever the other half of the bird brain is. The recipient of the message being underground may present some difficulties but the bird will wait and follow the signal given off by the sister piece until the task is complete.
How it works:
The bird brain works by taking advantage of birds sense of the earth's magnetic field. Instead of sensing the magnetic field of the north and south pole, they instead sense the field as if the poles were the two ends of the bird brain. These fields are programmed by the birds seeing the imperfections in the gem (hence the need for good eyesight). These imperfections are identicle in each piece of the bird brain.
After this is complete, its simple messenger training to teach them to be patient and allow the recipient to take and respond to the message before returning from their very short migration.
How to use it:
Use it as a way to keep NPCs in touch with players over long distances. Also, let PCs use it as a way to maintain relationships with NPCs after leaving town.
I failed Biology in high school. If my knowledge of migration and magnetic fields is waaaaaay off, feel free to just wave your hands in the air and say "cuz magic." It's more of a cool way to let my PC's communicate. Plus it has been fun figuring out what birds each faction in my world would use.