Sadly, you can't just buy a random hummingbird. You have to buy a gift certificate only at AJ Outfitters! Here is the information.
You have to buy a gift certificate (like a lion). It will give you one month membership, 1500 gems, a hummingbird, and a Birdhouse and a Nectar Feeder. Comment if you have bought this certificate!
Here is what a humming bird pet looks like.
Here are the items that you get with the certificate.