Michael
12-9 gauge galv wire. Buy it at the hardware store. Many use 12 and it is fine, I have used it on many birds. On Swans, I like 9 gauge as it give extra strength. It is a little harder to bend (which of course gives it the extra strength) and a bit heavier. I have gone back to 12 on some recent birds to save weight, and have many in use, in my rig that were made with 12 gauge. Unless you are making big geese or swans, I'd recomend starting with 12 as you learn to shape it.
Fire away with any more questions,
Chuck