I am no food/spoilage expert, but I would think you could hang a bird for the better part of a day in a cool spot and not have to worry about it spoiling. Certainly, if you can throw the whole bird in a freezer, or even a refrigerator, it would keep just fine for a while. Field dressing will allow the meat to cool more rapidly and, as such, would be preferrable to leaving it uncleaned. If you throw it in a freezer, and it freezes solid, it will take some time to thaw out to process, so it is probably better not to let that happen unless it is going to be several days before you can process it. However, there have been times when I have had to freeze game and leave it for a while, and everything turned out fine when I got around to taking care of it. You don't want to wait too long, though, as it will eventually begin to freezer burn.