Yes, in the large hatcheries there is a tendency for the rare and heritage breeds of all birds to have a little (or a lot) or something else mixed into their bloodlines. They do this in order to get vigour back into their flocks because they did not use proper breeding processes in the first place. If a breeding flock is managed properly this is not necessary, if it was there would be no purebred breeds of chickens in existence today. If you have a couple of breeds in mind then post them here and I can get you the contact info of breeders who breed purebred birds and can ship. Often the prices for the birds are similar to the large hatcheries and the quality is always higher than that of hatcheries (in my experience).