In order to keep up with certain bloodlines or strains we mark each of our chickens soon after birth. When you have several different breeds or strains being born at the same time it can be quite easy to get them mixed up, especially if the strains resemble one another after they are older. For example if we have pure Radios hatching at the same time as a crossed breed such as Hatch/Radios, we have to mark each bird with a breed specific toe marking so they do not get mixed up. Our School Teacher Radios come straight combed and yellow legged, and our Deadend Blueface Hatch come straight combed and green legged when bred pure. If we cross a Radio rooster and a Hatch hen, the chicks come straight combed and yellow legged (a few can crop up green legged). If we don't mark the chicks at birth we have no way of telling the chickens apart when they are older as they look similar.
You can use this method using a toe punch tool that can be purchased at some farm supply stores or Randall Burkey (http://www.randallburkey.com/.) They are easy to use and very inexpensive. It works the same way a hole puncher is used to put a hole in a piece of paper. The web between the chick's toes are placed into the puncher hole side down, tooth side up. The puncher is then clamped firmly through the web. I relate the pain to that of getting your ears pierced. It hurts the chick for a second, but doesn't bother them after it's done. A few different mark examples would be "In and In" or both inner webs punched, "out and out" or both outer webs punched, but there are tons of different toe punch combinations.
Even more marking combinations can be added by nose marking (left nose, right nose, or both). The nostril covers of a chicken are clipped using this method with a small pair of scissors, fingernail clippers, or cuticle trimmers. We nose mark soon after birth.
Leg and Wing Banding
We use leg and wing bands to identify older chickens. The bands have a specific number on them and the numbers are recorded in a book. Leg bands come circular and are opened up and slipped over the chicken's leg or come in metal strips that are fitted around the chicken's leg and clamped in place. Wing bands are clipped through the wing of the chicken at the shoulder using a wing banding tool.