Our 13 newest chicks are just over one week old. They all have the same father (our BCM rooster), but they are by three different hens (one BCM hen, one blue-laying Ameraucana and one pink-laying Ameraucana). Because we are breeding for egg color, we need to track which chick came from which hen, and this is pretty difficult to do when all but 3 chicks hatched out black with white bellies.
The initial solution (thanks to some tips on Backyard Chickens), was to dab a tiny bit of food coloring on a light part of their fuzz. This worked for the first few days; we had some marked green, some marked red, and some left blank. However by day 4, I was having trouble telling them apart again since the dye was wearing off quickly, so I decided to go ahead and leg band them like we do our adult birds.
Instead of buying pricey poultry leg bands, we use cheap colored zip ties for banding. A big 100 piece bundle of assorted colored ties costs $1 from The Dollar Tree, and the bundle includes two different sizes. While the ties are cheap, and not ideal for heavy duty jobs, they are perfect for leg banding. The larger size is great for adult birds and the small size is used for babies. The four colors give us lots of combination options for tracking specific things in our birds... egg color, hatch year, etc.
When using ANY kind of leg band, it is very important to remember (especially in young growing birds) that bands will need to be checked and replaced very often to keep up with the bird's growth. If dealing with "teenage" or adult birds, we find that it is easiest to work on leg bands in the evening when they are docile and pretty cooperative. Regardless, it is usually a two-person job to cut off and replace bands.
Bands should fit so that there is a big enough gap to allow it to move freely around and up and down, but not so big that it can come off. Feather-legged breeds may require a bigger gap. If you are banding chicks, you must check the bands every day... I can not stress this enough!! Baby chicks grow at an incredible speed and you don't want any band getting tight on their leg. After you've sized the zip tie as needed on your bird's leg, carefully cut off the "tail" of the tie so all that is remaining is the band.
We banded our chicks on day 4, and after a few minutes of all the chicks being fascinated with their new jewelry, they pretty much ignored their bands after that. To give you an idea of how fast they can outgrow them, we put theirs on fairly loose and four days later, we replaced nearly all of them.
Companies make all kinds of leg bands for birds/poultry so there are tons of options out there. Some are permanent bands, some "slide" to grow with the bird, and so on. I'm not here to say any method is wrong. We like the zip tie method because they are super cheap, easy and we can pick them up locally in town if we need more (after 3 years, we are still on our first bundle though).
Do you use any methods to identify your chickens? If so, we'd love to hear about it!