Leg bands allow the flock owner to mark certain chickens for easy identification. The problem with metal bands, a tool is needed to put them on and take them off, and the bands may not be widely available.
Instead of using metal bands, why not use cable ties?
Cable ties can be found at just about any hardware store. To put the cable tie on all you need is a pair of wire cutters to cut the slack off.
Leave enough slack so the cable tie will not cut off the circulation.
If you want to keep track of the chickens age, then use different colors:
Black for 2013
White for 2014
Yellow for 2015
,,, and so on.
Use red to mark chickens that have bad habits that you may want to cull.