A leg band is one of the easiest and most common ways of marking a bird for identification. Domestic birds are banded to mark ownership and provenance. Wild birds, especially those of vulnerable species, are banded by researchers interested in tracking bird migration patterns, behaviour and populations. The colours, materials and sizes of bird bands vary according to the type of bird banded, the purpose of the band and the organisation doing the banding. Bird banders almost never make their own bands --- they are typically obtained from commercial manufacturers or agencies that have bands specially made for them. But when commercial leg bands don't work --- for instance, if a bird requires a band size that's not available on the market --- you can custom-size a commercial band.
Use leg calipers to measure the bird you wish to band at the widest part of its tarsus (the long segment of the leg directly connected to the foot).
Fill a heated chafing dish with hot water (just below boiling) and keep the heated water nearby as you work.
Locate a nail or piece of stiff wire of the same diameter as the bird's tarsus.
Use sharp scissors to snip off a thin section from the end of a commercial plastic bird leg band slightly larger than what you require. Make the cut at an exact right angle so that the newly cut end fits seamlessly against the other end when the band is closed.
Wrap the trimmed plastic band around the nail or wire. If it is too small, discard it or set it aside for use on a smaller bird. If it is too large, trim it again to fit.
Pinch the band firmly but gently around the nail with needle-nosed pliers and dip the nail and band into the chafing dish of hot water. Hold the nail and band in the water until the band is flexible enough to be pinched completely shut.
Remove the band from the water and from the nail.
Measure the inside diameter of the shortened band with leg calipers. Do not use it on your bird if it is more than three millimetres off the targeted diameter.
Even if you need only one or two bands, make sure to have several commercial bands to work with. It can get tricky sizing your bands correctly.
Specialised training and a federal permit are required to handle and band wild birds in the U.S. Do not attempt to band any wild bird if you have not been trained by a qualified bander. Many states also require individuals who band wild birds to carry state as well as federal permits. Know the requirements for the jurisdiction in which you are working. Birds can be dangerous. Large birds struggling to escape a bander can cause serious injury, and even small birds can bite and scratch if they feel threatened. Measure carefully. Too-small bands can injure birds, and too-large bands can get caught on objects in the bird's environment and put the bird at risk.