Bird Leg Rings
A reputable shop or breeder will have fitted their finches with plastic or aluminium ID leg rings, or leg bands, in the bird's second week. The rings have letters and numbers, identifying the issuing organisation and the breeder, plus a unique code for the individual bird, including its year of birth. Bands have different colours, depending on the country in which the breeder lives.
The leg ring on this Lavender waxbill records information about its origins
Rings are a legal requirement in some countries, although not in Britain. It’s still standard practice to fit them, however. UK rings are stamped with the breeder’s registered number, the year in which the bird was born, and other numbers indicating the individual bird itself. Many breeders also fit blank, coloured plastic tags, as a means of tracking family lineages in their aviaries.
A leg ring is not something you should fit yourself unless you are a bird breeder and know what you're doing. The main thing to observe in a bird you intend to buy is whether the ring is sufficiently loose – you should be able to rotate it around the bird’s leg with ease.