The bands you noticed are the official bands of our German Poultry Breeder Association. They are closed plastic bands of different sizes according to the breed and sex of the birds, and they have imprinted numbers and of course year dates. Registered poultry breeders will get these bands every year for their poultry. They work as identification bands, as every bird can be identified by it, the year of hatch, the breeder, the country etc. Every year they will be a different colour, so you will be able to see the age of the bird already from afar without having to catch it to take a closer look at the numbers on the ring.
These official leg bands/rings are required for official exposition purposes, if your bird does not wear those, it will not be accepted to be shown at the official German or International Poultry shows.
But they have to be put on at a young age (6-9 weeks).