There’s a sweet spot when you can fit a closed ring of the correct size. It may only be a few days and the age differs for each breed and species. Once the ring is fitted, the bird gets used to it very quickly but it’s good practice to monitor how it is coping. The ring should stay between the hock and the hallux for waterfowl as they grow.
Closed rings are available from various sources. EE coloured plastic rings are available through PCGB, and rings which identify the breeder will be available from Aviornis UK for the next breeding season.