Through out the world, animals are banded, usually around the ankles, with different colours that help to identify them. Some bands will have have individual numbers on them. On Tiritiri Matangi you will see many birds with coloured bands. This helps everyone to identify and track their movement. See page 159.
Small birds have legs that are as fragile as a piece of uncooked spaghetti. Imagine how careful you would have to be. Banding takes a lot of skill.