I am a goose, not a number

While enjoying the famous Unterer Schlossgarten deep, deep within the bowels of Stuttgart I happened across about a zillion migrating geese, and almost all of them were wearing bands like this one:

I think hardly a goose – except maybe a crazy goose – would be inclined to wear a bracelet, but an absurd question remains: who would invest time putting bracelets on the legs of around a zillion migrating geese?  And for what purpose?