The Brits require that every wild bird in the United Kingdom be registered with the government, and display a registration number to prove that it is up to date on its taxes, etc..

Truthfully, the birds are tagged for biological research. (just like our condors, here in the American west) Red Kites are a closely monitored species of concern in the UK. They used to be quite rare and in decline. 20 years ago, when I was working for Nick Fox in southern Wales and northern England, Nick's operation was still producing captive bred Red Kites for release to the wild, in an effort to support the population. As an aside, they are extremely docile and calm birds to handle.