Why has this gull got a tag?

[Skyline]

I don't know much about birds but I am curious why this one would have a tag on its leg?
This was by the River Thames at Greenwich today. There was about 20+ gulls hanging around for dropped food when I noticed that one had a tag on its leg. Who would have put it there and why?

 

[morfis]

Around one million birds are ringed in the UK each year and ringing has been going on for around 100 years.
They allow the collection and collation of information on life-cycle, measurements, movement, etc. for scientific study.

In the UK the British Trust for Ornithology oversee the system and there are around 2500 trained ringers

 

[Skyline]

Around one million birds are ringed in the UK each year and ringing has been going on for around 100 years.
They allow the collection and collation of information on life-cycle, measurements, movement, etc. for scientific study.

In the UK the British Trust for Ornithology oversee the system and there are around 2500 trained ringers

Ok, cool. Now I've noticed one I'll probably see them all the time now

 

[lightshipman]

I work at sea maintaining navigation buoys, about 8 years ago we found a dead tern on a buoy at Lizard Cove in Cornwall, we sent the tag to an address in London, the answer we got back, was that the tern was ringed on Newborough Warren 21 years earlier. The longest it could have been on the buoy was a year as this buoy is serviced annually and it definitely wasn’t there the year before.

 

[F1.2]

I am not happy that any bird is taged / ringed
Imagin the stress the bird goes through being trapped , man handled and having a attachment forced on it by a human
Probably measured weighed and generally pulled about , Arnt they supposed to be wild and free ?