We've had 2 very tired racing pigeons in our garden the last two years. if they go off course they get tired and thats when they land. It doesn't mean they will automatically get killed by their owner, both the ones we found we managed to return to their owner, and both men were very happy to get their birds back - one was to race again, the other was retired to stud as he was an older bird. What stuck on my mind was the way those men handled their birds with such affection.
Can you tempt the bird close enough with some maize to catch it? This is what we did, slowely gained the birds confidence and fed it maize (the type you use for pop corn) until it was close enough to grab. Having got the ring number we kept the bird safe in the cat carrier with food and water.
Once you have the ring number you can go to this website and follow the instructions:
http://www.homingpigeons.co.uk/lostpigeon.htm Only do this once you have caught the pigeon!!
Both owners who came to get 'our' pigeons travelled some distance and gave my little girl a fiver for her trouble, hardly the action of someone who was going to ring the pigeons neck imo.
Good luck, let us know how you get on!