almost definatly either coxsideous (I know the spelling is wrong on that one) or Canker
Canker is treated with sulpher based drugs and coxy is a dirty condition thing that allows the pathogens to take over.
When pigeons/doves go light and wont respond to treatment, often if you pull out all the tail feathers, they will slowly come right.... sounds like an old wives tail but it works.
with all due respect to vets here in NZ, very few know anything about pigeons so most treatments that work are what we (as pigeon racers) have found that works over seas.