Does ringing harm the birds?

Hardly ever. Very occasionally a captured bird may be so exhausted that it suffers trauma during handling. And, as in any interaction between humans and wild creatures, accidents can happen. But these are extremely rare, and in general ringing does no harm to the birds being caught. Certainly the benefits that ringing brings to bird conservation far outweigh the risk of harming individual birds. Incidentally, ringers are licensed to become one take years of rigorous training.