The end of a 20 ml syringe is cut off and replaced with something soft such as a piece of balloon or a piece of support bandage held in place by an elastic band.  An X slit is cut in the bandage/balloon and the pigeon inserts its beak.  The food is gently pumped in using the plunger.

Always remember to let the baby come up for air, and clean his nostrils as you go along.  Wash his face and beak when you have finished.