I personally have tube fed a chicken,drained and treated for ascites,with no issues at all,so yes it is possible to become knowledgeable regarding issues that can affect our birds,know what treatments should be applied and know what medications can be given.
I agree, it's also necessary for a lot of cases, treatment must be given at home as well or even solely because waiting on the vet will let the animal die.
But generally most vets, depending on their ethics and the knowledgability of the animal owner, will tell you when that's necessary. Some will have you calling them for anything and everything most poultry keepers do themselves, though, just to make an easy buck.