supplement to boost immune system?

[carolynP]

One of my pet pigeons has been fine for years but recently has cost me a lot of money in vets fees first with bad canker then some other kind of throat problem now it has sinusitis(not sure if thats how you spell it!). It gets fed fancy pigeon food and also layers mash, we got that when she was ill and couldnt manage seeds, a pot of grit and a pot with a hard type mineral block in it.
Is there anything that i could supplement her diet with that could boost her immune system and hopefully stop her falling ill again?