Not sure, other than vitamins readily available in different fruits and veggies (we try to give them a limited but diverse spectrum of healthy treats). Also, a little bit of plain greek yogurt every now and then helps with a probiotic boost. We do use rooster booster when the weather is particularly brutal and that always perks them up noticeably. Grain of salt though--after all we are the ones with the sneezy girls!
I've read that small amounts of crushed garlic or garlic powder and ACV can sometimes provide an immune system boost now and then, but I haven't tried the former. Let me know how the oregano and rosemary works out--we have that growing in the garden.
Best of luck!