Flax seed (and therefore linseed meal) should not be fed in high quantities because a compound present in it may inhibit pyridoxine (vitamin B6).
but then I was reading another site and it said that flaxseed is often a component of store sold bird seed and it should be fine? I would err on the side of caution and not do it (or use just a little) in my opinion.
Flax seed is very high in magnesium, zinc (not the toxic heavy metal zinc), iron, vitamin E, carotene, B-group vitamins, calcium, sulfur, potassium, phosphorous, manganese, silicon, copper, nickel, molybdenum, chromium, and cobalt. Also, Omegas 3 and 6, as well as lignans that enhance the immune system.