I would just say that the seeds they have in wild bird seed mixes are often just "filler" seeds. Things like sunflower, millet seed and there's one other that I can't recall the name of - it's a little black seed - those are all very high in fat and bad oils. It'd be like feeding your rats mcdonalds. (I keep birds and know waay too much about their food...)

So I would say a little bit is ok as a treat but I wouldn't recommend it. Seeds and fruits and whatnot must be good for them, but I would only buy the organic ones and only certain seeds. Namely, not too much sunflower or millet seed (which they can't usually grip well anyway). When I give my feeders a last meal (out of guilt I guess you could say) they get dried corn and other vegetables, an organic bird seed mix which I know to be as healthy as possible, and dried fruits.