The average bird will eat about 1-2 tablespoons of seed, twice a day. The range seems to be more based on their activity level (do they fly or just hang around) than on size. I have an 800 gm king pigeon who eats the same amount as my 305 gm Old German Owl hen. A good rule of thumb is to feed what they clean up in 15 minutes. A pinch of leftovers is okay, but if they leave a third, I'd cut back. If they clean up every crumb, I'd add a bit next time. Most birds also eat more in the winter to provide fuel for staying warm. Make sure that all the birds get to eat, and no lower ranking birds doesn't get a good chance to feed!