Specifically the birds that you might see at your feeder.
There’s a local, um, “bird supply” dealer, and his radio spots tout the high-quality bird seed that he sells. One of his claims is that birds (at least the kind that are likely to show up at your feeder) do not care at all for grains, like milo or wheat–and most “regular” bird seeds contain a lot of grains as filler. So, you end up later with wheat growing underneath your feeder where they kick it off.
So, what’s the real deal?
My experience (Northeast USA) has been that birds commonly visiting feeders (e.g., chickadees and titmice) will generally prefer one item in the mix and preferentially pick those items out The species I named choose black oil sunflower seeds, scattering the less desirable items on the ground.
However, there are numerous birds and other animals that may not regularly visit the feeder but forage underneath it instead, such as Mourning Doves, Dark-eyed Juncoes and Grey Squirrels.
In nature very little is wasted.