I mentioned this before, but I want to mention this again. If there is any rotation with managing the birds, sometimes tiling a section in their pasture, is beneficial. If there is a variety of native plants in the pasture, it would not be as helpful. In some pastures, their is a shortage of diversity. Sometimes just tilling a section brings a variety of native seeds to the surface where they sprout and grow. Left alone for a time, the insects will colonize it. I have found that in my circumstances, it is a cheap and effective way to improve the birds forage. A simple grass lawn is beneficial, but not as beneficial as it could be.
The more plant diversity, the more insects. A variation of short and tall thick forage aids the insect population, and still provides the birds access.
I have trees and leaf litter as part of my poultry pens. In the rotation, I maintain it as I do the green forage. I will add a layer of fallen limbs from the trees in order to discourage their scratching, in some places for a time. The sticks break down in time.
To keep it simple, left alone, the insects are allowed to reproduce in the leaf litter, and become food for the birds. It does not cost any money. It only costs time.
Another variation of this is to have a pen where leaves, clippings, straw, etc. can be accumulated. In different seasons, or at different times, the birds can be allowed access to harvest the invertebrates, weed seeds etc.
Simply planting bird seed works, and is cheaper than buying seed when you get it on sale. Most mixes contain red millet, but the red millets are not a problem as a supplement. Allowing it to set seed, and then cutting to let the birds harvest it themselves works, in a rotation.
I like ideas that are cost effective, and labor friendly. I do not want to spend more on the supplement than I do the feed.