I realize I'm too late for this year, but I recommend getting in touch with your local fish market, drying out the carcasses, blitzing them up, and adding to corn and sunflower seeds.
Truth is, aside from newly hatched birds, they probably won't keel over and die after a week of only corn...
I forgot to mention acorns. I gathered a fairly large number of them in the fall and forgot them in the fridge. They look like they've even been sprouting. They should be a good source of nutrients as well.
That's what I meant by any animal. Dogs, cats, whatever. As long as a I own the animal and need to feed it, it may not be fed grain products, even by someone else, because it monetarily benefits the me to not have to feed my animal as much.
Even if one avoids the issue of owning grain on Passover, feeding any animal under one's care is still considered benefit, as it keeps one from needing to buy more of whatever else they eat.
Owning or deriving benefit from the five grains (wheat, oats, spelt, barley, rye,) even whole, is problematic, as yeast fermentation may have begun already. (It must be kept super dry from the moment it was harvested.)
I do corn, sunflower seeds, oyster shell, fish-market waste, and table...