Bread typically becomes "stale" (texture change due to internal moisture redistribution) before it spoils (usually by mold growth). Stale bread is still edible, and there are recipes that prefer it over fresh bread, e.g. bread pudding and bread sauce. It does not have to be fed to chickens to avoid wasting it.
And wheat can be converted to pasta, which can be stored for years, and can be easily made edible by brief cooking. You can reduce wastage of wheat to negligible levels, and chicken farming has non-zero overhead, so it seems unlikely that the chickens are an efficiency win if you take reasonable care.
It's pretty reductionist to look at this from a pure calorie input / output point of view. Keeping a few pastured chickens to feed food waste can provide pest control in a garden, an additional source of nutrition for people (which is a complete protein) and a rich fertilizer from their waste.
And wheat can be converted to pasta, which can be stored for years, and can be easily made edible by brief cooking. You can reduce wastage of wheat to negligible levels, and chicken farming has non-zero overhead, so it seems unlikely that the chickens are an efficiency win if you take reasonable care.