Formulated feed like layers pellets have a distinct shelf life if stored properly in a dry dark environment. These feed contains fats and oils and processed ingredients which will spoil if given the chance or allowed to age past their dates.
Careless storage, allowing damp, pests or contamination can lead to mycotoxins which can kill in hours.
The solution is to buy your feed in the right size bags for your flock. buying huge sacks for 3 birds may be reverse economy that ends up costing more in the long run.