The seed you feed birds does has a shelf life. It will go stale. A reader asked that question recently. For an answer I called Melissa Block, manager of the All-Seasons Wild Bird Store in Wayzata. She told me that seeds do lose nutritional value as they age. Sunflower kernels, the nuts inside the shell, dry out, losing nutrients. Thistle seed has the same problem. She tells her customers to buy no more seed than they can use in six to eight weeks. She also said that the birds will know if your seed is worth eating or not. It they're not attending your feeders with the same frequency and in the same numbers this week as last, replace the seed. Clean the feeder, and fill it with seed just purchased. She said that where you buy seed and how it's packaged is an issue. Buy from stores where stock frequently revolves. I've always believed that the plastic bags of seed you see on super-market shelves are not good buys. The seed mixes often contain filler seeds that most of our feeder birds will not eat. And now, consider how long those bags might have been on that shelf.
Feeder birds, like this Black-capped Chickadee with the sunflower seed in its bill, know good, fresh seed from stale seed.