As a bird lover, (I have a cockatiel---resides on cage but not locked in a cage, can get in and out), I am happy to see that you have taken the trouble to ask about old bird seed. While I don't know the answer, I would think mold might be bad for some songbirds. I keep my seed fresh and enjoy all the attention the birds give my feeder. The fact that you care about wildlife in this way makes me believe you are a good person. I would err on the side of caution and get rid of it. Also, it was good advice to clean the bird feeder occasionally.