most any seeds or nuts or grains even coffee beans have oil in them. Oils and fats can go stale easily. what some people do with even stuff they use for themselves is store the flour or corn meal or coffee or even bird seed and nuts
oats or anything with oil in it in sealed bags or containers in a chest freezer. It keeps them from getting stsle or rancid. the other thing to do is buy in small portions and use up before they get old. Also if you live in a humid area things can mold easily the the freezer keep them from getting moldy.

Makes sense - I'll throw it all out.

our finches won't eat at all if the thistle is old/stale. If it rains heavily and the feeder gets damp, they also won't eat it.

We always pour the old seed on top of the new seed when we refill, so the bottom doesn't get full of old seed.

Fussy little devils-

I've got about 3 gallons of thistle left. (In old whiskey bottles, just counted the bottles and divided by 5). I pour it on the ground in the garden and it disappears so somebody seems to like it.

If you have turkeys around, they love thistle and might not mind a little staleness. Various native sparrows and juncos forage thistle on the ground here so you may be making them happy.

Claire