They're really tiny, barely 3.5 mm, 1 3/8 in., snout to tail. The bag was swarming with them, which I didn't notice until I had poured out a bowlful for my juncos. By then, many had escaped and were scattering across my kitchen counter.
I got rid of all I could catch (they run pretty fast for such tiny critters), but I was too late. Now, every evening a few come out from some hideout, and head across the kitchen floor.
Two in a plastic lid.
There was a pinhole in the bag. I began to wonder if they made it, and which way they were going; in or out?
I sent their photos in to BugGuide for ID, and heard back within the hour. Yay BugGuide! It's a granary beetle, one of the Sitophilus (grain loving) species. And my question is answered already; they were leaving the bag.
The adults, which are unable to fly, live for 7 to 8 months and during this period each female lays about 150 eggs. In egg-laying, the female drills a small hole in the kernel, deposits an egg in the cavity and seals the hole with a gelatinous secretion. There is only one larva in each infested kernel. The white, legless grub completes its growth, pupates and develops into an adult weevil within the kernel. After reaching the adult stage, it eats its way out of the kernel. (From Canadian Grain Commission)And after eating their way out of the kernel, they kept on going, out of the bag. So they're somewhere in hiding, and for at least 8 months, I'll have to make sure all grains are in bug-proof containers, not plastic bags. As for the weevil-infested seed, it's now stored in the freezer.
At least the birds will like the extra protein.