Indian Meal Moth caterpillar. Found in the birdseed from this photo:
Removed from the birdseed and placed on a piece of pine bark for the photo. The length was hard to gauge; the caterpillar would contract and extend its body during its movement and was very active. A guess of appox. 1cm in length when mostly extended.
(This individual was not reared into an adult but there's a lot of circumstantial evidence behind the ID)