What's the pile of birdseed I found under a floor?

The birdseed was brought into the area by mice, but the lack of fecal pellets is a bit puzzling. Mice are seed eaters and hoarders, it seems that they have stockpiled the seeds, but were excluded from their nesting site by the person that stuffed the steel wool into the holes. The amount of birdseed would be ridiculous for a pest control professional to insert into the area, so more than likely the entire amount was put there by a family of mice that are no longer living in the void.