The seed used to be a large part of the mix of packaged birdseed.
It still was and may still be. For a while it was still packaged in mixed birdfood until civvies found out and began planting the seeds. Afterward, manufacturers sterilized the seeds so they wouldn't grow (sterilized is what they said). I guess it worked, but I'm not sure if it is still packaged for the birds these days; I haven't checked.