Yep, the thistle/nyjer seed sold as bird feed has been treated. I'm
more concerned about squirrels getting at/ripping open the "thistle
sock" before the goldfinches figure out it's there.
Squirrels are why my regular seed feeder is a Brome Squirrel Buster
feeder. It really works to keep them from eating all the seed. (I
don't begrudge them snacking on what falls on the ground and the patio.)
I may just invest in one of the Brome Thistle Feeders if I do start
seeing goldfinches again. :)