Fresh Thistle Wildbird Seed Is A Favorite Of Finches And Other Small Birds. Iowa's state bird, the Goldfinch, loves this food. Thing about thistle is as it is a small seed it needs to be kept dry or it will mold. Most Thistle feeders have smaller openings. If you put it in a sunflower feeder it will pour out the larger opening. Thistle needs to be kept fresh. Unless you a going through it very fast, you want to buy it often in small amounts. You can mix or layer Thistle with the fine Sunflower chips for some very happy birds. Thistle is sold by the pound.