The Nyjer thistle sack bird feeder is the simplest feeder we offer. They are very popular because Goldfinches and Pine Siskins will feed on every available inch of the sock. The Nyjer thistle sack bird feeder from Songbird Essentials are made of a strong nylon mesh that resists damage from bird claws and features a slide-lock to close the sack. The black thistle sacks make the birds standout and shine with their brilliant colors. The black color also blends in well and hides dirt better than the white sacks. You can hang this in your yard and watch more and more birds come by as they discover a new food source, or you can add this thistle sack to supplement a nearby feeder.