Nyjer seed, or thistle, is a favorite of finches: Goldfinches, Purple Finches. The small black seed is often used in Canary mixes and now a wild bird food.
Finches are more attracted to the Nyjer than sunflower seed because of its smaller size. While the seeds do have a hull (and tend to pile up), the seeds do not sprout as the seeds are sterilized.
A tempting delight for all finches, plus many other small colorful songbirds. Rich in protein and fat makes nyjer seed a life supporting food, especially in cold weather. Use a feeder specifically designed for this small seed.