Nyjer seed, also often called thistle seed, is great for attracting all types of finches, including ever-popular goldfinches. Nyjer normally refers to non-germinating thistle seed.
Even though nyjer seed can be a little more expensive, the reward of attracting a large selection of goldfinches is well worth the additional investment. Nyjer can be served in any finch tube feeder with small-sized ports or in a traditional thistle/nyjer sock. Squirrels on occasion will eat nyjer, but it is low on their list of favorite foods.