Nyjer (also called thistle or finch) feeders are especially popular with American Goldfinches, Pine Siskins, and Common Redpolls. They come in two forms—tube feeders with very small feeding ports, and “thistle “socks”—fine-mesh bags to which birds cling to extract the seeds. The seed within thistle socks can become quite wet with rain, so only use large ones during periods when you have enough finches to consume the contents in a few days. The same caveats are true of nyjer tube feeders as other tube feeders.
Pesky Squirrels aren’t particularly attracted to nyjer seed, so normally leave them in peace and don’t damage even fairly flimsy plastic nyjer feeders.