They love Black Oil Sunflower sees, but so do many other birds that are larger and more aggressive and may hog the feeders. You can buy special feeders designed for use with Niger Seed, which is a tiny black seed, fairly expensive but very attractive to small birds. The feeders in question require the bird to cling to the side of the tube feeder while pulling the seeds out through the fine mesh. There are no perches provided. This keeps a lot of species from stealing the Niger seed because they aren't very adapt at clinging like that...which in turn keeps those feeders open for expert clingers like Chickadees, Goldfinches, Redpolls, Nuthatches, etc...and not much else.