Safflower has the added benefit of being too bitter for squirrels, and too hard to crack open for gregarious blackbirds, such as grackles and starlings - all species we associate with quickly depleting the backyard feeder stock!
While it is one of the more expensive seeds, I prefer to fill a feeder with just safflower to see which other birds it attracts. I have seen House Finches, Tufted Titmouse, chickadees, White-breasted Nuthatches, Mourning Doves and woodpeckers. A more affordable way to introduce safflower is to purchase a seed mix that includes safflower, and these species will readily enjoy picking them out.
photo © adrian binns