Flax seed attracts some birds. Few seem to care about wheat or oats or barley. Corn, either whole, cracked or on the cob, is popular with Jays, Titmice, Cardinals and Red-bellied Woodpeckers. Milleted sorghums are fine and inexpensive. Birds much prefer white to red millet according to a wholesaler of bird seed.
T here are dozens of other foods that birds like.
Dog biscuits pulverized with a rolling pin make a fine food. Nutmeats of any kind are good. Pumpkins, squash and cantaloupe seeds attract some birds. My window ledge feeder holds a smorgasbord of odds and ends. It's fun to see how birds vary their diet, given a choice.