The seed that attracts the widest variety of birds, and so the mainstay for most backyard bird feeders, is sunflower. Other varieties of seed can help attract different types of birds to round out your backyard visitors. In general, mixtures that contain red millet, oats, and other “fillers” are not attractive to most birds and can lead to a lot of waste as the birds sort through the mix.
There are two kinds of sunflower—black oil and striped. The black oil seeds (“oilers”) have very thin shells, easy for virtually all seed-eating birds to crack open, and the kernels within have a high fat content, extremely valuable for most winter birds. Striped sunflower seeds have a thicker shell, much harder for House Sparrows and blackbirds to crack open. So if you’re inundated with species you’d rather not subsidize at your black oil sunflower, before you do anything else, try switching to striped sunflower.
People living in apartments or who have trouble raking up seed shells under their feeders often offer shelled sunflower. Many birds love this, as of course do squirrels, and it’s expensive. Without the protection of the shell, sunflower hearts and chips quickly spoil, and can harbor dangerous bacteria, so it’s important to offer no more than can be eaten in a day or two.
Sunflower is very attractive to squirrels, a problem for people who don’t wish to subsidize them. Some kinds of squirrel baffles, and some specialized feeders, are fairly good at excluding them. Sunflower in the shell can be offered in a wide variety of feeders, including trays, tube feeders, hoppers, and acrylic window feeders. Sunflower hearts and chips shouldn’t be offered in tube feeders where moisture can collect.
For more information – https://www.allaboutbirds.org/types-of-bird-seed-a-quick-guide/