Unlike specific seed packets that contain only one type of seed, wild bird seed is a mixture. Normally, you will want to feed specific birds and will select your bird seed based on that. For example, Finches love Thistle seeds and Bluejays enjoy cracked corn.
However, when you want to attract wild birds (as opposed to those that are domesticated), you will need to buy bulk wild bird seed. This will contain a lot of different types of seeds to attract many different types of birds. The selection of seeds varies widely. A nice way to classify wild bird seeds is by determining what “category” of birds they attract. For example, one mixture might attract all ground based birds.
However, commercially available bulk wild bird seed is usually looked on with suspicion. This is because many bird seed suppliers introduce fillers into the mixture. Fillers are an inexpensive material that adds bulk to the seed and is almost universally unattractive to birds. Depending on your location this can vary. For example, red milo is unattractive to birds in the East, but not in the West.
This is an important point to remember when buying bulk wild bird seed online. Since the dealer can be on the other side of the country, ensure that you double check the type of bird seed, as it may not be suitable for birds in your area.