There’s nothing added to the seeds, but there’s a good chance that some things are not taken out of the original harvest.
Grains and other seeds can contain lots of foreign materials, from dirt and debris to seeds from other plants. They are grown on a field after all, there’s soil, wind, rain and then they are harvested and transported. Part of the processing steps for seeds and grains for food production is to “clean” the batches and do some tests with regards to quality standards. Overall, these contaminants are usually mostly harmless (although biting down on a stray stone can be very uncomfortable and keep your dentist busy, effectively eating up all potential savings from buying bird food over human food), but if you have a batch with grains/seeds that have gone bad or contain harmful other seeds, things can get nasty.
Cleaning steps for livestock feed are less meticulous and for bird seed, processing is down to the bare minimum (basically what’s necessary for storage purposes), because ultimately the birds will pick up the seeds one by one anyway - think Cinderella and the doves.
So in short, the choice is yours. The bird seed (assuming we are talking plants that are part of the typical food store range, not just any random seeds) should be edible, whether it meets your expectations of safe is up to you. If money is an issue, I would rather check out whether cheaper options like non-name-brands or buying in bulk are available. Although for the latter, time is a factor as especially oily seeds can go rancid quickly, so can be worth to calculate carefully.
