Some avian predators (some hawks and members of the falcon family) specialize on hunting and eating other, smaller bird species.

Feeders are both a resource for the seed-eating birds and for birds that prey on seed-eating birds. Some predation by birds is to be expected, however, you can make your feeders more "safe" for visiting birds by locating those feeders near escape cover, such as shrubs and trees. Providing a mix of safe escape cover AND food will likely increase the number and diversity of birds coming to eat at your feeders.