Chicken meal and poultry meal ingredients are commonly made by vacuuming WHOLE chickens into a grinder. Obviously, this would include the feathers (not to mention intestines and their contents). Feathers don't disintegrate completely during grinding and cooking, which means you can find pieces of feathers sticking out of the food.
When animals are rendered to make meat and bone meal, animal fat, animal digest, or meat by-product meal, the entire animal carcass is melted down. This includes the hide and hair. Once the grease or fat is removed from the cooking process, the remaining cooked meat is run over shaker screens that remove the largest chunks of bone and hair. However, the hair may not be completely removed, leaving pieces of hair large enough to be found in the final product. The hair from pigs is particularly coarse, which leads some pet food companies to claim that small metal filaments found in the food are actually swine hair.
The barnyard can also have pieces and bits of plastic that animals can ingest. Hay bales are bound with plastic or wire and trash from fields gets incorporated into the hay when it is baled. This all gets ground into animal feed, which then ends up in pet food when all is said and done.
So don't be surprised next time you find foreign material in the kibble. It's there.