Bird feeder filler is a term used to describe the materials that are used to fill bird feeders. The most common bird feeder fillers are SUET, SEED, and seeds. SUET is a type of animal fat that is often used as a high-energy food source for birds. SEED is a general term for a variety of small, dry fruits or grains that can be used as bird feed. Seeds are the same as SEED, but they are usually larger and more nutrient-dense than regular SEED.