Birdseed is a mix of small seeds sold to feed birds — in a feeder, on a tray, or sprinkled on the ground. It is not one plant’s seed; bags often mix millet, sunflower, and others, and different birds like different mixes. People also write bird seed, two words. You buy it at a pet shop or a garden shop. It is food for birds, not seed you plant in the ground hoping a bird will grow — the compound only tells you who the food is for. Wild birds and pet birds may need different bags, so the label matters.