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Day 2B: Peach Fuzz The first feathers to grow are called down. Down is fuzzy and warm, but doesn't look like real feathers yet! Also, notice how these feathers don't cover the birds' entire bodies. They only grow in special areas called feather tracts. The first feathers grow on the back because the tummy and sides are pressed tight against the nest and the other nestling. The back is where insulation is most needed. Q: Why do you think most birds don't grow feathers over their whole bodies? (Answer under photo.) Photo: Dorothy Edgington A. Except for a very few birds like penguins, who swim in VERY cold water, most birds don't need to grow feathers over their whole bodies. One individual feather is a lot wider than one hair, and can cover a bigger space on a bird's body than one individual hair can cover on a mammal's body. But feathers take a lot of energy to produce. So birds have just enough outer feathers to cover their bodies when the feathers are groomed properly. An adult hummingbird has only about 940 feathers on its whole body. That's many fewer feathers than a mouse has hairs!