Pin feathers are the small, undeveloped feathers that grow from the base of a bird's feathers. These feathers are crucial for birds, as they help them stay insulated and helps in their flying process. While "pin feathers" is the most commonly used term for these types of feathers, there are also other synonyms that can be used, including quills, spines, filoplumes, and plumules. Each of these words refers to different types of feathers, but they all serve a similar purpose in helping birds to stay protected and fly efficiently. So the next time you see a bird with small, developing feathers, you can use any of these synonyms to describe them accurately.