Feather is one of the unique characteristics for birds. It serves as the fur for mammals. The primary feathers, known as contour feathers are the most common type of feathers found in birds. It has the long flat shape with a hollow stick called a calamus supporting the root and connecting to the rachis. Lateral barbs then branch off from the rachis creating the soft and smooth shaft of feather. The inner part of the feather serves as insulation, and the outer part is for different purposes. As the feather goes inward toward the body, it becomes more rigid structure to support the shape and give insulation.
Comments
Articles
For the first sentence you could add the article the. So it would read "The feather is a unique characteristic in birds." or something else like "Feathers are a unique characteristic in birds."
suggestion
I would suggest to try using some transition words so that the sentences flow better with one anohter and are less 'choppy' or combine sentences. For example, "Feather is one of the unique characteristics for birds comparable to fur being exclusive to mammals."
Feathers not unique to birds
Current thinking seems to be that feathers actually appeared in an ancestor to dinosaurs. Birds are the only extant taxon that has them now, but feathers came before birds existed and so are not a unique characteristic of birds.