Figure 4. Feather branches provide functional forms for endothermy, flight and display.

A. Regional specificity of feather forms: birds have flight, contour, downy, and tail feather types (Lucas and Stettenheim, 1972). B. A schematic drawing of rachis morphogenesis with representative molecular control and morphological transition. C. A schematic drawing of barb morphogenesis. D. Birds with different flight modes show different bio-architectures of the rachis of their flight feathers. On the right is a feather trapped in amber dated 99 million years ago. Cross sections show ancient rachis shape (Chang et al., 2019; Wang et al., 2020).