Final answer:
The statement is false; down feathers of a bird do have a rachis, but it is not as pronounced as in contour feathers. Down feathers are structured to trap air for insulation, whereas flight feathers are adapted for aerodynamics of flight.
Step-by-step explanation:
The statement that down feathers of a bird do not have a rachis is false. Down feathers do have a rachis, but it's less prominent than in contour feathers. Contour feathers are the ones with a defined central shaft, the rachis, from which parallel barbs branch out. Down feathers, on the other hand, are fluffy because their barbules are not interlocked and do not form a structured vane like that of contour feathers, providing outstanding insulation. However, down feathers still have a central rachis; it's just that the structure is designed differently to serve its primary function of trapping air and keeping the bird warm.Within birds, the two main types of feathers are down feathers and flight feathers. Flight feathers, which include primary and secondary feathers, are tailored for the aerodynamics of flight, while down feathers are specialized for insulation. Both feather types are critical for a bird's survival, contributing to flight mechanics and thermoregulation.