It comes down to the feather’s structure. In most birds, feathers are connected by hooks or barbs that lock together and make it possible for birds to swim or fly without losing too much energy, even over very long distances.
Because kiwi do not fly, their feathers have evolved a unique texture to suit a ground-based lifestyle. They are warm, shaggy and hair-like, hang loose and are much fluffier.
The patterns on kiwi feathers camouflage the bird, letting them disappear in the dark, or among tussock and bracken. These patterns evolved to keep kiwi safe from aerial predators that hunted using sight and sound, such as the now extinct goshawk.
Different kiwi taxa evolved different feather patterns to suit their particular niche and the subtle colours and markings can be used to distinguish the different species and varieties. For example, brown kiwi have – not surprisingly – brown feathers, while the little spotted and great spotted kiwi have beautiful speckly patterns.