It comes down to structure. In most birds, feathers are connected by hooks or barbs that lock together. This continuous covering is like the body and wings of an aeroplane and makes it possible for birds to swim or fly without losing too much energy, even over very long distances.

Kiwi feathers aren't like this at all – they hang loose and are much fluffier. Because it does not fly, it has evolved its feathers to suit its ground-based lifestyle.

Perfect Camouflage
Kiwi feather patterns are effective camouflage and allow the bird to disappear in the dark, or amongst tussock and bracken.  This evolved to keep the kiwi safe from aerial predators that hunted using sight and sound, such as the now extinct goshawk.

Different kiwi types have evolved different feather patterns to suit the particular niche environment in which they live.  This means the subtle colours and markings can be used to distinguish the different species and varieties.  For example, the Brown Kiwi has - not surprisingly - brown feathers, while the Little Spotted and Great Spotted Kiwi have beautiful spotted speckly patterns on their feathers.

Along with its feathers, the kiwi has evolved strange cat-like whiskers on its forehead and at the base of its beak.

These super-sensitive way-finding whiskers have probably evolved to help the bird feel its way through the dark.