Kiwis are strange birds in many ways, and not least for their short stubby wings and long, hair-like feathers. They are the smallest living ratites (flightless birds, including ostriches, emus and cassowaries), with the largest species reaching maximum heights of just 45 cm. Unlike flying birds, kiwis have relaxed constraints on their weight, and this is reflected in aspects of their anatomy such as their more solid bones. Most strikingly, without needing to fly, female kiwis are also able to produce the largest eggs relative to body size known in any bird. At 450g, the egg is so large that it takes up to 30 days to produce, and the female must fast for the last few days before laying due to the compression of the gut. Read more about the kiwi’s unusual skeleton here.

X-ray image courtesy of the Whakamanu Reserve.