The flightless kiwi is so unbirdlike that many biologists call it an “honorary mammal.” Flightless and nocturnal, the kiwi’s feathers evolved into softened, fur-like filaments and its nostrils migrated to the tip of its long beak, which it uses to snuffle in the dirt of its forested New Zealand habitat for the archipelago’s famously giant earthworms. It’s a member of the ratites, the avian order that includes ostriches, emus, and rheas, but the largest kiwis are only the size of a plump laying hen, while the smallest is the size of a guineafowl. Strangest of all, it lays an egg that can weigh up to a quarter of its body mass. Proportionally, this is by far the largest of any bird in the world. Imagine a chicken laying a one-pound egg, or, more graphically, a human giving birth to a fully formed four-year-old. Yowza. An adaptation so bizarre is like a magnet for evolutionary biologists, and a slew of ideas about how the petite bird ended up with such a ginormous egg...