My colleagues incredulously rejected the notion that the chicken-sized kiwi lays eggs six times the size of a chicken egg. Well, it wasn’t just a fancy, it really is the case. A couple of days before the egg is laid, it assumes so gigantic proportions that it fills the female’s whole body cavity, forcing the bird to fast. It’s a huge burden for the kiwi, while developing the egg, it needs to eat three times as much as usual. The kiwi has the largest egg-to-body ratio (1:5) among all birds.
I wonder what adaptive advantage this bird gains by producing such a monster egg. I think the answer lies in the lack of natural predators. No native mammals exist in New Zealand (except for bats) that could prey on them, which is the primary reason for the loss of most of the birds’ ability to fly (think of the kakapo or the takahe, all flightless) on the island. The energy necessary to fly is turned over to producing enormous eggs. Not incidentally, the lack of predation allows for the birds to have very low reproduction rates, raising only one chick per season. As a matter of course, the whole agenda is turned upside down by the introduction of invasive species like rats, pigs and feral cats, wreaking havoc in the bird nests and effectively reducing the population size of these wonderful, but hapless birds.
An X-ray image: