The small flightless bird certainly doesn’t lay eggs any larger than its ancestors the emu and ostrich but in comparison to its small body the kiwi’s eggs are enormous. While an emu’s egg only takes up 2% of their mother’s body weight, kiwi eggs take up almost 25%.
The kiwi is about the same size as a chicken but kiwis lay eggs nearly 10 times the size of a chicken egg. On top of that, a kiwi bird may lay up to 100 eggs in a year. This painful sounding birth isn’t a total disadvantage for the mother. Having such a large egg means the yolk inside is big enough to provide nutrition for a well-developed and independent chick. This way the mother doesn’t have to invest so much energy into parenting after the chick has hatched.
It is believed that the reason the kiwi egg is so big is because once upon a time the kiwi was actually as large as its ancestors. The kiwi was most likely the same size as an emu but as evolution has taken its course the kiwi became smaller but its egg did not.
This strange bird has many mammalian qualities including functional ovaries and soft fur like feathers. The kiwi bird has been evolving over 70 million years and before the introduction of predators such as cats and possums, was perfectly suited to the New Zealand environment. Over 80 years ago there were more than 5 million birds that still existed, today that number is down to 50,000, which makes them a critically endangered species.