The ostrich lays the largest egg of any bird in the world today – it can be up to 15cm in diameter, and weigh as much as 1.3kg! Compare that to a hen’s egg – ostrich eggs are nearly 25 times the average weight of a chicken egg.
However, this particular claim to fame on the part of the ostrich is only because the ostrich itself is so big – in reality, the ostrich egg is one of the smallest compared to the adult size of any bird in the world, only about 2% of the adult’s body weight per egg. It is the biggest bird, so it lays the biggest egg. So far, so simple. Below is a display we have in the museum of some of the largest eggs in the world. The emu and the cassowary are similar to the ostrich and quite large, a species of albatross has the largest wingspan of any bird in the world (up to 3.5m, in case you were wondering) and is, as anyone who has visited our Living Worlds gallery can attest, also quite large, but wait… what’s that in the corner?
The kiwi is the national bird of New Zealand, and is best recognised as the nickname by which New Zealanders are affectionately known by other countries. The bird itself is, like the ostrich, flightless, and has distinctive nostrils right on the end of its long beak. To illustrate further, they look like this:
Brown Kiwi, from the Living Worlds gallery at Manchester Museum.
Yes, that’s right – they’re actually quite small birds, about chicken-sized. Yet while the ostrich egg is about 15cm, the kiwi egg is nearly that size at about 12cm. The kiwi’s egg is up to 20% of the adult bodyweight! It takes roughly a month for the egg to grow inside the female kiwi, during which time it eats much more than normal and then for the two to three days before it lays its egg, it often cannot physically eat anything due to lack of room. To illustrate, here is an x-ray:
Ouch! Their behaviour while pregnant suggests this just might be as painful as it looks – females’ bellies drag along the ground, particularly in the later stages, and they may seek out cool water to bathe in to relieve some of the pain and weight they are carrying.
After ‘ouch’, many peoples’ next reaction is ‘why?’, and with good reason – quite simply, there are several theories about why on earth kiwis have evolved to have such disproportionately large eggs, but no consensus.
What do you think? Come on by and see it for yourself – you won’t be able to believe your eyes!
Little Spotted Kiwi, Nature’s Library gallery at Manchester Museum. Ouch!
-Post by Bryony Rigby
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