Which Bird Lays the Largest Eggs for Its Size?

Kiwi birds have the largest egg-to-body size ratio in the animal kingdom;
the egg is about 20% as big as the mother.

The kiwi is the bird that lays the largest eggs for its size. On average,
the kiwi lays eggs that about 20% of the mother’s size. Although the kiwi
is about the same size as a chicken, it lays eggs that are roughly six
times larger. Although it is not known why the kiwi lays such large eggs in
proportion to its body size, a generally accepted theory is that the
species used to be larger. It is thought that the kiwi gradually became
smaller over time, while its eggs remained the previous size, but it is not
known why.

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