Both females will have laid a fertilised egg and randomly shunted one aside (Biology Letters, vol 4, p 323).

这两只雌鸟会各自产下一枚受过精的卵，并且随机扔掉一枚（《生物通讯》，第4卷，第323页）。

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The twins originate from a single fertilised egg, so they are always identical and of the same sex.

BBC: NEWS | Health | Medical notes | Conjoined twins

Conjoined twins are created just a few days after they are conceived - most likely by the incomplete splitting of the fertilised egg.

BBC: Twin separations: A history of risk

Putting only one fertilised egg into the womb avoids multiple births, which are known to increase the health risks of both mother and child.

BBC: Non-invasive embryo gene screen