Birds' eggs are not just beautiful - their form and function provide overwhelming evidence of the efficacy of natural selection. We are so familiar with the uniform appearance of supermarket hens' eggs that we barely give them a second thought, yet the sight of the sky-blue, sparsely speckled egg of a song thrush still fills us with wonder. In a way this is a superficialwonder, for it is focused entirely on the egg's appearance, with perhaps a nod to the fact that, all being well, it willhatch and produce a new thrush. However, more remarkable than an egg's external appearance is the way that it forms inside the mother's body, and how - once it is laid - the egg functions as an external life-support system. But just how is the miracle accomplished?
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