The image of a flu y little chick emerging from a hen’s egg after popping open the ‘lid’ is a sugary exaggeration of what really happens. The truth is no less remarkable: simply escaping the egg is the first potentially lethal challenge of many in the young life of birds.

A bird’s eggshell has to be strong enough to support the weight of the incubating parent, but not so robust that the chick cannot get out. How is this tricky compromise achieved? The answer is through a clever evolutionary trick in which the calcium-carbonate crystals that make up the eggshell are arranged like the wedge-shaped bricks of an arched stone bridge, such that they can be safely pushed down upon (by the incubating parent), but when pushed from the inside, are less strong, allowing the chick to escape.

The eggshells of small songbirds — like our robin — are