(Sky News) Scientists have discovered something unexpected about the way birds communicate with their unborn chicks.
One species sings a special song to their eggs to warn their offspring if the weather outside is getting too hot.
On hearing the song, baby zebra finches hatch sooner and gain less weight - meaning they're smaller and better able to keep themselves cool.
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Ducks and chickens also 'speak' to their eggs, meaning that when they hatch they already recognise their parents by their voices.