[Campbell-Staton] The male finch flew off, leaving the female on her nest, and Mylene noticed a very faint call... [Faint chirping] ♪ a call she had never heard before.

[Mylene] Some very unusual calls.

"Ch-ch-ch-ch-ch-ch-ch-ch."

[Faint chirping continues] Really sounds like crickets, actually, so it's really high-pitched, really fast.

[Faint chirping continues] It was quite hot on that day, and that's when I realized, "Oh, maybe it's related to heat."

[Campbell-Staton] And that was when Mylene made the kind of mental leap that leads to a scientific breakthrough.

♪ She had the notion that the mother was talking to her unhatched chicks.

♪ [Mylene] I decided to play those calls to the egg and see if they listen.

[Campbell-Staton] She set up an experiment with two incubators.

[Mylene] In one incubator, the embryos were hearing the heat calls... [Faint chirping] that the parents make when it's hot.

[Chirping] [Campbell-Staton] The other group were played a set of normal contact calls.

[Chirping continues] ♪ [Mylene] The only difference between the two groups of eggs was the sound that they heard.

♪ [Campbell-Staton] And then she sits back and waits.

[Shells cracking] ♪ As her young hatchlings grow, she measures them and records something incredible.

The chicks that were played the heat song in the egg are developing differently.

♪ [Mylene] Just being exposed to the environment for 24 hours, they already started to diverge.

and then by the time they are 12 days old, we can see a big difference in their growth.

This one has heard the heat song, so he's a bit smaller than the other one, which didn't hear the heat song, and so he's just grown bigger.

[Campbell-Staton] It turns out those that hear the heat call are smaller and are more likely to survive.

[Mylene] When it's hot, they actually produce less energy.

If you produce less heat, then you can grow better with less food.

They are better at coping with the heat for longer and to higher temperatures.