When it’s especially hot out, zebra finches sing a unique “incubation call” to their unhatched young—a song that seems to alter the chicks’ physical development after birth.
Mylene Mariette of Deakin University in Geelong, Australia first noticed the song while she was doing field recordings for her PhD. To find out its function, she collected 175 eggs from zebra finches at the university’s aviaries. During the final five days of the eggs’ incubation, she and her colleagues played some of the eggs a recording of this special song. For the rest, she played a regular parent-to-parent call.
When the eggs hatched, the chicks were returned to their nests and the scientists observed their development.
Here’s Jonathan Webb, reporting for BBC News:
“We found that, depending on whether or not they had heard the ‘hot call’ from their parents, they reacted differently to heat,” Dr Mariette explained.
“They adjusted their growth to temperature differently, and also solicited food from their parents differently.”
Specifically, the “heat song” seemed to make the chicks develop slower—and remain lighter—if their nest was in a hot corner of the aviary.
Being a lightweight is generally bad news in evolutionary terms, but when the researchers followed the finches’ fortunes into adulthood, it proved the opposite: those lighter birds that grew up in hot nests produced significantly more fledglings than their heavier childhood companions.
That lightness could help finches in hotter climes—warm weather places greater stress on tissues, so smallness would be an advantage. The chicks who’d been sung to also preferred toastier nesting spots once they hatched, suggesting that the song somehow helps prime chicks for dealing with warmer environments. The researchers also noted that in the wild, the finches only sing this particular song on days when the ambient temperature rises above 25 ° C, and at that, only in the few days before hatching occurs.
Of course, it’s not as though the unborn chicks are consciously picking up cues from their parents about the environment they’re about to live in—it’s much more likely that the acoustic signal triggers a hormonal response that modifies the bird’s prenatal development. Still, it’s a new way of conveying information that experts don’t quite yet understand.