Zebra finches are adept vocalists; mates often call to one another as they share parental care. But Mariette noticed that, sometimes, just a few days before their offspring hatched, the birds would sing when they were alone with the eggs, she told the BBC. The birds performed one particular song only during warm weather (above around 78 °F), she observed.
To examine the effects of this call, Mariette and her colleague Katherine Buchanan, also at Deakin, experimentally controlled the sounds that dozens of eggs heard, New Scientist reported. For three to five days before the eggs hatched, the scientists incubated them in warm conditions—around 100 °F—and played them socialization songs. For one group of unborn chicks, they also played the “heat call.”
Chicks primed with their parents’ hot-weather warnings grew more slowly and weighed less as they matured than did those that hadn’t heard the special song, Mariette and Buchanan found.
And the slower-developing offspring were more fit. The females that were primed for warmer weather had up to six fledglings in their first breeding season, whereas those that were not tended to have only one or two, New Scientist reported.
The fitness advantage may stem in part from the benefit of having a smaller size for more effective heat loss in a warmer climate, the authors wrote.
Whether developing avian embryos are directly affected by a song remains a mystery. “They’re not actually thinking, ‘Mum says it’s hot out there, I better take it slow when I get outside!’” Nicola Hemmings, who studies bird reproduction at Sheffield University in the U.K. and was not involved in the work, told the BBC. “But [this song is] having some kind of physiological effect on their body which is making their growth rate slow.”