In a superb piece of science, researchers have revealed more bird mothers teach their offspring a signature call while their babies are still inside the egg, which nestlings later use when begging for food.
A study by bird ecologists from around the world, including researchers from Flinders University in South Australia, demonstrates vocal teaching and learning begins in ovo (in the egg) across at least 7 species of fairywren and 1 grasswren. The findings are published in The American Naturalist.
Dr Diane Colombelli-Négrel from Flinders University, an author of the study, says the paper builds on a 2012 discovery that superb fairywrens (Malurus cyaneus) were teaching their offspring certain calls even before they hatched.
The new research shows the practice is common among similar species.
Researchers recorded 624 in-nest vocalisations of 125 female birds from 8 different species of Australasian wrens from the family Maluridae.
Species included: superb fairywren, splendid fairywren (M. splendens), red-backed fairywren (M. melanocephalus), white-winged fairywren (M. leucopterus), red-winged fairywren (M. elegans), variegated fairywren (M. lamberti), and purple-crowned fairywren (M. coronatus), as well as related malurid called the thick-billed grasswren (Amytornis modestus).
Colombelli-Négrel tells Cosmos, the first step was to find out whether and when other females from the Maluridae family sang to their eggs, and what these calls sound like.
Using video and audio devices, researchers observed and recorded females’ in-nest calls, categorising them into different elements, which the researchers classed as A, B, C or D.
While each bird’s call combined different characteristics, all 8 species shared the B element. Colombelli-Négrel describes this as a soft, small up-and-down trill, and is the same sound repeated by nestlings in their begging calls.
The results show most birds began teaching their embryos when the egg was around 10 days old.
“It makes sense, because before that age, the embryo is not fully developed […] So in this case, the mother only starts calling and teaching when the embryo is receptive,” Colombelli-Négrel says.
After the eggs hatched (at about 14 to 16 days), the nestlings repeated the B element.
The researchers also tested whether embryos responded differently to certain call elements while still inside the egg. In superb fairywrens they tested this by playing back recordings.
To measure the embryo’s response inside the shell, the researchers used a device from the poultry industry (used to determine whether an egg is alive or not) to measure heart rate based on light variation through the shell.
A lower heart rate indicated the embryo was paying more attention to the sound, Colombelli-Négrel says.
She says “a lower value generally indicates that they’re paying more attention to the sound, while a higher value, they’re not paying attention.”
The superb fairywren embryos responded more to playbacks of the B element than to other call characteristics.
Overall, the study shows that across the 8 species “the mother is teaching the embryos the particular elements, which they reproduce when they are begging for food,” Colombelli-Négrel says.
The next step in the research is to determine the function of the other call elements (the A, C and D parts).