PhD student Sarah Dobney holding savannah sparror chickPhD student Sarah Dobney found that dense vegetation over their nests can affect what songs young savannah sparrows hear and learn from their elders.

Nest environment affects birdsong acoustics, study finds

Savannah sparrows living on a remote island off the coast of New Brunswick build their nests on the ground under dense vegetation. Sarah Dobney, a PhD student in the Department of Integrative Biology, wanted to know if this thick covering limits what nestlings can hear, potentially altering song learning — and set out to find out.

Nestlings learn to sing by listening to adults nearby. But Dobney says it was unknown what range a nestling can hear from inside the nest.

She devised a unique experiment to measure their hearing threshold with an acoustic recorder called a song meter. She took a typical omnidirectional microphone, with two back-to-back microphones, and split them up. She placed one microphone directly in the nest and clipped the second on a pole a meter above the nest, about where an adult would perch.

“I waited and let birds sing naturally and honestly, it was really awesome. We found songs are much quieter inside the nest,” says Dobney.

“I expected there to be a big difference, but I wasn’t prepared with how big that effect would be by just moving the mic into the vegetation that surrounds the nest — it was cool to see how drastic that was.”

The recordings were done after the young birds had already left the nest, so as not to disturb them during this sensitive phase of their lives. 

Dobney found that songs recorded from inside the nest were an average of 11 dB quieter than when the same songs were recorded just one meter above the nest, outside of the dense vegetation.

“This substantial reduction in amplitude means that nestlings can likely only detect song a maximum of 78.6 m away from the nest, which is much smaller than previously estimated,” she says.

“To learn their songs, savannah sparrows must hear tutor songs early in life. Our results suggest that nestlings hear 27 per cent fewer tutors than previously recognized.”

Male savannah sparrows use their songs to attract females and to ward off other males, so their song has huge implications into how they defend their territory, their resources, and how well they can attract mates, Dobney says.

Like human infants, savannah sparrows go through a critical period learning to make sounds.  When they are young, they memorize everything they hear and imitate to make their own individually distinctive sounds.

“I became interested in vocal learning because it is not a common behaviour,” she says.

“In this case, the nest may be surrounded by 15 singing adult males but say five of those males, nestlings aren’t even hearing.”

Individuals in this population return to breed in the same spot where they hatched as a baby. Dobney says this island population allows for lifelong studies of the savannah sparrows. Next, she will study whether early life social factors influence song complexity of young birds.

“Before answering questions about social factors, first you must understand what a young bird hears so you can know which adult males (aka tutors) to include and exclude from analyses,” she says.

“These results are really important and totally change my approach from how I initially planned to investigate this topic.”

Dobney’s research was recently published in the journal Avian Research in a paper called “Quiet in the nest: the nest environment attenuates song in a grassland songbird.” Integrative biology professors Dan Mennill and Stéphanie Doucet, her supervisor and collaborator respectively, co-authored the paper.

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