A new study from the University of Southern Denmark has revealed that songbirds have extremely high-resolution control over their vocal output. The experts report that birds can control single vocal muscle fibers while singing.
Songbirds have a unique vocal organ they use to produce their beautiful songs. This organ is surrounded by muscles that contract at high speeds.
“We found that songbirds have incredible fine control of their song, including frequency control below one Hertz,” said study lead author Professor Iris Adam.
A motor unit refers to a motor neuron and all of the muscle fibers that it activates. The researchers found that a large portion of the motor units must be very small – even as small as single muscle fiber.
“Motor units vary in size from several hundreds or thousands of muscle fibers in our leg muscles down to only 5-10 in the muscles controlling eye position and the muscles in the larynx,” explained study senior author Dr. Coen Elemans.
“In zebra finch song muscles our models predicted that 13-17% of the motor neurons innervates a single muscle fiber.”
The team developed a new technique to measure the activity of all muscle fibers within a small muscle.
“Our new method allowed us for the first time to activate single motor neurons and visualize and record the activity of all responding muscle fibers simultaneously,” said Professor Adam.
“Like this we were able to show that the song muscles of zebra finches indeed contain motor units as small as one muscle fiber.”
The researchers wanted to know what effect such small motor units may have on birdsong. They measured the amount of stress the muscles can make and how such stress changes the frequency of the sound.
“Next to small motor units, we discovered that songbird vocal muscles have the lowest stress measured in any vertebrate,” said Professor Adam.
“To be able to study how changes in muscle force alter the sound made by the birds’ vocal organ, the syrinx, we had to invent a new setup,” said Dr. Elemans. “This setup blows air through the syrinx while we can control the muscles with small motors.”
Professor Adam noted that songbird vocal muscles are among the fastest muscles known, and now we show that they are also the weakest with the highest level of control possible.
It is important for the songbirds to maintain this level of control. Previous research has shown that females can detect small changes that influence whether they are attracted to a male or not.
“We think that next to a special syrinx and their amazing ability to imitate sounds, the fine gradation of the song features such as pitch has increased the amount of different sounds a bird can make,” said Professor Adam.
The study is published in the journal Current Biology.