We may finally know why birds sing at dawn

Morning birdsong has inspired poets and nature lovers for thousands of years, but the reason why birds around the world start the day this way remains an enduring mystery.

Now a series of experiments on zebra finches (Teniopygia guttata) suggests that although darkness interferes with singing, birds have a stronger motivation to sing at night, leading them to sing at dawn. The study also hints that exercising the vocal muscles in the morning helps birds hone their songs.

Satoshi Kojima from the Korea Brain Research Institute in Daegu, South Korea, and colleagues studied lab-reared zebra finches under carefully controlled lighting conditions to manipulate the timing of light and dark cycles.

The team first pushed back the time the finches were exposed to sudden bright light to 3 hours later than the actual dawn. The birds did not sleep, but were silent in the artificial darkness, and when the lights finally came on, the birds sang louder than usual.

When the light came on 3 hours before the real dawn, the birds still burst into chorus, but without the same intensity as when they were forced to wait.

In other words, Kojima says, the more time passes between the birds waking up and dawn, the more intense the morning chorus.

“The birds were already awake in the dark before the lights came on,” he says. “But their spontaneous singing is suppressed by the darkness. This suppression increases their motivation to sing, resulting in a high frequency of rebound singing immediately after the lights are turned on.”

The researchers then trained the birds to press a lever to receive 10 seconds of artificial light. When artificial daylight was delayed 3 hours, birds often pressed the lever, but they rarely did so when artificial daylight arrived 3 hours earlier.

Next, scientists introduced the drug luzindole, which blocks the action melatonina hormone produced at night that helps regulate wakefulness cycles in many animals. Birds that were given this drug 5 hours before the usual light-on time woke up faster and began singing earlier than those that were given saline instead.

Kojima and his colleagues also analyzed the birds' songs to see how they changed throughout the day. They found that there were rapid changes in song structure in the first hour after dawn compared to the second.

“Due to the lack of singing at night, the vocal-motor system and acoustic structure of the song may deteriorate somewhat, and the morning chorus serves to quickly restore or optimize them,” Kojima says.

Although this study only looked at one species, similar factors may apply to other bird species, he said. “Given the simplicity of these mechanisms and the functional implications, we hypothesized that they may largely underlie morning chorus in different bird species.”

But Diego Gil at the National Museum of Natural Sciences in Madrid, Spain, urges caution. “There are many differences between species in how, when and which birds sing in the dawn chorus,” he says. “This explains why 11 different hypotheses have been proposed to understand this phenomenon.”

Gil says the study is smart, and the researchers do show that birds' desire to sing increases in the pre-dawn hours. “If zebra finches are discouraged from singing by giving them longer nights, their motivation to sing increases,” he says. “They also show that the nights are too long for them to sleep and that they would rather sing if they had the chance.”

However, the study doesn't prove that the dawn chorus' function is to enable birds to improve their songs, Gil says. “These changes in song structure are hypothesized to improve singing, but there is no evidence that females actually prefer these changes,” he says. “The study simply shows that the song gradually changes over time of singing.”