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The dawn chorus of birdsong has inspired poets and nature lovers for thousands of years, but the reason why birds all over the world start the day this way is an enduring mystery.

Now, a series of experiments in zebra finches (Taeniopygia guttata) suggests that while darkness inhibits singing, birds build up a stronger motivation to sing in the night that causes them to burst into song when the dawn breaks. The study also hints that a morning workout for the vocal muscles helps birds finesse their songs.

 at the Korea Brain Research Institute in Daegu, South Korea, and his colleagues studied laboratory-raised zebra finches in carefully controlled lighting conditions to manipulate the timing of light and dark cycles.

The team first pushed back the time at which the finches were exposed to sudden bright light to 3 hours later than the actual dawn. The birds were awake but stayed silent in the artificial darkness, and when the lights eventually did come on, the birds sang more intensely than usual.

When the lights came on 3 hours earlier than true dawn, the birds still broke into a chorus, but without the same intensity as when they were forced to wait.

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In other words, says Kojima, the longer the time lag between when the birds wake up and the onset of light, the more intense the dawn chorus.

“The birds are already awake in the dark before the lights come on,” he says. “But their spontaneous singing is suppressed by the darkness. This suppression elevates their motivation to sing, leading to a high rate of singing as a rebound immediately after the lights are turned on.”

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

Next, the scientists administered the drug luzindole, which blocks the effects of melatonin, a hormone released in the night-time that helps to regulate wakefulness cycles in many animals. Birds that were given this drug 5 hours before the normal lights-on time woke up more quickly and began singing earlier than those given a saline injection instead.

Kojima and his colleagues also analysed the birds’ songs to see how these changed over the course of the day. They found there were rapid shifts in the structure of the songs in the first hour after dawn, compared with the second.

“Due to the absence of singing during the night, the vocal motor system and song acoustic structure may slightly deteriorate, and the dawn chorus serves to quickly restore or optimise them,” says Kojima.

While this study only looked at one species, similar drivers may also apply to other bird species, he says. “Given the simplicity of these mechanisms and functional implications, we proposed that they may broadly underlie the dawn chorus across bird species.”

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

Gil says the study is clever and the researchers do show that the birds’ desire to sing builds in the hours before dawn. “If zebra finches are refrained from singing by giving them longer nights, their motivation to sing increases,” he says. “They also show that nights are too long for their sleeping needs, and that they would rather sing if given the opportunity.”

However, the study doesn’t prove that the function of the dawn chorus is so the birds can refine their songs, says Gil. “These changes in song structure are assumed to improve song, but no data whatsoever is given to show that, indeed, females prefer these changes,” he says. “The study just shows that song changes progressively with singing time.”