Zebra finches are commonly studied in captivity by biologists
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The dawn chorus of birdsong has inspired poets and nature lovers for thousands of years, but why birds around the world start the day this way is an enduring mystery.
Now, a series of experiments with zebra finches (Taeniopygia guttata) suggest that while darkness inhibits song, birds build up a stronger motivation to sing at night that prompts them to burst into song at dawn. The study also suggests that a morning workout for the vocal muscles helps birds fine-tune their songs.
Satoshi Kojima at the Korea Brain Research Institute in Daegu, South Korea, and his colleagues studied laboratory-reared zebra finches under carefully controlled lighting conditions to manipulate the timing of light and dark cycles.
The team first pushed back the time when the fins were exposed to sudden bright light 3 hours later than the actual dawn. The birds were awake, but remained quiet in the artificial darkness, and when the lights finally came 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.
In other words, says Kojima, the longer the time lag between the birds waking 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. “However, their spontaneous singing is suppressed by the darkness. This suppression raises their motivation to sing, leading to a high song rate as a rebound immediately after the light is turned on.”
The researchers then trained birds to press a lever for 10 seconds of artificial light. When the simulated daylight was delayed by 3 h, the birds pressed the lever frequently, but rarely did so when the artificial daylight was 3 h earlier.
Next, the researchers administered the drug luzindole, which blocks the effects of melatonin, a hormone released at night that helps regulate wakefulness cycles in many animals. Birds given this drug 5 hours before the normal time of ignition woke up faster and started singing earlier than those given a saline injection instead.
Kojima and his colleagues also analyzed the birds’ songs to see how these changed throughout the day. They found that there were rapid changes in the structure of the songs in the first hour after dawn compared to the second.
“Due to the absence of song during the night, the vocal motor system and the acoustic structure of song may be slightly degraded, and the dawn chorus serves to quickly restore or optimize them,” says Kojima.
While this study only looked at one species, similar drivers may apply to other bird species as well, he says. “Given the simplicity and functional implications of these mechanisms, we suggest that they may broadly underlie the dawn chorus across bird species.”
But Diego Gil at the National Museum of Natural Sciences in Madrid, Spain, is calling for bail. There are “countless 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 that the study is clever and that the researchers show that the birds’ desire to sing builds up in the hours before dawn. “If zebra finches are deterred from singing by giving them longer nights, their motivation to sing increases,” he says. “They also show that the nights are too long for their sleep needs and that they would rather sing if given the chance.”
But the study does not prove that the function of the dawn chorus is to allow the birds to refine their songs, Gil says. “These changes in song structure are thought to improve song, but no data is provided at all to show that females actually prefer these changes,” he says. “The study just shows that the song changes gradually with singing time.”
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