According to Zollinger, a bird group with a small gene pool might adapt less quickly to new diseases and could be wiped out.

　　A study of the dawn chorus found that birds in Berlin sang up to 14 decibels(分贝) louder than those in the forest. The birds sang loudest on weekday morning.

　　“ By trying to sing over the sound of the city, birds are risking vocal injury,”

　　Said Zollinger. “All this puts the same strain on a birds vocal cord as when a human tries to shout to be heard in a noisy pub----except the birds are doing it all day, every day,” she said.

　　“Singing under such pressure means birds have less control over the sound they produce. Their sound may lose quality and become more rough-sounding,” said Zollinger, “ This could make them appear less attractive to female birds.”

　　Mark Constantine, author of The Sound Approach to Birding, said,” Birdsong is important for quality of life and has been proved to reduce our blood pressure. When we live in the centre of large, urban areas, we get stressed and it’s extremely good to have birdsong around us.The impact on humans

　　of birdsong is massive. It harms us, as well as the birds, if their songs become louder and simple.”

　　43. Some birds in cities now choose to sing at night time because________.

　　A. they want to attract more birds of opposite sex

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