Birds in cities are damaging their health by trying to sing above the noise of urban life.     New research shows that male birds are trying to compete against traffic and city sounds.They are now singing louder and at higher frequencies,which could harm their vocal cords .Some birds are choosing to sing at night instead of during the day.This makes them more open to attack and also creates stress and exhaustion.    "The difference between urban and rural birdsong is becoming so big that the two groups could now be unable to communicate.This could lead to inbreeding and a weak gene poo1."said Dr Sue Anne Zollinger of the.University of St Andrews.     According to Zollingar,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 louder than those in the forest.The birds sang loudest on weekday mornings. "By trying to sing over the sound of the city,birds ale rising vocal injury,"said Zollinger."All this puts the same strain on a bird's vocal cords as when a human need to shout to be heard--except that 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 songs 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 our 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 simpler. Some birds in cities now choose to sing at night time because  _  .
they want to attract more birds of opposite sex they are more likely to be discovered by their family they can't adapt to the loud noises during the daytime they have to frighten their enemy away