Living in the city will change you sooner or later. You often get louder, ruder, and use slang that outsiders don't understand. These classic 16 of urban living can also be seen in birds. Like people, birds that live in the city are louder, 17 .and more stressed out than their country cousins. A study found that city birds start singing earlier than 18 in the countryside. Researchers observed that sparrows start singing earlier 19 not because of light pollution, but to avoid noise from the traffic. 20 every city resident, the birds have learned that you need to start your day early if you want to avoid the rush hour. City-dwelling species like blackbirds 21 their volume or alter their songs to 22 amid the noise of the bustling metropolis. 23 rural birds may not be able to make sense of 24 their big-city cousins are singing about. Rural birds may also find their city cousins rather rude. In crowded places, urban sparrows are more 25 because, although there is more food in cities, it is often more work to claim it and keep it away from other 26 . All this hustling makes city birds, like its people, smart. In a recent study, bullfinches living in cities 27 their country cousins in cognitive tests, by acting more boldly and creatively when presented with unfamiliar or difficult-to-reach 28 of food. City birds might be louder, tougher, and even smarter. Compared to their rural 29 .they also die younger. Researchers found that their cells aged 30 than their country cousins. They have to cope with a lot of stress.________
A.symbols B.sights C.symptoms D.signals正确答案C
