填空题

The Effects of Urban Noise on Bird Species
A The growing clamour of cities and roads may be annoying to humans, but for many birds it can mean the difference between life and death. Background noise can mask both the sounds of approaching predators and the alarm calls that warn of danger. They can also rob individuals of reproductive success by drowning out the songs that male birds use to attract mates and demarcate their territory.
B If noise levels continue to rise, it seems inevitable that urban bird life will change dramatically. You can already hear the changes, if you know what to listen out for. One giveaway is birds unexpectedly singing outside their traditional peak times of morning and evening. At these prime times of day, wind noise and turbulence are at their lowest, so sound carries further—but not if you factor in the impact of rush-hour traffic. Richard Fuller of the University of Sheffield in the UK has found that some local robins have abandoned the traditional dawn chorus and taken to singing at night to avoid the daytime din altogether. This shift had previously been attributed to the confusing effects of light pollution, but Fuller’s analysis reveals that daytime noise has a much stronger effect: the parts of Sheffield with nocturnal singers were an order of magnitude noisier by day than other areas of the city.
C It remains to be seen whether singing at night is a successful way to combat noise pollution. It is not the only option, however. Nightingales, when not singing at night, have opted for an approach that seems at odds with their delicate melodies—they simply shout louder. When Henrik Brumm, at the University of St. Andrews in the UK, recorded nightingales singing between 5 am and 10 am he found that those in Berlin sang up to 14 decibels louder than their forest counterparts, achieving volumes of up to 95 decibels—enough to send humans reaching for ear protection. The loudness of their vocals was proportional to the level of background noise, with birds singing particularly loudly on weekday mornings.
D Changes in the timing or volume of songs are fairly obvious solutions to the problem, but some songbirds have taken a more subtle approach. Urban noise is particularly loud at low frequencies between about 1 and 3 kilohertz. By avoiding these frequencies, birds can make their songs more audible. Blackbirds, song sparrows and house finches have adapted in this way, but the most well studied practitioner is the great tit. For the past five years Hans Slabbekoom, of Leiden University in the Netherlands, has analysed the ways in which great tits deal with noisy cities. He found that those inhabiting noisier parts of Leiden sing melodies with higher minimum frequencies than those in quieter areas of the city. When he looked at populations of great tits in 10 European cities, including London, Pahs and Amsterdam, he found that every one of them sang higher-pitched tunes than their forest-dwelling counterparts, raising the minimum frequency by 200 hertz on average, to around 3,500 hertz.
E The ability to change one’s tune is a valuable asset in the growing urban hubbub. Unlike some birds that learn their entire repertoire while in the nest, great tits, song sparrows and others regularly modify their songs throughout their lifetime. They have far more tunes than they require and select different songs depending on the context. By monitoring which songs work best in a particular situation, individuals can learn from experience and adapt to local changes. Such flexibility is an asset in natural conditions too, according to Slabbekoorn: forests can vary greatly in how loud they are and those birds that live near noisy areas like waterfalls and river torrents also sing at higher frequencies, in a similar way to urbanites. By chance, their flexible singing has put them in a good position to cope with the artificially noisy conditions caused by humans. Alternatively, birdsong may become better adapted by default: if younger birds cannot hear the low-frequency segments of their tutors’ songs, they may never learn tunes containing these lower notes, which could then drop out of local repertoires altogether.
F Behavioural flexibility is what distinguishes species coping with noise pollution from those that are struggling. The relatively recent rise of urban noise means that most of the vocal strategies used by city birds are likely to be learned responses rather than the result of evolution. In the long term, however, genetic changes are likely to occur because of the role that songs play in survival and reproduction. Songs are primarily sexual traits that influence the mates females choose and so the success of males. If females come to see the ability to counteract acoustic masking as an indicator of mate quality, they will prefer to mate with males that can do this and the trait will be boosted by sexual selection. In addition, individuals whose hearing is attuned to picking out the songs of other birds amid urban noise are also at a selective advantage, which will ultimately increase their proportion of the population.
G If singing and hearing diverge enough, urban birds may be less likely to find the vocals of rural birds attractive, or even to recognise them as members of the same species. These changes could serve to eventually split populations into genetically distinct urban and rural species. Alternatively, different populations of the same species might adopt differing strategies to cope with urban noise, leading eventually to a species split occurring in birds living in the same neighbourhood. "It would be absolutely fascinating to see this kind of sympatric speciation," says Fuller.
Reading Passage 2 has seven paragraphs.
Choose the correct heading for each paragraph B-G from the list of headings A-I.
List of Headings
A The benefits of versatility
B A case for bird sanctuaries
C Possible developments within species
D Adjusting the timing of singing
E Ways to reduce urban noise
F Predicting evolutionary adaptations
G Modifications to the pitch of songs
H Modifications to the volume of songs
I Changing functions of birdsong Paragraph F