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