Reading passage
More than half of the world's people now live in cities, and the number is rising every year. As streets, buildings and car parks replace fields and forests, most wild animals disappear. Birds, however, are an interesting exception. A walk through almost any city centre reveals pigeons on the pavements, sparrows in the hedges and gulls circling above the shops. Scientists estimate that around one fifth of all bird species can now be found in urban areas.
Not every bird can make this move. The species that succeed are usually generalists, meaning they can eat many different foods and nest in many different places. Specialists, such as birds that depend on one type of insect or one kind of tree, often vanish when their habitat changes. Generalists like pigeons, crows and starlings treat the city as a new kind of countryside, full of ledges, roofs and open bins.
City life is noisy, and this has changed the way birds sing. Traffic produces a constant low rumble, so a normal song can be hard to hear. Researchers in several countries have found that urban great tits sing louder and at a higher pitch than their relatives in quiet forests. Some species have even shifted their main singing time to the early morning, before the roads become busy, so that rivals and mates can still hear them clearly.
Food in cities is different too, and clever birds take full advantage of it. Pigeons and gulls survive partly on scraps that people drop or leave in open bins. Crows in Japan have learned to place hard nuts on roads so that cars crack the shells, and some even wait at pedestrian crossings for the lights to change before collecting their meal. Behaviour like this shows remarkable problem-solving ability, and scientists now study city birds to understand how intelligence develops.
Life in town is not easy, though. Millions of birds die each year after flying into glass windows, which they cannot see. Bright lights at night confuse migrating birds and can lead them off course. Domestic cats kill large numbers of sparrows and blackbirds, and air pollution damages the insects that many birds eat. Because of these pressures, some once-common city species, including the house sparrow, have declined sharply in parts of Europe. Scientists estimate that in some countries, window strikes kill more birds than cats do, and simple stickers or patterned glass can cut the number sharply.
Birds have also changed where and when they raise their young. Tall buildings serve swifts and peregrine falcons as artificial cliffs, and many pairs nest on ledges high above the streets. Studies show that some city birds start breeding earlier in the year than countryside birds, probably because towns are warmer and food is available all winter. Street lights may also extend the time parents can spend searching for food for their chicks.
Interestingly, cities can even act as refuges for some species that are struggling elsewhere. When farmland is sprayed with chemicals and hedges are removed, countryside bird numbers fall, and towns sometimes offer safer conditions. Gardens with flowers, lawns and ponds provide insects and seeds throughout the year. In Britain, for example, goldfinches and wood pigeons are now more common in leafy suburbs than in many farming areas. Researchers call this the urban refuge effect, and it shows that the story of city wildlife is not always one of loss and decline. House sparrows, for example, have declined steeply in parts of the countryside yet remain common in many towns.
People can do a great deal to help their feathered neighbours. Simple actions such as putting up nest boxes, planting native shrubs and keeping feeders clean can make a real difference. Architects are beginning to design bird-friendly buildings with special glass that birds can see. City parks that include long grass, ponds and dead wood support far more species than neat lawns alone. With a little thought, urban areas can become places where both people and birds flourish.