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托福阅读真题第58篇TheMostCommonBirdonEarth
The red-billed quelea is small, mostly brown bird found in the annual grasslands of Africa. Other than ornithologists and the people who see the bird in its native habitat, almost no one has ever heard of the little quelea, although, with a population estimated at 10 billion, it may be the most common bird on Earth.
Given the seemingly stressful conditions in the environments where the birds are found, it is surprising that the red-billed quelea survives at all, much less that it is so remarkably successful. Queleas feed on native annual grasses. Any animal that feeds primarily on the seeds of annual grasses has a potentially major problem-its food supply periodically appears and then disappears. A wide range of adaptations allows annual-seed eaters to survive. For example. rodents in annual grasslands often store their extra seeds to survive lean times, an adaptation that was taken up by human populations once they domesticated and became dependent on annual grasses, such as barley and wheat. If they are to survive, queleas must also cope with the challenging feast-or-famine situation attending annual-grass feeders. One might expect such species to be regularly wiped out (at least locally)when fluctuations in the growing season result in seed failures of their food. The red-billed quelea copes well enough to be the world's most common bird.
Part of the quclea's success arisces from its life-history adjustments to its environment. Over the African dry scason, the species has to subsist on the ever-diminishing supply of seeds produced by the annual grasses at the end of the last wet season. As food becomes scarcer, the species feeds actively and gains sufficient weight reserves to be able to migrate to more
auspicious areas. With the onset of rains in the African wet season, the seeds that queleas eat germinate (develop) and so are no longer available for food. The resulting severe food shortage lasts six to eight weeks. The birds are forced to migrate to better situations (if they can find them). For red-billed qucleas, the savannas in which they live are an ever-shifting mosaic of patches of varying suitability depending on their recent rainfall history: some areas have dry seeds: others have immature grasses, mature grasses with abundant green seeds or old grasses with no seed. By moving over distances of 30 to 120 miles (50 to 200 km). the birds subsist in an inhospitable universe by moving from one patch of habitat with suitable food to another.
The so-called early-rains migration ends when queleas return to formerly abandoned locations. By this time the rains have come and grass seeds have germinated. The new grasses mature, and eventually fresh green seeds become available. ending the local food shortage. Then queleas establish communal breeding colonies in appropriate locations. They typically nest in thorny acacia trees. Their breeding colonies can contain several million pairs of breeding birds and can cover tens of hectares. The entire breeding cycle, from nesting colony to independent fledged young, requires six weeks-an exceptionally short interval for birds. The biological synchrony found among the birds in a breeding colony can be remarkable. Millions of eggs in millions of nests hatch on the same day. The fall of eggshells from subsequent dropping of the shells from the nests after these synchronous hatchings has been likened to a snowstorm. Sometimes the dry season comes early and the breeding areas dry out prematurely. In such times, the breeding colonies are abandoned. In other instances, the rains may sustain
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2023-04-22
