2013年3月22日托福考试阅读真题及解析（网友版）_第2页

　　第二篇：

　　生物学，果蝇和青蛙成长的不同时期需要什么

　　第三篇：

　　bird nesting,说鸟喜欢在colonies筑巢，说了有什么好处，有一种鸟有自我保护意思，可以群体攻击那些天敌，有些seabird都在哪里筑巢比较安全，同时，集体筑巢又有什么隐患，例如吸引大量天敌。etc.

　　Bird Nesting Colonies

　　In many species, including herons, swallows, and most seabirds, individual birds come together each year to build nests near the nests of many others of the species. The resulting aggregations are called nesting colonies. Colonial nesting involves a number of factors. Seabirds, for example, often forage widely over the ocean surface, where the only available nesting land may be an island of limited area. The birds may also prefer an island to mainland nesting sites because it is safer, being inaccessible to most land predators. Also, by watching their neighbors returning to the colony with food for their chicks, colony-nesting gulls, or puffins may learn from one another where they can forage most successfully. Bank and Cliff Swallows, for example, build their nests in sites protected from ground predators such as foxes, skunks, and weasels.

　　Because residents in a colony usually share their feeding sites, colonial nesters are not, strictly speaking, territorial birds. They do, however, defend their nests against the adjacent birds, and with good reason: Colony members are known to sometimes steal nesting material from one another. They have also been known to sneak eggs into other birds' nests and to seduce other birds' mates. On the positive side, a few colony nesters have been known, on rare occasions, to feed another neighbor's chicks.

　　Bird Colonies

　　The habit of nesting in groups is believed to provide better survival against predators in several ways. Many colonies are situated in locations that are naturally free of predators. In other cases, the presence of many birds means there are more individuals available for defense. Also, synchronized breeding leads to such an abundance of offspring as to satiate predators.

　　For seabirds, colonies on islands have an obvious advantage over mainland colonies when it comes to protection from terrestrial predators. Other situations can also be found where bird colonies avoid predation. A study of Yellow-rumped Caciques in Peru found that the birds, which build enclosed, pouch-like nests in colonies of up to one hundred active nests, situate themselves near wasp nests, which provide some protection from tree-dwelling predators such as monkeys. When other birds came to rob the nests, the caciques would cooperatively defend the colony by mobbing the invader. Mobbing, clearly a group effort, is well-known behavior, not limited to colonial species; the more birds participating in the mobbing, the more effective it is at driving off the predator. Therefore, it has been theorized that the larger number of individuals available for vigilance and defense makes the colony a safer place for the individual birds nesting there. More pairs of eyes and ears are available to raise the alarm and rise to the occasion.

　　Another suggestion is that colonies act as information centers and birds that have not found good foraging sites are able to follow others, who have fared better, to find food. This makes sense for foragers because the food source is one that can be locally abundant. This hypothesis would explain why the Lesser Kestrel, which feeds on insects, breeds in colonies, while the related Common Kestrel, which feeds on larger prey, is not.

　　Colonial behaviour has its costs as well. It has been noted that parasitism by haematozoa is higher in colonial birds and it has been suggested that blood parasites might have shaped adaptations such as larger organs in the immune system and life-history traits. Other costs include brood parasitism and competition for food and territory. Colony size is a factor in the ecological function of colony nesting. In a larger colony, increased competition for food can make it harder for parents to feed their chicks.

　　The benefits and drawbacks for birds of nesting in groups seem to be highly situational. Although scientists have hypothesized about the advantages of group nesting in terms of enabling group defensive behavior, escape from predation by being surrounded by neighbors (called the selfish herd hypothesis), as well as escaping predators through sheer numbers, in reality, each of these functions evidently depends on a number of factors. Clearly, there can be safety in numbers, but there is some doubt about whether it balances out against the tendency for conspicuous breeding colonies to attract predators, and some suggest that colonial breeding can actually make birds more vulnerable. At a Common Tern colony in Minnesota, a study of Spotted Sandpipers observed to nest near the tern colony showed that the sandpipers that nested nearest the colony seemed to gain some protection from mammalian predators, but avian predators were apparently attracted to the colony and the sandpipers nesting there were actually more vulnerable. In a study of a Least Tern colony in Connecticut, nocturnal avian predators in the form of Black-crowned Night Herons and Great Horned Owls were observed to repeatedly invade a colony, flying into the middle of the colony and meeting no resistance.

　　For seabirds, the location of colonies on islands, which are inaccessible to terrestrial predators, is an obvious advantage. Islands where terrestrial predators have arrived in the form of rats, cats, foxes, etc., have devastated island seabird colonies. One well-studied case of this phenomenon has been the effect on Common Murre colonies on islands in Alaska, where foxes were introduced for fur farming.