新托福考试阅读练习：鸟蛋的各种色彩

发布时间：2011-04-27来源：查字典留学网

新托福考试阅读练习：鸟蛋的各种色彩

复活节快到了，制作复活节彩蛋成为近期热门的活动。在自然界中，鸟下的蛋其实也是天生的“彩蛋”，颜色千差万别，这是为什么呢?今天的新托福考试阅读练习资料会告诉我们原因。

Depending on the species, bird eggs can be any color ranging from chicken’s plain white to a robin’s blue, streaked, spotted, bright, pale, and anything in between. We take a look at some of the reasons this might be the case.

But first, how do eggs get their color? As the egg moves down the female’s oviduct, the tube that connects the ovary to the outside, it squeezes against glands that produce colored pigments that combine to form every color in the egg shell spectrum. If the egg is stationary at the time that it comes in contact with these glands, it’ll be spotted. If the egg is in motion, it will be streaked.

So why go through all the trouble of producing colored eggs? For many species of birds that nest out in the open, the eggs’ colors help to conceal them by enabling them to blend into the background. Another way that colored eggs come in handy, is that in some species, like murres, individual birds learn to recognize their own particular markings and reject eggs that don’t match. In contrast, birds that nest in cavities, or that start incubating as soon as the egg is laid and consequently don’t need to hide their eggs, tend to lay unmarked white ones.

Given all this, how do scientists explain the fact that robin’s eggs are an eye-catching, unmarked blue? Perhaps this enables robins to spot when a parasitic bird tries to add its own eggs to the nest. For some species, this may pose a greater threat than that of nest robbers.

不同鸟类蛋壳的颜色千差万别，鸡蛋是纯白色，知更鸟蛋是蓝色，其它蛋壳的颜色迥异：有条纹的、有斑点的、亮色的、淡色的或者是介于之间的。那么，让我们看看颜色不一的原因吧!

那么，蛋壳的颜色是如何形成的呢?鸟蛋经由母体的输卵管(连接卵巢和外界)排出，分泌色素的腺体受挤压，给蛋壳着色。如果此刻鸟蛋是静止的，触碰腺体的蛋壳形成斑点;如果是运动的，蛋壳形成条纹。

为什么那么大费周章地形成不同色彩呢?一些鸟类的巢在露天野外，蛋壳的色彩有助于和外界混为一体，便于隐藏。蛋壳颜色还在某些地方能排上用场：例如海鸦，通过蛋壳颜色来识别，排除它类。另外一些鸟类在树洞中筑巢的，或下蛋后马上孵化的;鸟蛋不用隐藏，蛋壳颜色通常是白色。

终上所述，科学家如何解释知更鸟蛋是耀眼的纯蓝色呢?也许这样便于知更鸟识破其巢寄生的鸟类将自己蛋放进知更鸟巢吧!巢寄生的行为对其他鸟类更具威胁性!

巢寄生(Brood parasitism 或 Nest parasitism)是某些鸟类将卵产在其他鸟的巢中，由其他鸟(义亲)代为孵化和育雏的一种特殊的繁殖行为。

有些巢寄生的鸟类喜欢找与其亲缘关系近的鸟作为宿主，比如非洲维达雀亚科的鸟;而另外一些则选择其它科的与之生活习性大致相近的鸟类为宿主。无论是种间的还是种外，寄主都要选择与自己孵化期和育雏期相似、雏鸟食性基本相同的宿主。