英語聽力文摘 千差萬別的鳥蛋

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How Eggs Get Their Color

鳥蛋顏色的形成

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.

不同鳥類蛋殼的顏色千差萬別，雞蛋是純白色，知更鳥蛋是藍色，其它蛋殼的顏色迥異：有條紋的、有斑點的、亮色的、淡色的或者是介于之間的。那么，讓我們看看顏色不一的原因吧!

那么，蛋殼的顏色是如何形成的呢?鳥蛋經(jīng)由母體的輸卵管(連接卵巢和外界)排出，分泌色素的腺體受擠壓，給蛋殼著色。如果此刻鳥蛋是靜止的，觸碰腺體的蛋殼形成斑點;如果是運動的，蛋殼形成條紋。

為什么那么大費周章地形成不同色彩呢?一些鳥類的巢在露天野外，蛋殼的色彩有助于和外界混為一體，便于隱藏。蛋殼顏色還在某些地方能排上用場：例如海鴉，通過蛋殼顏色來識別，排除它類。另外一些鳥類在樹洞中筑巢的，或下蛋后馬上孵化的;鳥蛋不用隱藏，蛋殼顏色通常是白色。

終上所述，科學家如何解釋知更鳥蛋是耀眼的純藍色呢?也許這樣便于知更鳥識破其巢寄生的鳥類將自己蛋放進知更鳥巢吧!巢寄生的行為對其他鳥類更具威脅性!

巢寄生(Brood parasitism 或 Nest parasitism)是某些鳥類將卵產(chǎn)在其他鳥的巢中，由其他鳥(義親)代為孵化和育雛的一種特殊的繁殖行為。

有些巢寄生的鳥類喜歡找與其親緣關系近的鳥作為宿主，比如非洲維達雀亞科的鳥;而另外一些則選擇其它科的與之生活習性大致相近的鳥類為宿主。無論是種間的還是種外，寄主都要選擇與自己孵化期和育雛期相似、雛鳥食性基本相同的宿主。