本篇文章共分 5 段，從鳥類每窩生蛋數量的觀察、到學者理論提出、理論推導到實驗證明等諸步驟，詳細說明鳥類每窩產卵量乃是物競天擇後的最佳選擇成果。

本篇考題英文原文與對應之中文翻譯整理如下。練習作答解題時若有對語意不清楚之處，請仔細查閱對照，以提升閱讀理解能力。

Clutch Size in Birds 鳥類的卵窩大小

1. 鳥類產卵數乃天擇適應

   Each year the emperor penguin lays one egg, the pigeon lays one or two eggs, the gull typically lays three eggs, the Canada goose four to six eggs, and the American merganser ten or eleven eggs. What determines clutch size in birds? The ultimate factors that determine clutch size are the requirements for long-term (evolutionary) survival. Clutch size is viewed as an adaptation under the control of natural selection.

   帝王企鵝每年產一個蛋，鴿子產一到兩個蛋，海鷗通常產三個蛋，加拿大鵝產四到六個蛋，美洲秋沙鴨產十到十一個蛋。是什麼決定了鳥類的窩卵量？決定窩卵大小的最終因素是對長期（進化）生存的要求。窩卵量的大小被看作是天擇控制下的一種適應。

2. 限定與非限定數量

   Natural selection will favor those birds that leave the most descendants to future generations. At first thought we might hypothesize that natural selection favors a clutch size that is the physiological maximum the bird can lay. We can test this hypothesis by taking eggs from nests as they are laid. When we do this, we find that some birds, such as the common pigeon, are determinate layers; they lay a given number of eggs, no matter what. The pigeon lays two eggs, if you take away the first, it will incubate the second egg only. If you add a third egg, it will incubate all three. But many other birds are indeterminate layers; they will continue to lay eggs until the nest is “full.” If eggs are removed once they are laid, these birds will continue laying. When this subterfuge was used on a mallard female, she continued to lay one egg per day until she had laid 100 of them. Evidence from other, similar experiments suggests that most birds under normal circumstances do not lay their physiological limit of eggs but that ovulation is stopped long before this limit is reached.

   天擇將有利於那些為後代留下最多後裔的鳥類。乍一看，我們可能會假設天擇傾向於選擇鳥類生理上能產下的最大窩卵量。我們可以通過在產蛋時從巢中取蛋來檢驗這一假設。當我們這樣做時，我們會發現一些鳥類，如普通鴿子，是限定數量的產卵鳥類；無論什麼情況，它們都會產下一定數量的蛋。鴿子下了兩顆蛋，如果你拿走第一顆，它就只孵化第二顆蛋。如果你增加第三顆蛋，它就會孵化所有三顆蛋。但許多其他鳥類是非限定數量的下蛋者；它們會繼續下蛋，直到鳥巢「滿」。如果蛋一產下就被移走，這些鳥就會繼續下蛋。當對一隻野鴨母鳥使用這種詭計時，它每天繼續產一顆蛋，直到產下 100 顆蛋。來自其他類似實驗的證據顯示，大多數鳥類在正常情況下不會產下生理極限的蛋，但在達到這個極限之前很久就會停止排卵。

3. 產卵數與食物供給有關

   The British ornithologist David Lack was one of the first ecologists to recognize the importance of evolutionary thinking in understanding adaptations in life history traits. In 1947 Lack put forward the idea that clutch size in birds was determined by the number of young that parents can provide with food. If his hypothesis is correct, the total production of young ought to be highest at the normal clutch size. And if one experimentally increased clutch size by adding eggs to nests, increased clutches should suffer greater losses because the parents could not feed the extra young in the nest.

   英國鳥類學家大衛．拉克是最早認識到進化思想在理解生活史特徵適應性方面具有重要性的生態學家之一。1947 年，拉克提出了一個觀點，即鳥類的巢穴大小是由父母能夠提供食物的幼鳥數量決定的。如果他的假設是正確的，那麼在正常的窩口大小時，幼鳥的總產量應該是最高的。如果在實驗中透過向鳥巢中增加鳥蛋來增加窩口大小，增加的窩口應該遭受更大的損失，因為父母無法餵養巢中的額外幼鳥。

4. 成本與收益的平衡

   One way to think about this problem of optimum clutch size is to use a simple economic approach. Everything an organism does has some costs and some benefits. The benefits of laying more eggs are very clear—more descendants in the next generation. The costs are less clear. There is an energy cost to make each additional egg, and there is a further cost to feed each additional nestling. If the adult birds must work harder to feed their young, there is also a potential cost in adult survival—the adults may not live until the next breeding season. If adults are unable to work harder, there is a risk of reduction in offspring quality. Models of this type are useful because they help us think about the costs and benefits for a particular ecological strategy. No organism has an infinite amount of energy to spend on its activities. The reproductive rate of birds can be viewed as one sector of a bird's energy balance, and the needs of reproduction must be maximized within the constraints of other energy requirements. The total requirements involve metabolic maintenance, growth, energy used for predator avoidance, competitive interactions, and reproduction. According to Lack's hypothesis, if enough additional eggs are placed in a bird's nest, reproductive energy requirements escalate and the whole brood will suffer from starvation so that, in fact, fewer young birds will fledge from nests containing large numbers of eggs.

   思考最佳窩口大小問題的一個方法是使用一個簡單的經濟方法。一個生物體所做的每件事都有一些成本和一些收益。多產蛋的好處是非常明顯的—在下一代中有更多的後代。成本則不太清楚。如果成鳥必須更努力地工作來餵養幼鳥，那麼成鳥的生存也會有潛在的代價—成鳥可能活不到下一個繁殖季節。如果成鳥不能更努力地工作，就會有後代質量下降的風險。這種類型的模型是有用的，因為它們幫助我們思考特定生態策略的成本和效益。沒有任何生物體有無限的能量可用於其活動。鳥類的繁殖率可以被看作是鳥類能量平衡的一個部份，繁殖的需要必須在其他能量需求的約束下達到最大化。總體需求涉及新陳代謝的維持、生長、用於躲避捕食者的能量、競爭性互動和繁殖。根據拉克的假說，如果在鳥巢中放置足夠多的額外的蛋，生殖能量需求就會升高，整個鳥群就會遭受飢餓，因此，事實上，從含有大量蛋的鳥巢中羽化的幼鳥會更少。

5. 實驗證明

   In England, the blue tit normally lays a clutch of nine to eleven eggs. What would happen if blue tits had a brood of twelve or thirteen? A researcher artificially manipulated broods at hatching by adding or subtracting chicks and found that the survival of the young blue tits in manipulated broods was poor. Blue tits feed on insects and apparently cannot feed additional young adequately, so more of the young starve. Consequently, it would not benefit a blue tit in the evolutionary sense to lay more eggs and the results are consistent with Lack's hypothesis. Individual birds appear to produce the clutch size that maximizes their reproductive potential.

   在英國，藍山雀通常會產下一窩 9 到 11 個蛋。如果藍山雀有一窩 12 個或 13 個蛋，會發生什麼事？一位研究人員通過增加或減少雛鳥來人為地操縱孵化時的雛鳥，發現在被操縱的雛鳥中，小藍山雀的存活率很低。藍山雀以昆蟲為食，顯然不能充分地餵養額外的幼鳥，因此更多的幼鳥會餓死。因此，從進化的意義上講，藍山雀產更多的蛋對它沒有好處，結果與拉克的假說一致。個別鳥類似乎會產生使其繁殖潛力最大化的窩口大小。