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  • 鸟类结构和功能 - 高级

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    What is significant about the structure-function relationship in ?
    ::结构-功能关系的意义何在?

    Obviously, most birds are built to fly. In fact, there are only a handful of birds that don't fly. But there is more to it than that. Look at this eagle. What other structures are evident? What structures, in addition to the wings , allow this bird to function as necessary?
    ::很明显,大多数鸟类都是为飞行而建造的。事实上,只有少数鸟类不飞行。但还有更多。看看这只鹰。其他哪些结构是显而易见的?除了翅膀外,还有哪些结构允许鸟发挥必要的功能?

    Structure and Function in Birds
    ::鸟类的结构和功能

    Compared with other vertebrates , birds have bodies that show many unusual adaptations - mostly to allow for flight . Some of these adaptations are shown in  Figure and  Figure .
    ::与其他脊椎动物相比,鸟类的体体显示出许多不寻常的适应-主要是允许飞行,其中一些调整在图和图中显示。

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    The external anatomy of birds. All birds have a similar body plan to this blue tit (Cyanistes caeruleus). However, flightless birds do not have primary feathers.

    Skeleton and Muscles
    ::骨骼和肌肉

    The of a bird skeleton are very lightweight. The bones have large air-filled cavities, making them very light. The skull bones are fused, as are many other bones in the bird skeleton. These fused bones give the bird skeleton more rigidity than other skeletons, allowing for more stability during flight. Because of the many fused bones, birds usually have fewer bones in their skeleton than other vertebrates.
    ::鸟类骨骼的重量非常轻。 鸟类骨骼有巨大的空气填充的洞穴,让它们变得非常亮亮。 头骨和鸟类骨骼中的其他许多骨头一样,都装了引信。 这些骨骼使鸟类骨骼比其他骨骼更僵硬,使得飞行期间的稳定性更大。 由于许多骨骼都装满了引信,鸟类骨骼中的骨骼通常比其他脊椎动物的骨骼要少。

    The spine of birds has a range of different movements . The vertebrae of the neck region are especially flexible, but movement is reduced in the vertebrae of the chest area and absent in the lower vertebrae. Some of the last few vertebrae are fused with the pelvic bones to form the synsacrum. The final few vertebrae are fused into a structure called a pygostyle. The pygostyle supports the tail feathers and . The ribs are flattened, and the sternum, or breastbone, has an extension called a keel, to which the large flight muscles are attached. Flightless birds do not have a keeled sternum; instead, their sternum is flat. The forelimbs (front legs) of birds are modified into wings. Birds do not have teeth nor do they have a true jaw. They have evolved a lightweight beak instead.
    ::鸟类的脊椎有各种各样的运动。 颈部的脊椎特别灵活, 但胸部的脊椎运动会减少, 下脊椎则没有。 少数少数脊椎与骨骨结合, 以形成结骨。 最后少数脊椎结合成一个叫做俾格格风格的结构。 俾格风格的脊椎支持尾羽毛和。 肋骨是扁平的, 胸骨或胸骨的延伸称为, 大飞行肌肉都附着在其中。 无飞行的鸟类没有骨; 相反, 它们的脊椎是平的。 鸟类的前腿( 前腿) 被调整成翅膀。 鸟类没有牙齿, 也没有真正的下巴。 它们进化了一个轻量的。

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    A skeleton of a dove. Bird skeletons are light, yet strong. This skeleton shows the many adaptations for flight such as the keeled sternum for the attachment of large flight muscles and the fused vertebrae of the synsacrum and pygostyle. Pigeons are a type of dove.

    Most birds have approximately 175 different muscles, mainly controlling the wings, skin, and legs. The largest muscles in a bird are the pectorals, or the breast muscles, which control the wings and make up about 15 to 25 percent of a flighted bird’s body weight. They provide the powerful wing strokes that are essential for flight. The muscles under the pectorals raise the wings between wing beats. Together, these flight muscles make up about 25 to 35% of the bird's full body weight.
    ::大多数鸟类都有大约175种不同的肌肉,主要是控制翅膀、皮肤和腿。 鸟类中最大的肌肉是胸肌,或胸部肌肉,它们控制翅膀,占飞行鸟类体重的15%至25%。 它们提供了飞行所必需的强大的翅膀中风。 胸下肌肉将翅膀的翅膀放在翅膀的两侧之间。 这些飞行肌肉加起来占鸟类全身重量的25-35%。

    The skin muscles help a bird fly by adjusting the feathers, which are attached to the skin muscle and help the bird in its flight maneuvers. There are only a few muscles in the trunk and the tail, but they are very strong and are essential for the bird. The pygostyle controls all the movement in the tail and controls the feathers in the tail. This gives the tail a larger surface area, which helps keep the bird in the air.
    ::皮肤肌肉通过调整羽毛帮助鸟类飞翔,羽毛与皮肤肌肉相连,有助于鸟类飞行操作。尾部和尾部只有几根肌肉,但肌肉非常坚固,对鸟类至关重要。尾部和尾部的所有运动都由俾格风格控制,尾部的羽毛则由它控制。这使尾部的表面面积更大,有助于鸟类在空中活动。

    Feathers and Plumage
    ::羽毛和管道

    Feathers are among the most complex structural organs found in vertebrates. They are formed by the controlled replication of epidermal (skin) that produce the keratin . Feathers, beaks, and claws (and the claws, scales, and shells of reptiles) are made of β-keratin. The molecules in the β-keratin strands are bound and twisted into very tough structures. This β-keratin structure is much tougher than the α-keratin of mammalian hair, horns, and hooves.
    ::羽毛是脊椎动物中发现的最复杂的结构器官之一,它们是由生产氯丁酸的上皮瘤(皮肤)的受控复制而形成的。羽毛、嘴和爪子(以及爪子、天秤和爬行动物的贝壳)是用β-克赖廷制成的。β-克赖廷线上的分子被捆绑并扭曲成非常坚硬的结构。这个β-克赖廷结构比哺乳动物毛、角和蹄子的α-克赖廷要硬得多。

    Feathers are a feature unique to birds. They allow flight, provide insulation that aids in thermoregulation, and are used in display, camouflage, and signaling. There are several types of feathers, each having its own set of uses. The arrangement and appearance of feathers on the body, called plumage, may vary within a by age and sex. Feathers are regularly molted. A small number of species, such as ducks and geese, lose all of their flight feathers at once and are flightless for a time.
    ::羽毛是鸟类独有的特征。 它们允许飞行,提供绝缘,辅助温度调节,并用于展示、伪装和信号。 有几种羽毛,每种都有自己的用途。 羽毛在身体上的安排和外观,称为羽毛,可能因年龄和性别的不同而不同。 羽毛经常被缠住。 少数物种,如鸭子和鹅,立即失去所有飞行羽毛,暂时没有飞行能力。

    There are two basic types of feather: vaned feathers, which cover the exterior of the body, and down feathers , which are underneath the vaned feathers. Flight feathers, which are vaned, are smooth and stiff and are much longer than they are wide, as shown in  Figure . Vaned feathers usually have two vanes, one on each side of the rachis. Down feathers are fluffy and soft. They trap warm air close to the bird’s body and are therefore good insulators.
    ::羽毛有两种基本类型:覆盖身体外部的假羽毛和位于假羽毛下面的下羽毛。 如图所示,圆羽毛是光滑和僵硬的,并且比其宽度长得多。 假羽毛通常有两条面包,两边各一个,下羽毛是毛和柔软的。 它们把温暖的空气困在鸟的身上,因此是很好的诱导器。

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    This Tail feather has two vanes, one each side of the rachis.

    Barbs are a series of branches that are fused to the rachis (stem) of a feather. The barbs themselves also have branches, called barbules, which have hooks that lock the barbs together. Barbules have many tiny hooks called barbicels that interlock with the barbules of nearby barbs. When all the barbules are interlocked, the feather has a smooth appearance, as can be seen in  Figure .
    ::刺布是一系列的树枝,它们与羽毛的拉奇(stem)结合。 刺本身也有树枝, 叫做巴布, 它们有钩子, 将巴布锁在一起。 芭布有许多小钩子, 叫做巴布, 与附近巴布的条形交叉。 当所有条形相互交织时, 羽毛的外观很光滑, 如图所示 。

    The barbs of down feathers are not as stiff and straight as those of vaned feathers. They also do not have barbicles. This is why down feathers, like the ones shown in  Figure , look and feel so fluffy.
    ::羽毛下方的柱子不像羽毛上的柱子一样僵硬和直。 它们也没有圆柱子。 这就是为什么下方的羽毛,像图中显示的那些羽毛,看起来和感觉是如此的松散。

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    Down feathers. These fluffy feathers are used primarily for insulation. You can see the individual barbs as they puff out from the rachis.

    Feathers need to be kept clean and tidy, so birds groom them daily. A bird uses its beak and head to brush away both dirt and parasites and to apply waxy secretions from their uropygial gland, or preen gland. The preen gland is found near the base of the tail of most birds. The gland secretes an oily substance called preen oil. While preening, a bird will usually transfer this oil to its feathers by rubbing its head against the oil and then around the rest of its body. Preen oil protects the feathers' flexibility and acts as an antimicrobial agent, inhibiting the growth of feather-digesting . Some birds also place ants in their feathers in a behavior called anting. The ants produce formic acid which kills bacteria and mites that may be parasitizing the bird. Other birds get rid of parasites by taking dust baths.
    ::羽毛需要保持干净和整洁, 所以鸟儿每天都要培养它们。 一只鸟用它的嘴和头来刷去泥土和寄生虫, 并用毛细细的分泌物从它们的极旋腺或前腺中抹去, 发现在大多数鸟类尾巴的底部附近。 腺分泌一种油状物质, 叫做先生油 。 在孵化时, 鸟通常会通过摩擦它的头来将这种油移到羽毛上, 摩擦它的油头, 然后在它的身体周围。 羽毛会保护羽毛的灵活性, 并起到抗微生物剂的作用, 抑制羽毛的生长。 一些鸟类还将蚂蚁放在它们的羽毛中, 一种叫作蚂蚁的行为。 蚂蚁会产生致菌酸, 杀死可能诱鸟的细菌和植物。 其他鸟类会通过洗尘浴来除寄生的寄生虫。 其他鸟类会摆脱寄生虫。

    Other theropods also had feathers. of Archaeopteryx include well-preserved feathers, but it was not until the early 1990s that non-avian dinosaur fossils were discovered with preserved feathers. Today there are more than a dozen genera of dinosaurs with fossil feathers.
    ::其他的类栖动物也有羽毛。 考古切片内含有保护良好的羽毛,但直到20世纪90年代初,才发现有保留羽毛的非禽类恐龙化石。 如今,有十几个恐龙基因带化石羽毛。

    Respiratory System
    ::呼吸系统

    Due to the high metabolic rate needed for flight, birds have a high oxygen demand. Birds have evolved one of the most complex respiratory systems of all animal groups. They ventilate their lungs by means of air sacs , structures that are unique to birds (and possibly dinosaurs). The air sacs do not play a direct role in gas exchange, but they store air and act like a bellows, pushing air into the lungs. Therefore, the lungs maintain a fixed volume of oxygenated air.
    ::由于飞行需要高代谢率,鸟类的氧气需求很高。鸟类是所有动物群中最复杂的呼吸系统之一,它们通过鸟类(可能还有恐龙)独有的气囊结构,通过空气囊使肺通风。空气囊在气体交换中不发挥直接作用,但是它们储存空气,就像一个钟头,把空气推入肺部。因此,肺部保持固定的氧气量。

    Three distinct sets of organs perform respiration : the anterior air sacs, the lungs, and the posterior air sacs, shown in  Figure  . When the bird inhales, about 75% of the fresh air moves past the lungs and flows directly into air sacs that are located around the lungs. The other 25% of the air goes directly into the lungs. When the bird exhales, the used air flows out of the lungs and through the nostrils. The stored fresh air from the posterior air sac is forced into the lungs. In this way, a bird's lungs receive a constant supply of oxygenated air during both inhalation and exhalation.
    ::三组不同的器官进行呼吸:前额空气囊、肺和后部空气囊,如图所示。当鸟吸入时,大约75%的新鲜空气穿过肺部,直接流入肺部周围的空气囊。其他25%的空气直接进入肺部。当鸟吸入时,用过的空气从肺和鼻孔流出。从外部空气囊中储存的新鲜空气被迫进入肺部。这样,鸟肺在吸入和呼吸期间就不断得到氧气供应。

    Bird lungs do not have alveoli as mammalian lungs do. Instead, they contain millions of tiny passages known as parabronchi, connected at either end by the dorsobronchi and ventrobronchi, as shown in  Figure  . Air flows through the honeycombed walls of the parabronchi and into air capillaries where oxygen and carbon dioxide are traded with cross-flowing capillaries.
    ::鸟类肺部没有哺乳动物肺部那样的肺泡。 相反,它们含有数百万个叫做parabronchi的小孔口,两端都由多尔索布罗诺奇和喷口罗布罗诺奇连接,如图所示。 空气流穿过鹦鹉的蜂窝壁,进入气动螺旋,氧气和二氧化碳与气动螺旋交叉交易。 气动气动气动气动气动气动气动气动气动气动气动气动气动气动气动气动气动气动气动气动气动气动气动气动气动气动气动。

    Since fresh air flows through the lungs in only one direction, there is no mixing of oxygen-rich air and oxygen-poor, carbon dioxide-rich air as in mammalian lungs. Birds have more efficient gas-exchange of both oxygen and carbon dioxide than mammals do.
    ::由于新鲜空气仅从一个方向流过肺部,因此哺乳动物肺中没有富氧空气和缺氧、二氧化碳含量高的空气混合。 鸟类的氧气和二氧化碳交换效率比哺乳动物要高。

    The posterior air sac extends from the lungs and connects with air spaces in the bones, filling the bones with air.
    ::后天空气囊从肺部延伸,与骨头的空气空间相连,用空气填充骨头。

    Birds also do not have a diaphragm . The entire body cavity acts as a bellows to move air through the lungs. The active phase of respiration (requires muscle contraction) in birds is exhalation.
    ::鸟类也没有隔膜。 整个身体的洞口都起到钟头的作用,通过肺部输送空气。 鸟类的活性呼吸阶段(需要肌肉收缩)是呼气。

    A recent study carried out by researchers at Ohio University and Harvard University showed the presence of a very bird-like pulmonary (lung) system in predatory dinosaurs. Such findings provides more evidence of an evolutionary link between dinosaurs and birds.
    ::俄亥俄大学和哈佛大学研究人员最近进行的一项研究表明,在掠食性恐龙中存在着一种非常像鸟的肺系统。 这种发现为恐龙和鸟类之间的进化联系提供了更多证据。

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    Bird respiration. Birds' lungs obtain fresh air during both exhalation and inhalation.

    Nervous System and Sensory Organs
    ::神经系统和感知器官

    Birds have a large brain to body mass ratio. This is reflected in birds' advanced and complex intelligence and behavior. The most developed part of the brain is the one that controls the flight-related functions, while the cerebellum coordinates movement, and the cerebrum controls behavior patterns, navigation, mating , and nest building. Most birds have a poor sense of smell .
    ::鸟类的大脑与身体质量比率相当大。这反映在鸟类的先进和复杂的智力和行为中。大脑中最发达的部分是控制飞行相关功能的部分,而小脑坐标的移动和大脑控制行为模式、导航、交配和筑巢。大多数鸟类的嗅觉很差。

    Sight
    ::视觉视觉

    Birds usually have good eyesight. For example, hunting birds, like the raptors (eagles and hawks), have eight times sharper than humans. Their occupy a large part of their skull and are surrounded by a ring of tiny bones. This ring of bones is a characteristic also seen in . birds have special flexible lenses, allowing them to see in air and underwater. Birds have higher densities of photoreceptors in their retinas. Hawks can have up to one million photoreceptors per square mm of retina , compared to 200,000 per square mm for humans. Birds also have ultraviolet (UV) sensitive cone cells in their eyes, as well as green, red, and blue cone cells. This allows them to see ultraviolet light. Many birds show plumage patterns in ultraviolet that are invisible to the human eye. Some birds whose sexes appear similar to the naked eye look different under UV light. Kestrels can search for prey by detecting the UV-reflective urine trail marks left on the ground by rodents .
    ::鸟类通常有良好的视力。 例如,猎鸟,如猛禽(鹰和鹰),比人类更锋利八倍。 它们占据了大部分头骨,周围环绕着一圈小骨头。 这种骨骼环也是在. 鸟类有特殊的柔性透镜,允许它们在空气和水下看到。 鸟类的视网膜有较高的光受体密度。 鹰每平方视网膜有高达100万个光受体, 而人类每平方毫米有20万个光受体。 鸟类眼中也有紫外线(UV)敏感锥细胞,以及绿色、红色和蓝锥形细胞。 这使得它们能看到紫外线光。 许多鸟类在紫外线中表现出的羽毛模式, 人类眼睛看不见。 有些鸟类的性别看起来与裸眼在紫外光光下不同。 Kestels 可以通过探测到鼠在地面留下的紫外线反光性尿痕来寻找猎物。

    The eye is lubricated by the nictitating membrane, shown in Figure  . The nictitating membrane is a third eyelid that moves horizontally across the eyeball. The nictitating membrane also covers the eye and acts as a contact lens in many birds when they dive underwater. Most birds cannot move their eyes, although there are exceptions. Birds with eyes on the sides of their heads have a wide visual field, while birds with eyes on the front of their heads, such as owls, have binocular vision and good depth perception.
    ::双膜是垂直穿过眼球的第三只眼皮; 双膜是双眼,在很多鸟类潜水时也覆盖眼睛,作为隐形眼镜; 大多数鸟类无法移动眼睛,但有例外。 头部两侧有眼的鸟类有宽视场,头部前部有眼的鸟类,如猫头鹰,有望远镜的视力和深度的洞察力。

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    The nictitating membrane of a black-crowned night heron. As the heron blinks, the nictitating membrane can be seen moving across the eyeball.
    Sound
    ::声音

    The avian ear lacks external pinnae but is covered by feathers, although, in some birds, such as owls, these feathers form tufts which resemble ears. The inner ear has a cochlea , but it is not spiral as in mammals.
    ::鸟类耳朵没有外部针刺,但被羽毛覆盖,尽管在诸如猫头鹰等鸟类中,这些羽毛形成与耳朵相似的尖牙。 内耳有科氏膜,但不像哺乳动物那样螺旋。

    Circulatory System
    ::循环系统

    Like crocodiles and mammals, birds have four-chambered hearts. The septum between the left and right ventricles is complete, which means oxygenated and deoxygenated blood does not mix. Bird hearts are relatively large for their body size, compared to other vertebrates. These adaptations allow for efficient nutrient and oxygen transport throughout the body and provide birds with enough energy to fly and exert high levels of activity. A Ruby-throated Hummingbird's heart beats up to 1,200 times per minute (about 20 beats per second). Larger birds have slower heart rates. Unlike mammals, the red blood cells in birds have a .
    ::与鳄鱼和哺乳动物一样,鸟类有四组的心脏。左和右腹腔的肺部完整,这意味着氧和脱氧血液不会混合。与其他脊椎动物相比,鸟类的心体大小相对较大。这些适应使鸟类能够在整个身体中高效地传递营养和氧气,并为鸟类提供足够的能量来飞翔和进行高水平的活动。红宝石蜂鸟的心脏每分钟跳动1200次(大约每秒20次)。大鸟的心跳速度较慢。与哺乳动物不同,鸟类中的红血细胞有一个。

    Digestive System
    ::消化系统

    The of birds is unique; a crop for storage and a gizzard that contains swallowed stones for grinding food make up for the lack of teeth. To aid with flight, most birds are highly adapted for rapid digestion . Some migratory birds have the ability to reduce parts of their intestines prior to .
    ::鸟类是独一无二的;一种储存的作物和一种含有用于研磨食物的吞咽石块的灰沙,弥补了牙齿的缺乏。 为了帮助飞行,大多数鸟类都高度适应快速消化。 一些迁徙鸟类有能力在之前削减其部分肠子。

    As birds have high metabolic rates, they need to eat often, and their food needs to be broken down efficiently and quickly. Birds do not chew their food, though it may be torn up or crushed by their beak. Birds cannot swallow like mammals can; instead, they have to tilt their heads back to move the food from their mouth to their esophagus . From here the food moves through the bird by contractions of the smooth muscles of the digestive system. Many birds have a muscular pouch, called a crop, along the esophagus. The crop functions to both soften food and regulate its flow through the system by storing the food temporarily. The size and shape of the crop vary among bird species. Birds possess a ventriculus, or gizzard, composed of four muscular bands that rotate and crush food by shifting the food from one area to the next within the gizzard. The gizzard of some species contains small pieces of grit or stone swallowed by the bird to aid in the grinding process of digestion, essentially serving the function of mammalian or reptilian teeth. The use of gizzard stones is a similarity between birds and dinosaurs, which left gizzard stones called gastroliths as trace fossils.
    ::由于鸟类的代谢率很高,它们需要经常食用,而且食物需要高效和迅速地分解。鸟类不咀嚼食物,尽管食物可能被其嘴唇撕碎或粉碎。鸟类不能像哺乳动物那样吞食;相反,它们必须把头朝后倾斜,以便把食物从嘴部移到食道。从这里,食物通过消化系统光滑的肌肉缩缩缩穿过鸟。许多鸟类有一个肌肉袋,称为作物,沿食道吞食。作物的作用是软食食物,并通过暂时储存食物来调节食物在系统中的流动。作物的大小和形状因鸟类种类而异。鸟类拥有一个通风室,或,由四个肌肉带组成,通过将食物从一个地区转移到另一个地区来旋转和压碎食物。有些物种的有小块被鸟吞食的石块,小块小块的,被鸟吞食,以帮助消化过程,主要是满足哺乳动物或爬行鸟的牙齿的功能。使用石质石块的石块是相似的磁石块。

    Many species of birds, such as owls, regurgitate pellets. A pellet is the mass of undigested parts of a bird's food that some bird species occasionally regurgitate. The contents of a bird's pellet depend on its diet but can include the exoskeletons of , indigestible plant matter, bones, fur, feathers, bills, claws, and teeth.
    ::许多种类的鸟类,如猫头鹰、复古颗粒。 颗粒是鸟类食物中未经消化的部分质量,有些鸟类偶尔会加以再生。 鸟的颗粒的成分依赖其饮食,但可以包括稀释的植物物质、骨头、毛、羽毛、纸、爪子和牙齿。

    Passing pellets allows a bird to remove indigestible material from its proventriculus, or glandular stomach . In birds of prey, the regurgitation of pellets serves the bird's health in another way: by "scouring" parts of the digestive tract including the gullet. Pellets are formed in the bird's gizzard within six to ten hours of a meal.
    ::传球粒子可以让鸟儿从它的被证明的睾丸或腺胃中去除不可稀释的物质。 在猎物的鸟类中,卵粒的再熔化以另一种方式为鸟的健康服务:通过“涂抹”消化道的部位,包括河沟。 在鸟食后六至十小时之内,在鸟的凝胶中会形成颗粒。

    Organ system adaptations for flight
    Organ System Adaptations for Flight. The intricate passageways in a bird’s lung are adapted for efficient gas exchange. Find the crop and gizzard in the digestive tract diagram. What are their functions?

    Excretory System
    ::散逸系统

    Like those of reptiles, birds’ extract nitrogenous waste from their bloodstream and excrete it as uric acid instead of urea or ammonia. Uric acid is excreted along with feces as a semisolid waste, since birds do not have a separate bladder or urethra . This material leaves the body through the bird's cloaca . The cloaca is a multi-purpose opening; waste passes out of the body through it, birds mate by joining cloaca, and females lay eggs from it.
    ::像爬行动物一样,鸟儿从血液中提取氮化废物,将其排出为酸尿而不是尿素或氨。 乙酸与粪便一起作为半固体废物排出,因为鸟儿没有单独的膀胱或尿道。 这种物质会通过鸟儿的阴道离开身体。 木薯是一个多功能的开口;废物会通过它从身体中流出,鸟儿会加入木薯,雌性会从中产卵。

    Summary
    ::摘要

    • Most of a bird's unusual adaptations have to do with flight.
      ::鸟儿大多数不寻常的适应 都和飞行有关
    • The bones of a bird skeleton are very lightweight.
      ::鸟骨的骨头非常轻
    • The largest muscles in a bird are the pectorals, or the breast muscles, which control the wings.
      ::鸟体内最大的肌肉是胸肌, 或控制翅膀的胸肌。
    • The feathers, beaks, and claws of birds are made of β-keratin.
      ::羽毛、嘴和爪子 鸟儿都是用β-克拉丁制成的
    • Birds' lungs obtain fresh air during both exhalation and inhalation.
      ::鸟类的肺在呼吸和吸入期间都可呼吸新鲜空气。
    • The digestive system of birds is unique; a crop for storage and a gizzard that contains swallowed stones for grinding food make up for the lack of teeth.
      ::鸟类消化系统是独一无二的;储存的作物和含有碾碎食物的吞咽石的灰沙弥补了牙齿的缺乏。

    Review
    ::回顾

    1. What allows a bird's skeleton to be light but also rigid enough for flight?
      ::是什么让鸟的骨骼很轻 但也够僵硬可以飞行?
    2. What are the largest muscles found in birds? Why is this?
      ::鸟体内最大的肌肉是什么?
    3. How is the molecular structure of feathers different from mammalian hair?
      ::羽毛的分子结构与哺乳动物的头发有何不同?
    4. What are the two types of feathers found in birds? Which category do flight feathers fall into?
      ::在鸟类中发现的两种羽毛是什么类型的?飞行羽毛属于哪一类?
    5. How and why do birds preen?
      ::鸟儿们怎么和为什么做牛排呢?
    6. Describe the unique structure of a bird's respiratory system and how it functions.
      ::描述鸟类呼吸系统的独特结构及其功能。
    7. Why do some birds regurgitate pellets?
      ::为什么有些鸟会反转颗粒呢?