The Bird Skeleton: A Perfectly Structured Flying Machine

photo credit: stoc.xchng

We appreciate and revere birds for a variety of reasons. Aesthetically speaking, they appeal to our inner sense of beauty because they are gorgeous creatures with an amazing array of color, shape, and structure. Birds also impress us with their grand diversity of behavior, form, and function. But the one thing that all of us marvel at and dream about are bird’s magnificent flying ability. Beautiful color and diversity in form and function exists throughout the plant and animal kingdoms, but creatures that can fly are rare and of those relatively few species that are able to fly, none come close to being able to match a bird’s mastery of the aerial domain. We all know that a bird’s feathers are key to their fantastic flying abilities. But is that all? Not even close! There are a number of key characteristics that combine to give birds’ this special skill. In fact, you could say that their entire bodies are perfectly structured flying machines. So let’s look at this bold statement in more detail. First, for a flying machine to work properly, a lightweight structure is crucial.

One characteristic common to all birds is their bill, which has no exact parallel among other extant vertebrates, and its total lack of teeth. Most evolutionary biologists agree that the toothless avian bill is a clear case of weight reduction as an adaptation for flight. The gizzard, sometimes referred to a muscular stomach, evolved to compensate for the lack of teeth. The gizzard serves essentially the same function as teeth, to grind food into smaller bits and initiate the digestive process, but it is much, much lighter because it is just muscle tissue and does not require a heavy jawbone to support it as teeth do. This brings us to bones or rather the entire avian skeleton, which is completely modified to be very light and still strong enough for flight. The actual structure of bird bones varies to some degree, but all of them can be described as hollow, spongy, or strutted, or any combination of the three weight saving adaptations depending on which bone or even which species we are talking about. A bird’s skeleton is also made stronger by specific fusions, reductions, and/or enlargements in the bones of hands, feet, chest cavity, vertebrae, pelvis, and head. For instance, the hand and wrist bones are highly reduced and some are fused together to be light and strong, resulting in forelimbs that are entirely devoted to flying and serve essentially no other significant purpose. A bird’s tail vertebrae are reduced into a unique structure called the pygostyle, while the lower back vertebrae are fused together with a highly modified pelvis, again for lightweight and strength.

A bird’s ribs even have lateral projections that overlap and reinforce the chest cavity. This is not to mention the special wishbone, keeled sternum, or the array of adaptations in muscles, joints, internal organs, basic senses, physiology, or the possession of unique air sac structures, all of which contribute to making bird’s the spectacular flying machines that they are. But those are topics for another day.

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