The first signs of life emerged nearly 3.5 billion years ago. Since that time, life has evolved into a multitude of organisms that have come and gone over the aeons. It tells us how varied life is. Of the billions of species on earth, birds (as well as bats) are rather unique vertebrates – these are the only vertebrates to evolve true/ powered flight.
Birds are now considered as the living relatives of dinosaurs, especially the theropods (meat-eating, bipedal dinosaurs). And just like their ancestors, they have pneumatic bones. It means that the bones are hollow and have dense struts to provide strength without excess weight. And also, there are air sacs present in their body to support the structure.
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Light But Strong
Most birds have to work against gravity and weight is one of the biggest factors that limit their ability to fly. Evolution and natural selection have resulted in birds having bones that are hollowed out and filled with struts to increase strength. This adaptation has resulted in them having a lighter, but stronger structure that helps during flight. However, most diving birds like penguins do not have hollow bones as they do not fly.
Birds have special structures associated with the lungs, called air sacs. Some of the hollow bones have these air sacs extended into it. To put it in the simplest way possible, these air sacs help in respiration by allowing the birds to take in oxygen while inhaling or exhaling. Basically, humans and most other mammals have bi-directional airflow. This means fresh air is mixed with the existing air in the lungs which has lower oxygen content. Birds, on the other hand, have unidirectional airflow, that is, air flows in one direction, and as a result, have higher oxygen content. This enables them to fly quickly and more efficiently.
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