What makes birds unique for flight? 

Sparrows make flight look easy. As a sparrow darts among forest trees, it never touches the branches or leaves. That’s because its feathers are continually adjusting to the surroundings. Side winds are no problem; the feathers are instantly adjusted. Sparrows are made to fly. 

For birds to fly, they need to be specially designed.

Bones need to be lightweight, so they are hollow and filled with air. If they were filled with marrow, they would be too heavy, and flying would be difficult or impossible. Birds have a very short digestive tract. Nutrition moves in and out as quickly as possible, often while flying. Extra weight would hold the bird down. Birds have no bladder because that would add unnecessary weight. Birds concentrate their urine. Uric acid needs very little water to be eliminated. We often observe this white paste on our cars. The birds are constantly unloading extra weight. 

Birds do not give birth to live young.

A flying pregnant bird would weigh too much. Instead, birds lay eggs. Their lungs are a one-way system that circulates air almost throughout the entire body using large air sacs and hollow bones. The air is exchanged in one breath. This system allows maximum oxygen absorption and maximum carbon dioxide elimination. These are but a few of the design features that birds needed right from the beginning in order to fly. If one were missing, flight would be impossible. 

(Source: Inspired Evidence- Job Martin, Incredible Creatures that defy evolution)