Eat like a bird? Flight burns a lot of fuel! A 150-pound human would have to consume approximately 23,000–31,000 Calories per day if he or she had the same metabolic rate as the average 35g budgie! But birds consume small meals throughout the day and their fluid intake is minimal compared to mammals.

Many bird species' kidneys produce dry, lightweight urates and relatively little heavy, liquid urine, and they don't store wastes very long. You may notice that a bird often "lightens the load" just before taking off.

Birds' respiratory systems extend throughout their bodies — skin, skull, limbs, beak, bones, and abdomen — in a series of interconnected air sacs in order to efficiently move oxygen into the blood to power flights requiring enormous physical endurance.

Birds can breathe in and out at the same time in a circular fashion, efficiently moving oxygen in and carbon dioxide out of the bloodstream. Birds replace nearly all the air in their lungs with each breath.

To lighten their body weight, birds only have one developed ovary or testis, and their reproductive organs atrophy during non-breeding season.

Birds have hollow, lightweight, yet strong bones that are often fused to support strong flight muscles without adding extra weight.

Wing feathers or "flight feathers" form a strong but lightweight extension of the wing that moves the air to propel the bird upward and forward when a bird flaps his or her wings.

Birds' jaws are reduced to bills with no teeth — another weight reducer.