Why Do Birds Withdraw Their Feet While Flying?

Birds fly very fast. If they kept their feet outstretched while flying, they would expand their body contact with air, thus increasing air resistance and reducing their speed. So before starting to fly, they naturally withdraw their feet closely to their belly.

Some birds, such as the egret and the red-crowned crane, have such long legs that they cannot be withdrawn to their belly. So, they extend their feet parallel to their bodies so as to lessen air resistance and fly faster.