In the series A world without gravity it became very obvious that we all need gravity because our daily life is designed on this incredible force. But what about birds why do they need gravity?
Birds need gravity to eat without it they can not process food. Birds do not really masticate (chew) and their esophagus can be large in diameter as they swallow large meals. So, the birds depend on gravity to push their food through the esophagus from throat to the crop, then later propelled into the stomach by waves of peristalsis in the crop.
If you want to know more about how birds need gravity to eat watch the video below.