Clearly, without the ability to fly, almost all birds would quickly become extinct. A bird that wants to stay alive does not have the option of 'putting on a little weight'. Birds’ anatomy is designed appropriately. The large flight muscles, anchored onto the breastbone, make up from 30 to 40 percent of a bird’s body weight. Obviously these muscles power the wings, so a bird's body simply can't be too much for the flight muscles.