Imagine a massive bird, size even larger than pterosaur (Quetzalcoatlus), at about 300 kg. It's able to use a magic that allows it to make its bones have negative gravitational mass(but not negative inertial mass) and rest of its body become lighter. When antigravitating, the bird's weight thus goes down to only 5-10%.
However, the bird burns through its magic power at a rate, being able to antigravitate for only two hours without rest and requiring 3 hours of rest/6 hours of land based functioning per an hour of magic flight. What anatomic features would this large bird develop, since it wouldn't have need to generate as much lift?