Byakuroku wasn't hungry the day he met Hiwa. He was still digesting a much heartier meal than one small skin-and-bones bird would have provided. But that wasn't the reason Byakuroku took Hiwa in.
Byakuroku isn't a natural provider. He'd never had a child before Hiwa, though he had of course observed animals and humans alike interacting with their young. He knew that Hiwa would rely on him, that Byakuroku would provide him with food and protection. After all, such a small, weak creature would never survive on his own. In a world full of more powerful ayakashi and humans, Hiwa would have been easily killed off or trapped.
But Hiwa didn't want that protection. Hiwa still doesn't.
Hiwa scowled when Byakuroku brought back food, no matter if he ate it or not. Hiwa spurned his name, which Byakuroku had so kindly given him, along with the promise of Byakuroku's name and the protection it offered. Hiwa refused to follow Byakuroku through the forest and instead flew out on his own, coming back with bruised pride and skin.
Byakuroku didn't understand it, understood it even less when Hiwa returned smelling of blood and cooked food and the holy princess, two jagged scars where his wings had been. He didn't understand it when Tsuyukusa came into their lives and started behaving as Byakuroku had always thought a child should, because his bird was so different.
Tsuyukusa followed Byakuroku and Hiwa around like there were strings tied to his horns as soon as he could assume a human enough form to move on his own. Hiwa, meanwhile, grew more and more distant, hiding himself away and biting back all the questions he'd asked Byakuroku before. Hiwa grew up, and he still insisted on standing alone.
Byakuroku loves them both: his tragic, beautiful bird, who cannot let himself be loved; and his bird's master, who will as stand tall and proud as Byakuroku's. And even if Hiwa refuses to rely on Byakuroku, Byakuroku will protect him, because that is what a parent should do.