I have a 4 day old duckling, who hadn't fully absorbed its yolk when he hatched. He stayed in the incubator an extra 2 days, and came out when it had been absorbed. He's not quite as active as the other ducklings, which I assumed was because of the yolk not being fully absorbed. But I'm not so sure now, because he has vomited 2 times in the last 1/2 hour. There's no blood in his vomit, or poop. Does anyone know why he threw up?