Sometimes there are things we just cannot see, and baby birds are pretty fragile for the first little bit. As I understand it, they have almost no immune system for a few weeks and are susceptible to stress reactions, not to mention internal issues. A bird, or a person for that matter, can seem big and healthy and still have a defect in a heart valve or a liver that stops working right through nobody's fault. You are an attentive and caring parront, and I think you should give yourself kudos for that~