First of all, you need to get as much information about the food of a Fledgling bird as possible. Usually a Fledgling bird eats everything that you feed to dogs and cats. But you should soak the food in the water in order to make it soft and moist which will help Fledgling bird to eat it easily.
Pet food is best for Fledgling bird but you cannot provide it on constant basis because this little bird usually lives on its own and will rely on insects etc.
Fledgling bird opens its mouth when it feels hungry and if you find it in such condition, then try to attract it by building a nest in your lawn or yard and put the food in that nest which will attract this little bird.
Small birds usually require proteins in a huge quantity and feeding them with protein rich things will help them in growing fast. Fledgling bird also likes to have meat and all you need is to cut the meat in small pieces and put it on the ground or into a homemade nest for Fledgling bird.
You can also feed a Fledgling bird with the whites of white eggs. But make sure you have cut the white into small pieces so that a little Fledgling bird could easily eat it.
Always soak pet food in water and avoid dipping it into milk because birds cannot digest it properly which will cause them harm. Also avoid using all sorts of dairy products that contain milk as an essential ingredient.
Usually Fledgling bird keeps eating all day with a regular interval of 30 minutes. However, you need to make sure that you provide enough for a Fledgling bird that could help it in spending a few hours.